The Anaerobic Microverse: an In-depth Roadmap on AD Plant Operations, Microbiome Innovation, and Economic Impact

1 MICRO4BIOGAS — ROADMAP Cover 2 MICRO4BIOGAS — ROADMAP This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101006361 MICRO4BIOGAS is a new European research project that aims to develop a deeper understanding of anaerobic microbiomes in particular for biogas production. Here, new ways of manipulating anaerobic microbiomes are to be explored, especially with regard to bioaugmentation. The project includes a comprehensive analysis of the European biogas landscape followed by a policy analysis, which is to be published as an e-Book in the framework of MICRO4BIOGAS. Editorial Board: Manuel Porcar, Christian Abendroth, Pascal Otto, Camino Fernández, Adriana Polo, Ariadna Mendoza, Joana Branco DOI: 10.26127/BTUOpen-6923 This publication is published in parallel as a print edition under ISBN 978-3940471-84-0 3 MICRO4BIOGAS — ROADMAP MICRO4BIOGAS - ABSTRACT Since the 30.06.2021, the European Commission is funding the MICRO4BIOGAS research project. In the frame of the H2020 research programme, 14 partners from 6 European Countries will receive €5.7M for a duration of 4 years. The main goal of the project is to assess the manipulability of anaerobic microbiomes based on bioaugmentation. Before entering the anaerobic microverse, the present roadmap gives a detailed overview on the basic functionalities of anaerobic digester (AD) plants and as well the economic situation. This includes the technical functionality, different types of digester plants, but also inputs and outputs, such as substrates, biogas, emissions and digestate. To reflect the European situation in the biogas sector, MICRO4BIOGAS is focussing on the 6 countries, which are represented within the project: The situation of the respective countries is very different. Among the countries considered, Greece had the smallest number of biogas plants in 2020 (60 units). Outgoing from about 20 plants in 2011, more and more plants for the utilisation of agricultural residues have been built. Most of the Greek AD plants are now using substrates from agriculture. Finland has a similar situation to Greece. Spain and the Netherlands each had between 200 and 300 biogas plants in 2020. However, these are mostly landfills and digesters for sewage sludge. Belgium had a similar amount as Spain and the Netherlands in 2020 (approx. 200 plants), but about 50% received their substrates from agriculture. Germany plays a pioneering role in the European biogas landscape with currently 11,000 plants, which provide now more than 70,000 GWhs. More than 90% of these plants use agricultural residues. The rapid increase in the amount of AD installations was possible mainly due to legislative support. In Germany, the Renewable Energy Sources Act (EEG) established a feed-in tariff and grandfathering for biogas plants, which ensured a stable economic situation in the long term. It must be emphasised at this point that Germany started early with the construction of biogas plants. There were already more than 8,000 biogas plants in 2011. The case in Germany shows in a positive way what potential biogenic residues have. Supported by current political events (tensions with Russia and the war in Ukraine), this leads to continuously increasing prices per kWh, which in turn increases the competitiveness of biogas plants. This shows even more that biogas is a good alternative to fossil fuels. It also reduces the dependency of international fossil fuel trading. However, biogas plants are still dependent on economic support (e.g., through the feed-in tariff). Further developments are necessary to ensure that biogas plants have a long-term future. Here, new marketing strategies could take effect. Potential improvements could be achieved in relation to emissions trading, new kinds of substrates, or new ways to better integrate biogas plants into the bioeconomy (e.g., through hydrogen production or extraction of organic acids for fine chemistry). Another solution, which is the focus of MICRO4BIOGAS, is the optimisation of biogas plants based on bioaugmentation and microbiome manipulation. Bioaugmentation refers to the introduction of microorganisms into certain systems, such as contaminated soil, polluted water or even biogas plants. The challenge here, however, is to find microorganisms that could not only optimise the biogas process. These respective organisms must also be able to assert themself against the large number of existing, indigenous microorganisms. To find microorganisms that are both efficient and robust, the MICRO4BIOGAS project is specifically looking for microorganisms that are already native to biogas plants. Currently, the project partners are working on one of the most comprehensive metagenome studies in relation to biogas. About 80 different samples were collected from many biogas plants. Process chemical parameters were collected for the corresponding plants. Further analy- 4 MICRO4BIOGAS — ROADMAP ses are now being carried out using 16S-rRNA gene amplicon high-throughput sequencing or metagenome sequencing. Numerous microorganisms are to be isolated from the most promising samples and tested for suitability for bioaugmentation. In line with the goal of optimising anaerobic microbiomes, the underlying roadmap also describes the structure of anaerobic microbiomes and addresses their manipulability. The word microbiome stands for the totality of all microorganisms found in a defined habitat. The microbiome in biogas plants comprises about 300 key microbial groups and is thus a highly diverse habitat. Besides bacteria, archaea, fungi, protista and bacteriophages are found. While the work in MICRO4BIOGAS focuses on bacteria and archaea, the other microbial groups are also of interest for future projects. Both anaerobic fungi and bacteriophages have important functions in the anaerobic microbiome. Anaerobic fungi have outstanding degradation strategies and bacteriophages have considerable influence on the taxonomic composition. Nevertheless, the number of publications on these organisms in relation to biogas is extremely low. Amongst bacteria, the so-called Candida Phyla Radiation (CRP) could also play an importance in future research. One also speaks here of nanobacteria, and an interesting representative is Gracilibacteria. This genus is not viable on its own. They can physically dock to other microorganisms and are thus kept alive. They seem to have important functions in the degradation of substances that are difficult to break down, e.g. in petroleum-contaminated waters, and they have also been observed be present in taxonomic profiles, which comprise methanogenic archaea. The anaerobic treasure chest of the biogas sector certainly holds further surprises and MICRO4BIOGAS would like to help realise this potential. Independent of optimisation of anaerobic microbiomes, MICRO4BIOGAS would also like to give an update on the current situation of the European biogas landscape. In this context, an intensive cooperation with the European Biogas Association (EBA) is planned and an extensive survey is already in preparation. At least 200 people are to be interviewed, whereby stakeholders but also people without a connection to biogas are eligible. Of interest is the general level of knowledge about the biogas industry, the future potential of biogas plants for the bioeconomy, technical and legal approaches to improving biogas plants, and the need for new funding instruments. The keyword "microbiome" also plays an important role here. Finally, the cooperation with the town of Aras de los Olmos should be mentioned. The town is in the northernmost part of the Community of Valencia. Since the "Covenant of Mayors for Climate and Energy" in 2014, the town has committed itself to decarbonisation, capacity building, sustainable and affordable energy. The goal is a complete switch to renewable energy, and biogas will play a role here. Even though the goals of Aras de los Olmos are pursued independently of the MICRO4BIOGAS project, the project consortium accompanies Aras de los Olmos on its way and presents their project as a case study in this roadmap. Aras de los Olmos is currently building a biogas plant. Should this be completed within the project framework, synergies with the MICRO4BIOGAS project might be possible. The plant biogas plant might be available for test trials within the framework of MICRO4BIOGAS. 5 MICRO4BIOGAS — ROADMAP Specific tasks within work package 1 - MICRO4BIOGAS The MICRO4BIOGAS project is divided into 8 work packages. The MICRO4BIOGAS roadmap is part of work package 1, which is divided into 5 tasks. The following paragraphs are giving an overview of the specific tasks of work package 1. The MICRO4BIOGAS roadmap is the focus of task 1.5. Although task 1.1 – 1.4 are separate from the roadmap, they are partly reflected in the present roadmap as well. Task 1.1: State of the art analysis Biogas production is an industrial area with various structural solutions. There are different types of biogas reactors, but several reactors of different designs can also be combined with one another. In addition, there are additional process differences with regard to the selected process chemical environmental parameters. In particular, the number of possible substrates is highly variable. Finally, many industrial areas are known that are related to the biogas industry. The first chapter on the project roadmap is intended to show the current status in the biogas industry and to reflect its versatility. We would like to present the structural standards of different types of biogas plants against the background of biological processes, operational safety, environmental friendliness, and ecological considerations. We would also like to point out weaknesses and opportunities for improvement. Task 1.2: Policy Analysis A comprehensive overview of the political situation, political objectives, target markets and future development opportunities for the biogas industry is planned. The role of biogas plants as a networking element of the bioeconomy and industry should be emphasized. In connection with MICRO4BIOGAS, three fundamental questions arise: 1. Can bioaugmentation improve the efficiency of biogas plants and thus the market situation for biogas plants? 2. Can multi-stage biogas plants help to substitute petroleum-based raw materials at least partially? 3. Could bioaugmentation help to better interconnect the biogas industry with other branches of industry in the biorefinery or with the provision of biological resources? All three questions will be answered based on an extensive literature research. A survey with key players in the European biogas landscape is also to be carried out. Biogas production, as commented above, is linked to agriculture and cattle industry among other sectors, having thus a great influence over the development of several EU rural areas. MICRO4BIOGAS will study which existing EU policies can foster biogas production in rural areas and how to improve them, as well as the existing rural networks able to push towards Biogas production. In addition, and linked to WP6, we will analyse the public funding options and alternatives in such a low margin market. 6 MICRO4BIOGAS — ROADMAP Task 1.3. New technologies and innovations The rise of next-generation sequencing technologies in the last few years has stimulated the research of anaerobic digester microbiomes. This tremendous sequencing effort will now be gathered and exploited by the MICRO4BIOGAS consortium. With the help of curated literature and text mining strategies, all publicly available sequencing data and their associated scientific papers will be identified, integrated, and analysed, with a special focus on the manipulability of anaerobic microbiomes and on the accessibility and standardisation of the sequencing data, metadata, methods and protocols from each study. Once all possible data is integrated, several aspects and their effects on the anaerobic digestion process will be deeply analysed, for example, increase of hydrogen forming and synthrophic bacteria, DIET and its links with phototrophy, variation of typical process parameters (pH, Temperature, viscosity, etc.), increase of cellulase-producing bacteria, and alterations of signalling/quorum sensing. The final goals are to gain insight on potential bioaugmentation strategies, and to apply machine learning techniques to predict the behaviour of a specific microbiome and its implication in the biogas production process. Task 1.4. AD-Microbiomes vs. Europe The key contribution of this task will be the addition of industrial microbiome approach to the European biogas landscape (EBL). To do so, the EBL will be deeply characterized from several points of view: 1. Innovations, technological advances and in silico analysis associated to this field. 2. Public policies and public networks involving local governments. 3. Biogas stakeholders’ map. We will reinforce the biogas innovation scenario, particularly those aspects related to the industrial microbiomes of anaerobic digesters, including microbiome manipulation, big-data and bioaugmentation. The EBA (European Biogas Association), one of the most important bodies linked to the European Biogas Network, is supporting MICRO4BIOGAS in this task. T1.5. e-Book This task will see the compilation of an e-Book showing the roadmap of the project through the integration of data collected from all relevant contributions from partners. Summarized reports of each task will be aggregated and complemented with graphical abstracts towards turning the useful information into a user-friendly e-Book that will be publicly available through the project website. Furthermore, this e-Book with the project roadmap will be used for widespread dissemination among potential end-users, policy makers, and biogas plant operators, among others. 7 MICRO4BIOGAS — ROADMAP Involved partners: The MICRO4BIOGAS project is coordinated by Universitat de València (Spain) and comprises the following partners: • Gasterra BV (Netherlands) • ABS International (Belgium) • AEV Energy GMBH (Germany) • Ayuntamiento de Aras de los Olmos (Spain) • Bioenergie Verbund EV (Germany) • Technische Universität Dresden (Germany) • Draxis Environmental SA (Greece) • Bioclear Earth BV (Netherlands) • Universitat Politècnica de València (Spain) • Universiteit Gent (Belgium) • Finrenes OY (Finland) • Darwin Bioprospecting Excellence SL (Spain) • Scienseed SL (Spain) Contact For more information or interview requests, please contact: [email protected] You can also follow us on social media: @micro4biogas 8 MICRO4BIOGAS — TABLE OF CONTENTS MICRO4BIOGAS — TABLE OF CONTENTS Table of contents Chapter 1. State of the art .................................................................................. 14 1.1. Introduction......................................................................................................................17 1.1.1. What is biogas? ................................................................................................................. 17 1.1.2 Biochemical conversion to biogas ................................................................................. 19 1.1.3 Milieu conditions ............................................................................................................... 20 1.1.4 Inhibitors............................................................................................................................. 22 1.1.5 The technical production of biogas ................................................................................ 22 1.2. Substrates .........................................................................................................................25 1.2.1 Feedstocks for biogas production .................................................................................. 25 1.2.2 Delivery and storage of substrates................................................................................. 27 1.3. Anaerobic Digesters ............................................................................................................ 32 1.3.1 Types and designs of containers..................................................................................... 33 1.3.2 Construction types of tank heaters................................................................................ 36 1.3.3 Stirring systems ................................................................................................................. 37 1.3.4 Conveying technology for liquid digestate.................................................................... 45 1.3.5 Power2Gas.......................................................................................................................... 48 1.3.6 Two stage digestion .......................................................................................................... 49 1.4 Utilisation of biogas..........................................................................................................51 1.4.1 Intermediate storage in the gas storage facility .......................................................... 51 1.4.2 Conveying technology for biogas.................................................................................... 53 1.4.3 Gas treatment.................................................................................................................... 54 1.4.4 Combined heat and power plant.................................................................................... 58 1.4.5 Biomethane injection ....................................................................................................... 60 1.4.6 Thermal utilisation of biogas........................................................................................... 62 1.5 Digestate.............................................................................................................................63 1.5.1 Digestate storage .............................................................................................................. 63 1.5.2 Solid/liquid - separation of digestates ........................................................................... 65 1.5.3 Processing and utilisation of digestates........................................................................ 66 1.6 Environmental and plant protection .............................................................................69 1.6.1 Avoidance of gaseous emissions and dusts.................................................................. 69 1.6.2 Avoidance of liquid emissions......................................................................................... 69 1.6.3 Emission reduction measures......................................................................................... 69 1.6.4 Noise protection................................................................................................................ 71 1.6.5 Explosion protection......................................................................................................... 71 1.7 Potential for improvement..............................................................................................72 9 MICRO4BIOGAS — TABLE OF CONTENTS MICRO4BIOGAS — TABLE OF CONTENTS 1.7.1 Technical potential for improvement ............................................................................ 72 1.7.2 Biological improvement potential .................................................................................. 73 1.7.3 Potential for organisational improvement ................................................................... 73 References................................................................................................................................74 Chapter 2. Technological description ............................................................... 82 2.1 The goals of fermentation ...............................................................................................83 2.2 Technical parameters of operation management.......................................................84 2.2.1 Space load........................................................................................................................... 84 2.2.2 Retention time ................................................................................................................... 85 2.2.3 Productivity, yield and degree of degradation ............................................................. 86 2.2.4 Gas composition, gas volume measurement and gas analysis ................................. 87 2.2.5 Fermentation temperature ............................................................................................. 88 2.2.6 Dry matter content and viscosity of the fermentation mixture................................ 88 2.2.7 FOS/TAC............................................................................................................................... 88 2.2.8 pH value and volatile fatty acids .................................................................................... 90 2.3 Process variants ...............................................................................................................91 2.3.1 Dry matter content .......................................................................................................... 91 2.3.2 Number of process stages .............................................................................................. 92 2.3.3 Temperature control ........................................................................................................ 94 2.3.4 Auxiliary materials/additives........................................................................................... 95 2.4 Automation and control technology .............................................................................99 2.4.1 Sensors ............................................................................................................................... 99 2.4.2 Power feed-in ..................................................................................................................104 2.4.3 Plant control .................................................................................................................... 104 2.4.4 Emergency power supply .............................................................................................. 105 References..............................................................................................................................107 Chapter 3. Economical and ecological aspects.............................................. 110 3.1 The Economic Biogas Framework in Europe ..............................................................112 3.2 Exemplary biogas frameworks of selected countries ...............................................113 3.2.1 Finland .............................................................................................................................. 113 3.2.2 Spain ................................................................................................................................. 115 3.2.3 Netherlands ..................................................................................................................... 117 3.2.4 Belgium .............................................................................................................................118 3.2.6 Greece ..............................................................................................................................120 3.2.7 Germany .......................................................................................................................... 121 16 CHAPTER 1 — STATE OF THE ART Wastewater treatment: Sewage treatment produces sewage sludge, which can be treated anaerobically to produce biogas. However, the technical requirements (tanks, agitators, separators, etc.) for this process are fundamentally different from the biogas production described below in the context of agricultural plants. The legal framework conditions are also completely different in most cases. In the case of anaerobic wastewater treatment, the focus is more on the decomposition of organic substances and economic advantages when combined with aerobic processes. Biogas production from wastewater is therefore not considered in the following. New bioproducts: The primary components of biogas (CO2 and CH4) can in principle be separated and further utilised. In the case of methane, this is done by biogas upgrading. The biomethane produced can be fed into the natural gas grid and is mostly used to produce electricity and heat. In the European Union, the production of biomethane has been steadily increasing since 2010. In 2020, 32 TWh of energy was produced in this way. CO2 is also a potential feedstock for syntheses of carbon-containing compounds. However, the cost of carbon dioxide production is too high compared to other chemical processes to make it economically viable. There is also research into the production of other chemical substances during fermentation. However, these are not part of the state of the art, can be omitted, and the waste can be freed from organic fractions in this way. The potential for producing biogas from waste varies greatly from region to region. 17 CHAPTER 1 — STATE OF THE ART 1.1. Introduction 1.1.1. What is biogas? Biogas is a combustible gas mixture and consists mainly of methane and carbon dioxide. Biological processes form the basis to produce biogas. A combustible gas mixture is produced from organic biomass under anaerobic conditions. The formation of biogas can be observed in natural processes. These take place in bogs, in sediments at the bottom of water bodies and in the faeces and stomachs of cattle (Fachagentur Nachwachsende Rohstoffe, 2013). Biomass is decomposed by various microorganisms in several steps, mainly to carbon dioxide, methane and water. The gaseous components form the so-called biogas. This gas mixture consists of a methane content of 50 to 75% by volume and a carbon dioxide content of 25 to 50% by volume (FachagenCH4 CO2 ORGANIC SUBSTRATES Different biodigester formats pose different challenges and offer different benefits Anaerobic biodigester (hosts an anaerobic microbiome and maintains specific environmental requirements for biogas production) Biogas plants are available in many different designs, with different solutions for every feedstock and every industry. In this capter, we provide a clear summary that compares the various process engineering solutions and shows the advantages and disadvantages. 18 CHAPTER 1 — STATE OF THE ART tur Nachwachsen stoffe, 2013). In addition, it contains other components such as nitrogen, water (as water vapour), ammonia, hydrogen sulphide and hydrogen in small quantities. In the following, only the technical production of biogas is considered. The raw materials of the biomass are referred to as feedstocks. Plants for the targeted and priority production of biogas are called biogas plants. In biogas plants (also referred to as anaerobic digesters), biomass is used as the starting material for microbial degradation. Feedstocks include excrements from farm animals, biowaste, residues from food production as well as energy crops (Streicher et al., 2016). The resulting biogas can be used in combined heat and power plants to produce electricity and heat, or it can be purified and fed into the natural gas grid. Biogas plants make it possible, especially for companies that have unused biomass available, to save costs for disposal, prevent emissions and generate income through the sale of electricity and heat or gas. Composition of biogas: The composition of biogas is determined by various influencing factors, which also influence each other. The most important factors are the microorganisms involved, the process conditions and the chemical composition of the feedstock used. The following table (table 1) gives an overview of common gas compositions (Rasi, S., 2009). Table 1: Composition of biogas (Fachagentur Nachwachsende Rohstoffe, 2013). Methane – CH4: Methane is the main combustible component of biogas and therefore the target product in technical production. The higher the methane content in biogas, the higher its calorific value. The more methane is produced, the more electricity and heat can be generated, and thus the more revenue. For utilisation in CHPs, the methane content must normally not be below 40% - 45%. Under normal process conditions, this methane content is achieved without any problems. Biochemically, methane is formed in various metabolic pathways (Rasi, 1984). Carbon dioxide – CO2: Carbon dioxide is the second main component of biogas after methane. Since it is not combustible, it is a by-product from a technical point of view. Carbon dioxide behaves like an inert gas during combustion in the CHP unit and does not contribute anything to the calorific value. If the biogas is to be processed and fed into the natural gas grid, the CO2 must be separated. Carbon dioxide is formed at many points during microbial degradation. It is formed during hydrolysis/acidification and during methane formation. It is the end point of the oxidative degradation of carbon compounds (Abdeen et al., 2016). Water – H2O: Water is found in biogas, mainly as water vapour. After its formation, biogas usually has a relative humidity of almost 100%. In (inactive) combustion engines, this moisture can condense and, in combination with hydrogen sulphide, lead to corrosion. Biogas must therefore often be dried and H2S should be removed. Nitrogen - N2: Nitrogen is not formed during the decomposition of biomass under anaerobic conditions, but it reaches the biogas plants in small quantities when air is added. Biochemically, nitrogen behaves inertly and is therefore negligible for the technical process. Oxygen – O2: In the presence of oxygen, the energy-rich hydrocarbons in the biomass are completely degraded to carbon dioxide and water without methane being Gas Concentration Methane (CH4)50 - 75 Vol.-% Carbon dioxide (CO2)25 - 45 Vol.-% Water (H2O) 2 - 7 Vol.-% (20-40 °C) Hydrogen sulphide (H2S) 20 - 20,000 ppm Nitrogen (N2)< 2 Vol.-% Oxygen (O2)< 2 Vol.-% Hydrogen (H2)< 1 Vol.-% 19 CHAPTER 1 — STATE OF THE ART formed. Biogas production therefore takes place in the absence of air. The addition of very small amounts of oxygen with the substrates can hardly be prevented technically, but oxygen is often added to the biogas in a controlled manner to remove gaseous sulphur compounds. Under anaerobic conditions, small amounts of oxygen are consumed very quickly, which is why it is usually only found in very small quantities in biogas. It generally has an inhibitory effect on methane-forming microorganisms. In larger concentrations, oxygen forms an explosive mixture with methane, which is why the oxygen content is kept as low as possible in technical biogas production. Hydrogen sulphide – H2S: H2S is formed as a gaseous end-product during the decomposition of sulphur-containing compounds of biomasses. It is technically undesirable due to its corrosive properties. The content of H2S in biogas depends strongly on the sulphur content of the outgassing products. Sulphur compounds can have an inhibitory effect on microorganisms. However, such concentrations are usually not reached in agricultural biogas plants. Hydrogen sulphide is toxic to humans, and thus provides an increased hazard potential when found in biogas (Guidotti, 1996). 1.1.2 Biochemical conversion to biogas In the following, the biochemical basics of biogas formation will be briefly presented. Further information and an introduction to the microorganisms involved can be found in chapter 5. Biogas formation is also known as anaerobic digestion (AD). The anaerobic digestion can be divided mentally into four sub-steps. The process is shown schematically in figure 1. In technical language, all sub-steps are usually summarised as methane formation, since all biochemical processes often take place together in one container. (Weiland, 2010) Hydrolysis: The first phase of biochemical conversion to biogas is hydrolysis. In the chemically broader sense, hydrolysis means the splitting of molecules under the reaction with water (Planet Biogas, online accessed Complex biopolymers (proteins, polysaccharides, fats/oils) Broken down monomers and oligomers (sugars, amino acids, peptides) Propionate, butyrate, etc. (short-chain volatile organic acids) CH4 + CO2 H2 + CO2Acetate Fermentative bacteria Fermentative bacteria Fermentative bacteria Acetogens (H2 producing) Acetogens (H2 consuming) Fermentative bacteria Acetoclastic methanogens CO2 reducing methanogens PHASE 1 Hydrolysis PHASE 2 Acidogenesis PHASE 3 Acetogenesis PHASE 4 Methanogenesis Figure 1: Schematic representation of anaerobic fermentation (Adapted from Dutton, online accessed 2021). 20 CHAPTER 1 — STATE OF THE ART 2021). Hydrolysis can take place under anaerobic conditions. Polymers present in the biomass such as cellulose, proteins and fat (complex macromolecules) are split in this phase to oligo-, diand monomers (shorter split products) (Nwokolo et al., 2020). Bacteria involved in this process release enzymes that biochemically decompose the substrates (Bauer et al., 2009). Acidogenesis: In the second phase, also called the acidification phase, the hydrolysis products (mainly from the sugars, fat, and proteins) are converted into hydrogen, carbon dioxide, alcohols, and fatty acids by acid-forming bacteria. The fatty acids are short organic fatty acids such as acetic, propionic, and butyric acid (Schnürer, 2016). For the process, it should be noted that ammonia is formed from nitrogen compounds. In excessive quantities, this is toxic for the microorganisms and thus has a process-inhibiting effect (Aberle, 2016). Acetogenesis: The third phase of anaerobic digestion is acetogenesis (acetic acid formation). The initial products of acidogenesis are converted into even smaller molecules by acetogenic bacteria. Acetic acid, hydrogen and carbon dioxide are formed. A too high hydrogen content inhibits the conversion of the intermediate products of acidogenesis (Drake, 1994). Methanogenesis: In the final phase of biogas formation, the acetic acid, hydrogen and carbon dioxide are converted into methane by means of strictly anaerobic methanogenic archaea. The starting materials of methanogenesis are methane, carbon dioxide and water. The production of methane can be divided into two groups. Hydrogenotrophic and acetoclastic methanogenesis. In hydrogenotrophic methanogenesis, methane is produced from hydrogen and carbon dioxide; in acetoclastic methanogenesis, methane is produced by acetic acid cleavage (Schnürer, 2016). The microorganisms involved in methanogenesis are called methanogenic archaea. These are light and temperature sensitive (Wu et al., 2002; Olson et al., 1991). In anaerobic digester plants, mainly acetoclastic and hydrogenotrophic methanogenesis is relevant for methane production. The respective methane formation follows the following reactions (Zengler et al., 1999): Acetic acid-splitting (acetoclastic methanogenesis): • CH3COOH ⟶ CO2 + CH4 Hydrogen-utilising (hydrogenotrophic methanogenesis): • CO2 + 4H2 ⟶ CH4 + 2H2O Different microorganisms are involved in the individual degradation stages, and these have different optima in respect to process chemical conditions (e.g. temperature, pH value). From a process-technical point of view, a compromise must therefore be found which takes into account above all the methanogenic microorganisms. These have a low growth rate and react fragilely to disturbances. Therefore, the conditions of the biocenosis must be adapted to their requirements. The biocenosis must, therefore, allow fermenting bacteria and methanogenic archaea to coexist. The milieu conditions and operating parameters required for this are discussed below. Depending on the type of feedstock and the design of the biogas plant, different environmental conditions can arise in the individual fermenter stages. The environmental conditions have an impact on the microbial biocenosis and thus influence the metabolic products (Boe et al., 2010). 1.1.3 Milieu conditions Oxygen: Methanogenic archaea formed about three to four billion years ago. Therefore, they are amongst the oldest living organisms of our planet. Since the atmosphere had a completely different composition at that time, archaea depend on an oxygen-free environment. Even small amounts of oxygen can already have a toxic effect on archaea. An input of oxygen into the fermenters cannot be completely avoided, but this does not lead necessarily to inhibition of the involved methanogenic archaea. These archaea live 21 CHAPTER 1 — STATE OF THE ART in symbiosis with oxygen-consuming bacteria from the previous degradation steps. Therefore, oxygen is consumed before causing toxic effects on the archaea involved (Pedone et al., 2004). Temperature: Although the reaction rate of chemical reactions increases with temperature, the temperature optimum must not be exceeded. There are different temperature optima for the metabolic products of microorganisms (table 2). If these are exceeded or not reached, the underlying processes can be inhibited, and the microorganisms can be irreversibly damaged. The temperature optima for biogas plants can be divided into three groups (Dobre et al., 2014). Table 2: Temperature optima of participating microorganisms (according to Dobre et al., 2014). Due to the low temperatures in the case of psychrophilic microorganisms, it is not necessary to heat up the fermenter. Nevertheless, the gas yield per unit of time is significantly lower under psychrophilic conditions due to the lower reaction rates. Mesophilic and thermophilic microorganisms have a significantly faster gas production (Zinder et. al., 1984). Compared to mesophilic and psychrophilic temperature, the thermophilic temperatures allow a faster biogas formation, but it has a lower process stability. Under thermophilic conditions, cultures exist in a lower species range than in the mesophilic group. This means that the fermentation process is more sensitive to the introduction of new substrates, as well as to the operation and certain disturbances. The fermenter must also be brought to a high temperature level, which leads to an increase in maintenance costs. The most widely used biogas plants are those that operate in the mesophilic temperature range. They offer a good compromise between fast biogas production and good process stability (Briones and Raskin, 2003). For the cultures involved, the temperature should be kept as constant as possible. The microorganisms can react and adapt to slow temperature changes, but this is not possible with too high fluctuations in short intervals (Chae et al., 2008). When microorganisms break down carbohydrates, self-heating occurs, which influences the temperature in the fermenter. The temperature can rise to a level of 43 - 48°C if the fermenter is operated in a mesophilic mode (Lindorfer et al., 2006). pH value: Characteristic for the biodegradation process are the decrease of dry matter and an increase of the pH value. Different microorganisms require different pH values for optimal growth. The pH optimum of the individual bacteria and archaea is listed in table 3. Table 3: pH values optima of microorganisms involved (Fachagentur Nachwachsende Rohstoffe, 2013). The pH value within a fermenter is self-adjusted due to the underlying biochemical processes. The alkaline and acidic metabolic products formed during anaerobic degradation are decisive for this. The pH value can drop if the acidic metabolic products of acidogenesis accumulate. This is possible, for example, if the substrate supply is too high or if methane formation is inhibited. If the pH value drops, the inhibiting effect of hydrogen sulphide and certain organic acids may increase, and the fermenter may tip over. Thermal stage Temperature range Psychrophilic < 25 °C Mesophilic 37 – 42°C Thermophilic 50 – 60°C Microorganims pH-value Hydrolysing and acidifying bacteria 5.2 - 6.3 Acetic acid-forming bacteria and methanogenic archaea 6.5 - 8 22 CHAPTER 1 — STATE OF THE ART 1.1.4 Inhibitors Inhibitors can interfere with processes of the individual degradation stages. If an inhibitor reaches a too high concentration, the degradation process can even be completely stopped. Inhibitors can either enter the fermenter via the substrate or they arise as intermediate products of the individual degradation stages. Excessive addition of substrates can also be classified as inhibitory. Microorganisms can adapt to inhibitors, but their adaptability is limited. Examples of inhibitors that are introduced via the substrate include antibiotics, disinfectants or solvents, herbicides, salts, or heavy metals, which inhibit the process even at low concentrations (Theuerl et al., 2019). Some of the inhibitors are described below as examples. Nitrogen / Ammonia is a prerequisite for the growth and activity of microorganisms. If the added substrate is rich in nitrogen compounds (especially proteins), this leads to an increase in the ammonia concentration in the fermenter. Ammonium (NH4+) and ammonia (NH3) are produced during hydrolysis and are in equilibrium with each other (Purwono et al., 2017). • NH4 + + OH- ⇄ NH3 + H2O An increase in pH increases the NH3 content. Also the temperature influence the ammonia concentration, as with rising temperature there is a shift from ammonium to ammonia (Fuchs et al., 2018). Sulphur / Hydrogen sulphide is an essential building block for the microorganisms involved and thus essential for the production of biogas. Dissolved hydrogen sulphide (H2S) can be toxic and have an inhibitory effect at high concentrations. Elevated H2S concentrations cause corrosion and can lead to damage in the CHP. H2S can be converted with the help of sulphur bacteria, which biologically convert the H2S into harmless sulphur (Aberle, 2016). • Sulphur hydrogen oxidation: H2S + O2 ⟶ S2 + 2 H2O 1.1.5 The technical production of biogas According to Weiland (2010), an agricultural biogas plant can basically be divided into four process steps: » Feedstock management (delivery, storage, processing) » Biogas production » Digestate management » Biogas utilization (incl. storage and treatment). The individual process steps are shown in detail in figure 2. The individual process steps are not decoupled from each other but are closely connected. For example, process step four provides the required process heat for the second step (Fachagentur Nachwachsende Rohstoffe, 2013). Components of a biogas plant: The components used in a biogas plant vary depending on the design of the plant. In the following, only the core components of a biogas plant will be discussed. Further information on the individual plant components can be found in chapters 1.2 - 1.5. Figure 3 shows the schematic structure of a biogas plant. Feedstocks/Substrates: Feedstock are plant and animal substances that can be fermented in biogas plants (Person et al., 2019). They originate from agriculture as well as industry, commerce, and municipalities. Feedstocks differ in their degradability and biogas yield due to different material properties. In addition to residual materials, renewable raw materials are also used, which are produced specifically for the use in biogas plants (Person et al., 2019). The different substrates are discussed in further detail in section 1.2. 23 CHAPTER 1 — STATE OF THE ART Preliminary pit, silo and receiving feeder: The liquid substrates are temporarily stored in the preliminary pit. This is usually a round container or tank, which is embedded in the ground (Person et al., 2019). The building material is usually ferro concrete, which is often used in the form of prefabricated parts or as cast-in-place concrete. The preliminary pit is located before the digester and has the task of storing several days’ rations of liquid substrate. The respective tanks can be open or closed. In addition to liquid, pumpable and agitable substrates, it is also possible to introduce solid substrates to a limited extent. However, it is more common to store solid substrates in silos. Silos are usually piles of solid substrates that are covered with a membrane to protect them from rain. Energy crops (or parts thereof) are stored in silos after harvesting and then continuously removed over the course of the year. During storage in the silo, lactic acid fermentation occurs. Due to the lowered pH from lactic acid fermentation, siloed substrate becomes durable and more usable for anaerobic digestion. Solid substrates are conveyed from the silo to the receiving dosing unit using a wheel loader or a similar machine. A receiving feeder consists of a receiving container, a dosing device and often also crushing and weighing technology. The substrates are are further conveyed via either a chute or a solid’s feeder such as a screw conveyor (Person et al., 2019). The comminution of the substrates serves to protect the system from blockages and also significantly increases Figure 2: General process of biogas recovery (Adapted from Fachagentur Nachwachsender Rohstoffe, 2013). Delivery and storage Preparation infeed Feeding, metering Preparation and pretreatment (optional) Sorting comminution, wetting down to mash, homogenisation Biogas recovery Fermentation in digester Solid/liquid separation (optional) Field spreading, compositing Liquid fertiliser Field spreading or compositing without solid/liquid separation Biogas treatment and storage Drying, desulphurisation Biogas utilisation Electricity and heat generation (CHP) Digestate storage and/or secondary fermentation Digestate treatment Digestate 2nd process step 3rd process step 4th process step 1st process step Biogas Biogas 24 CHAPTER 1 — STATE OF THE ART the usable surface area for the microorganisms (Sreekrishnan et al., 2004). Fermentation tank/digester: The biochemical degradation process takes place in the fermentation tank. Different microorganisms gradually decompose the introduced substrates under exclusion of oxygen. In the process, biogas is produced in the fermentation tanks. The decomposition takes place under exclusion of air and light. Fermentation tanks are often made of ferro concrete, but constructions made of steel enamel or stainless steel are also common. The airtight cover of the tanks is provided by a flat solid or foil roof with integrated gas storage. To facilitate the heating of fermentation tanks, they are usually insulated. Heating is usually provided by the waste heat from a CHP unit. Systems for mixing, usually agitators, are installed in the tanks; these prevent differences in concentration and temperature within the fermentation substrate. In doing so, they mix the bacteria and archaea in the tank with the fresh biomass. Agitators also prevent sedimentation and flotation of the fermentation substrate and, depending on the viscosity, are also important for gas discharge. Secondary digesters: In two-stage processes, a secondary fermenter is connected down-stream of the fermentation tank/ digester. This is where substrates that are more difficult to degrade are converted. A secondary fermenter is often necessary to meet the requirements for a minimum retention time in the gas-tight system and thus avoid emissions (Schmidt-Eriksen, 2011). Separator: The digestate from the fermenter is separated into a solid and a liquid phase in the separator. The liquid phase is collected and discharged as liquid fertiliser after intermediate storage. If required, it can also be used to mash the solid substrates in the fermenter. The solid phase has a reduced volume and can be used as a highly concentrated fertiliser. Figure 3: Components of a biogas plant (Planet Biogas, accessed 07.10.2021). Slurry and solid biomass are suitable for biogas production. A cow weighing 500 kg can be used to achieve e.g. a gas yield of maximum 1.5 cubic metre per day. In energy terms, this equates to around one litre heating oil. Regrowable raw materials supply between 6 000 cubic metre (meadow grass) and 12 000 cubic metre (silo maize/fodder beet) biogas per hectare arable land annually. Biogas System Are used as fertiliser or are composted. This substantially reduces the use of mineral fertiliser in agriculture. Fermented residual materials 1 ha energy crops, e.g. maize, grain, reeds Feed Biogas Gas motor Generator Heat Electricity Energy crops of biowastes Livestock farming Biowaste Slurry or manure If the biomass has been fermented in the digester, it is first placed in the digestion residue storage facility from where it can be removed later and used as high-quality fertiliser. Digestion residue storage The methane content and the quality of the biogas are increased to make it like conventional natural gas. Gas treatment plant In the CHP the biogas is incinerated to produce electricity and heat. Combined heat and power station (CHP) The treated biogas can be fed directly into existing natural gas networks... Natural gas network ... or can be used as fuel. Biogas petrol station The resulting biogas is stored in the top ("hood") of the fermenter, directly above the ermenting biomass. Gas storage is e.g. fed into the local heat supply network. Process heat Collection tank for biomass. Pit In this tank, with light and oxygen excluded, the biomass is digested by anaerobic micro-organisms. This digestion process produces methane and carbon dioxide - the biogas. Digester Heats the fermenter. Process heat Gas storage 25 CHAPTER 1 — STATE OF THE ART Digestate storage tank: The digested fermentation residues are collected in the digestate store. These are then used as fertilizer in agriculture. Digestate tanks are located downstream of the fermenter and the separator. Depending on the intended use, the digestates are stored in different ways (Effenberger et al., 2008). The digestate storage must be large enough to store the digestate for the next thinning in spring or autumn. The liquid phase of the digestate is stored in round basins or lagoons, the solid phase on paved areas. Gas storage: Gas storage tanks are used to buffer fluctuations in gas production and to store biogas for flexible operation. This enables flexible operation of the CHP units, which then do not have to be operated permanently, but can also be shut down for several hours. The plant can thus be operated according to demand and feed in electricity when it is needed and best remunerated. In the event of technical faults at the CHP unit or during maintenance work, gas storage facilities prevent the loss of biogas for a limited time. Gas storage tanks, as well as all gas-tight covered fermentation tanks, are equipped with an over-vacuum safety device. In the event of overpressure, they release biogas into the atmosphere and in the event of under pressure they let in air. This is to prevent damage. Safety shutdowns, operating methods and the gas flare must ensure that a release of biogas through the overpressure safety devices occurs as rarely as possible. Heat reservoir: Heat reservoirs are water tanks with an insulating layer. They store generated heat even during times when no CHP is in operation. The stored heat can be used, for example, to heat the fermenter or to dry substrates. Heat reservoirs are necessary if the CHP units are not applied fulltime, i. e. if they are operated flexibly. Gas flare: A gas flare is almost always connected to the gas pipe system of a biogas plant. This burns biogas in an emergency to prevent uncontrolled release in the event of overpressure. This is to prevent emissions and explosive atmospheres on the plant. Combined heat and power plant (CHP)/ gas utilisation: The utilisation of biogas usually takes place in so-called combined heat and power plants. There, electricity, and heat are generated from the biogas. Biogas utilisation is mostly carried out by means of gas Otto engines with flange-mounted generators. Thermal and electrical energy produced is reused in the biogas plant or fed into the public grid. The thermal energy produced is used in the fermenter, temporarily stored in buffer tanks, fed into a district heating network or used for drying plants. If heat utilisation is not possible (e.g. in summer when the heat demand is low), the CHP units have emergency coolers that dissipate the excess heat to the ambient air. The electrical energy is fed into the public grid via a transformer station. 1.2. Substrates 1.2.1 Feedstocks for biogas production The composition of the substrate mixes and the nature of the individual substrates (dry matter content, structure, origin, etc.) determine the design of the process technology (DBFZ Deutsches Biomasseforschungszentrum, 2021). The biogas yield is not only substrate-specific, as one could assume based on figure 4, but it also depends on the boundary conditions such as HRT, temperature and the plant operation. Therefore, different biogas yields can occur with the same substrate input (Weiland, 2010). When using renewable raw materials, the operator should make use of different feedstocks to be able to react to crop failures and price fluctuations. In Germany, biogas plants are often operated primarily based on animal by-products such as liquid or solid manure and renewable raw materials (Nwokolo et al., 2020). Substrates can be differentiated according to their material properties, degradability, and gas yield. The 32 CHAPTER 1 — STATE OF THE ART Feeding stackable substrates Rinse pit Feed screw Input poison Figure 9: Feeding of stackable substrates. Another possibility is to introduce the solids by means of a piston (figure 9). A hydraulically operated feed cylinder presses the substrates into a pressure channel that is below the liquid level of the fermenter (Hopfner-Sixt, et al., 2007). After the substrates have been introduced, the channel is closed again by a hydraulic slide valve. The introduction of the solids by means of feed pistons enables automatic dosing at any time intervals. With this method, there is a risk that the substrate introduced will form a sink layer and clump together, which means that it is no longer optimally degradable for the microorganisms (Kamarád et al., 2013). Containers with and without crushing tools are used to feed the feed screws and feed pistons (Kissel et al., 2014). Size reduction increases the substrate surface for microbiological degradation, which has a positive effect on methane production. The higher the comminution, the faster the degradation takes place, but the gas yield does not necessarily increase (Naegele et al., 2014). 1.3. Anaerobic Digesters The digester is the main component of a biogas plant. In this reactor, microorganisms convert organic biomass into biogas. The digester must provide optimal growth conditions for the microorganisms to ensure the highest possible biogas production. Essentially, a digester consists of a fermentation tank, which is thermally insulated, a heating system, mixing units and, if necessary, discharge systems for sediments and the fermentation substrate (Fachagentur Nachwachsende Rohstoffe, 2013). Biogas plants can be divided according to the type of fermentation process into continuous wet fermentation, continuous dry fermentation and discontinuous dry fermentation (Effenberger and Lebuhn, M., 2011). In 2016, an operator survey (reference year 2015) was conducted from 310 samples (Nwokolo et al., 2020). The result of the survey can be seen in figure 10. The most commonly used reactor types were stirred tank fermenters, followed by multistage systems in which the stirred tank fermenter is followed by another reactor type (usually as a secondary fermenter). With a three percent share, the plug flow reactor was named second most frequently. Newer reactor types such as ring-in-ring solutions or those operated according to the double-chamber process (Pfefferkorn principle) were only rarely mentioned. The small number of special processes can be attributed to the low rates of expansion of biogas plants after their market saturation (Fachagentur Nachwachsende Rohstoffe, 2013). 1.3.1 Types and designs Continuous stirred-tank reactor Pug flow reactor Continuous stirred-tank reactor-combination Others 5% 2% 3% Figure 10: Percentage distribution of the fermenter systems used (operator survey 2016, reference year 2015) (Adapted from Planet Biogas, accessed 07.10.2021). 33 CHAPTER 1 — STATE OF THE ART of containers General construction types of containers: Fermenter designs can be differentiated according to their geometry (round, angular, horizontal, or vertical), their mixing (stirred tank, plug flow), their process requirements (parallel, serial), their biological arrangement (single-stage, multi-stage) and their constructional design. In digesters with complete mixing, the substrate fed in is distributed evenly over the entire contents of the digester. When fresh substrate is added, the corresponding amount of fermentation substrate is removed from the reactor. With plug flow, the substrate is fed to the beginning of the reactor. A defined amount of substrate fed in will theoretically not mix with another defined amount until the end of the reactor. In practice, however, this does not apply due to recirculation flows. In batch digesters, neither mixing of the biomass nor continuous feeding of further substrate is necessary (Samer, 2012). Table 6 shows a comparison of the properties of different fermenter technologies. Biogas reactor with complete mixing (storage-flow process): As shown in figure 10, stirred tank fermenters are the most common type, with a share of 90%. They are continuous stirred tank reactors (CSTRs) and are mainly used in the field of agricultural biogas production. They are fully mixed reactors in cylindrical upright design. A scheme of a typical CSTR is shown in figure 11. The CSTR reactor is characterised by its simple, robust and reliable technology. (Weiland, 2010) CSTRs are designed to maximise the contact between the biomass and the substrate, which increases the fermentation performance. Feeding is automated to the greatest possible extent. The digester is designed as a tank with a concrete floor and walls made of steel or ferro concrete. It can be fully or partially sunk into the ground or built completely above ground (Mutungwazi et al., 2018). Foil roofs or concrete ceilings are built on top of the tanks for gas-tight covering. Mixing is achieved with one or more agitators located in or on the reactor. The agitators are arranged either centrally or laterally. Hydraulic fermenters without agitators are also possible. The mixing devices must be very efficient. Agitator systems are discussed in more detail in chapter 1.3.3. A size up to 6,000 m3 is possible. As the size increases, mixing and process control become increasingly demanding. In principle, all substrate types can be introduced, ideally pumpable substrates with a low to medium dry matter content. This type of fermenter is basically Figure 11: Stirred tank reactor with long shaft agitator (Adapted from Energypedia, accessed 15.10.2021). Geared motor Platform Large-blade rotor SMA Central column Heating pipes Discharge Fill level Gas-storage space Gas-retaining diaphragm Floating roof Insulation 34 CHAPTER 1 — STATE OF THE ART suitable for continuous, quasi-continuous and discontinuous feeding. Advantages are the low-cost construction, the variable operation as flow-through or flow-through-storage system and low maintenance efforts, which can be carried out without emptying the fermenter. Disadvantages are high costs due to energy-intensive agitators, short-circuit flows (parts of the fresh substrate can get into the outlet) as well as floating blankets and sink layers (Fachagentur Nachwachsende Rohstoffe, 2013). Plug flow biogas reactor: Plug flow fermenters are designed in the form of horizontal pipes with a round or rectangular cross-section. They use the displacement effect of the substrate to achieve a plug flow through the fermenter. The substrate is continuously fed into the reactor via the substrate feeder (usually with a screw pump). The feed device creates a pressure that guides the substrate through the reactor chamber. The plug flow leads to a separation of the fermentation stages within the tank. The schematic construction is shown in figure 12. Agitators are needed for mixing. Special agitators are needed due to high content of solids, mostly slow-running paddle shafts, which cause mixing across the direction of flow (direction of movement of the substrate), but not in the longitudinal direction. There are horizontal and vertical plug-flow fermenters; horizontal reactors are mostly used in agriculture. Standing reactors are rarely used; they also exist in designs without agitators (Veluchamy et al., 2019). Standing reactors use gravity and pumps to mix the biomass. Plug flow fermenters are mostly made of steel or stainless steel as well as ferro concrete. The size of horizontal digesters is about 800 m3 for horizontal digesters and up to 2500 m3 for vertical digesters. For economic reasons, horizontal reactors made of steel or stainless steel are usually only manufactured up to a volume of 300 m3; in these size regions, the advantages of the compact and cost-effective design outweigh the disadvantages. The fermenters are built in the factory and transported to their place of use, which limits the size of the tanks (Fachagentur Nachwachsende Rohstoffe, 2013). The tanks are thermally insulated, often offer several devices for gas extraction, substrate connections as well as digestate discharge, agitator, and heating pipes. Heating coils can be integrated into the paddle agitators. Biogas reactors with plug flow are suitable for wet fermentation, mostly pumpable substrate with high dry matter content is used. Quasi-continuous or continuous feeding of the tanks is envisaged (Veluchamy et al., 2019). The compact design is an advantage, which is why these digesters are preferably used in small plants. Due to the construction, floating ceilings, sinking layers or sediFigure 12: Plug flow reactor (Adapted from Persson et al., 2019). Biogas Extraction system Dewatering Press water Residue Agitators Pretreated waste Microorganisms and liquid are recycled Plug flow reactor Calibrator 35 CHAPTER 1 — STATE OF THE ART mentation of solids are usually avoided. The retention times are usually adhered to and can be determined relatively accurately, as short-circuit flows cannot occur as with other reactor types. Due to the small size, there is less heat loss, and the reactor can be heated effectively. The small size of the fermenters is a disadvantage, and the fermenter must be completely emptied for maintenance work. The process stability is lower than with stirred tank fermenters, and the operating and construction costs are usually higher (Edelmann and Engeli., 2015). Leachbed processes: For discontinuous operation, so-called boxes or garages are used. These can usually be opened via a gate and emptied and filled directly. The box fermenters are made of ferro concrete or steel. In practice, several containers are connected in parallel and operated at different times to be able to operate the downstream plant continuously. Two to eight units, usually four units, are used in parallel. This enables quasi-continuous gas production. Box digesters are particularly suitable for dry fermentation, often using pourable substrates such as corn and grass silage. This type of reactor is particularly advantageous for substrates that cannot otherwise be handled, such as organic waste with impurities (Qian et al., 2016). Figure 13 shows the process of a box fermenter from BEKON GmbH. In leachbed processes, the fermenters are filled with substrate, sealed airtight and only opened after outgassing. Both the filling process and the emptying are usually carried out with a wheel loader (Clarke, 2018). The ratio of fresh material to inoculation material is 40% to 60%. This ensures that the substrate is supplied with sufficient anaerobic bacteria. For humidification, the substrate is sprayed with percolate (leachate of the substrate, which is recirculated) with nozzles attached to the ceiling throughout the entire residence time. The percolate migrates through the substrate, is collected, and pumped into a storage tank. The temperature of the biomass is controlled by means of attached wall and floor heating. Gas collection pipes are attached to the fermenter, in which the biogas is collected and discharged (Fu et al., 2018; Weiland, 2010). Leachbed digesters are usually single-stage processes in batch operation and the various degradation reactions (hydrolysis, acid and methane formation) take place in one process step. Figure 13: THE BEKON PROCESS, Process of a box fermenter (Adapted from Bekon gmbH, accessed 18.10.2021). BiogasLean gasHigh methane content gas Fermenter with wall and floor heating 1,5% Fermenter door Gate Percolate drainage system Service room with modular rear panel Percolate fermenterBiogas utilization Heating system Gasholder Flare Purge air Exhaust stack 36 CHAPTER 1 — STATE OF THE ART Double chamber process (Pfefferkorn principle): Another approach to wet fermentation is the use of double-chamber processes. A biogas plant based on the Pfefferkorn principle (after the Austrian inventor Herbert Pfefferkorn) is a plant that works without an agitator in the fermenter. The reactor contains a main fermentation tank with an enclosing secondary fermentation tank with hydraulic mixing. This process eliminates the need for agitators, which saves electrical energy. In addition, the maintenance effort is reduced, as no agitator units must be changed or serviced, which also saves costs. The substrate used consists mainly of liquid manure and small amounts of corn silage and grain meal. The construction effort is much higher than with stirred Reactor Feeding Temperature Stirring Substrate Reliability Stirred tank reactor Continuosly Mesophilic or thermophilic Flow-through fermenter with agitators, hydraulic fermenter without Agitators Easy to pump, Use for various substrates Foreign substances can cause technical problems Plug flow reactor Continuosly Usually thermophilic, also mesophilic possible Longitudinal or trans-verse to the flow, vertical systems with-out agitators Pumpable, mainly used for municipal biowaste High toler-ance to Foreign substances Box fermenter Discontinuosly Mostly mesophilic, also thermophilic possible No agitators, Percolation system Stackable, mainly used for municipal organic waste Robust fermenter without moving parts Table 6: Pump comparison (BioBG, accessed 25.10.2021). tank fermenters. The fermenter volume is between 400 and 6,000 m3 (Fachagentur Nachwachsende Rohstoffe, 2013). Mixing takes place through pressure surges. The pressure surges are generated when the gas at the end is not extracted but displaces the liquid phase and thus leads to different filling heights of the two tanks. Due to a sudden pressure equalisation, the filling levels then balance out abruptly. A major disadvantage of the system, apart from the high construction costs, is the enormous technical effort required to deal with any blockages that may occur. 1.3.2 Construction types of tank heaters To ensure a stable fermentation process, there should be no temperature fluctuations in the fermenter. Temperature fluctuations must, therefore, be kept low in terms of temporal temperature fluctuations and as well in respect to temperature gradients within the fermenter. Excessive fluctuations as well as exceeding or falling below certain temperature limits can inhibit the process or even cause it to come to a standstill (Han et al., 2020). According to the German agen- 37 CHAPTER 1 — STATE OF THE ART cy for renewable resources (Fachagentur Nachwachsende Rohstoffe, 2013), temperature fluctuations can lead to: • The supply of fresh (cold) feedstock • Temperature stratification or temperature zone formation due to insufficient thermal insulation, ineffective or incorrectly dimensioned heating, insufficient mixing • Location of the heaters • Outside temperatures • Failures External or internal heat exchangers or heaters provide the required process temperature. They compensate for heat losses and heat the substrate. Accordingly, heating technologies can be divided into system-integrated and external systems. Integrated heating: For the integrated heating solutions, a distinction can be made between wall heating, floor heating and heated stirring unit. Wall heating is carried out by installing heating pipes inside the tank in the wall as well as on the outer wall (Persson et al., 2019). Stainless steel pipes, PVC or PE are used as material for the pipes. Stainless steel pipes have a better heat transfer than plastic pipes, turns can be saved, but they are also more expensive. In the tank, the use of stainless-steel pipes means that less heating surface is required, which in turn reduces the flow resistance. Plastic pipes are installed at Figure 14: Stainless steel heating pipes installed in the digester (inside) (left); installation of heating hoses in the digester wall (centre, right) (Fachagentur Nachwachsende Rohstoffe, 2013). the inner tank wall or inside the wall (figure 14). Table 7 shows an overview of integrated heaters. Figure 14 shows different types of integrated heating systems. External heat exchanger: When using external heat exchangers, the substrate is heated outside the fermenter. This can be done in wet fermentation plants by heating in circulation or in the substrate feed (Persson et al., 2019).Heated percolate can temper the fermentation substrates in solid-state fermentation plants. Table 8 shows an overview of external heat exchangers. External heat exchangers can be used to avoid temperature fluctuations during substrate introduction. As a rule, stainless steel is used as material for heat exchangers, and they are usually designed as spiral or double-tube heat exchangers. External heat exchangers are basically suitable for all types of fermenters; they are often used in plug-flow fermenters. Heat exchangers are well suited for thermophilic operation. External heat exchangers ensure good heat transfer, the entire biomass volume is reached by the heating. They can be easily cleaned and maintained and offer good temperature controllability (Fachagentur Nachwachsende Rohstoffe, 2013). 1.3.3 Stirring systems Functions of the stirring systems: According to Kissel et al, (2014), agitators must fulfil the following tasks in biogas plants: • Homogenization of the ingredients, this 38 CHAPTER 1 — STATE OF THE ART Type Description Range of application Advantages Disadvantages Wall heater Inside of tank All types of fermenters, rather standing Pipes accessible for work, e.g. for lime deposits, very good heat transfer Flow resistance, deposits, risk of tearing In the tank wall, heating hoses In-situ concrete, not segment construc-tion Pipes protected Temperature gradient in wall (stress cracks), reduced heat transfer Container exterior Steel tank only Pipes easily accessible Reduced heat transmission Floor heating Common floor heating cation lines All standing fermenter Longitudinal or transverse to the flow, vertical systems without agitators Low heat transfer, especially with sink layer formation Heating in stirring unit Double wall pipe system All digester types, rather horizontal No agitators, Percolation system Damage to welds difficult to detect, less heat transfer during standstill Type Description Range of application Advantages Disadvantages Heating in circulation Circulation of the fermenting liquid, e.g. via doublepipe or spiral heat exchanger Wet fermentation Tanks and walls are free of fix-tures Increased electrical energy consumption due to pumping; deposits possible Heating in the feed Design of the inlet as a doubletube or spiral heat exchanger Wet fermentation Flexible handling, e.g. combination with hygienization More elaborate tank insulation required; additional heating may be necessary; storage possible Heated percolate From temperaturecontrolled percolate storage for trickling over digestate in the solid’s fermenter Solid’s fermentation No or low-power heating devices required at the solid’s fermenter Carbonates in percolate liquid clog pumps, pipes and nozzles Table 7: Tank heating integrated (Postel et al., 2008). Table 8: External tank heating (Postel et al., 2008). 39 CHAPTER 1 — STATE OF THE ART enables a uniform conversion of substances for the formation of biogas. • Suspension of heavy inorganic substances that are introduced via the substrate. • Reverse suspension: light, floating substances must be fed back into the bulk phase at varying levels. This serves to avoid the formation of swimming layers. • Heat exchange in the container, uniform temperature distribution throughout the fermenter room for uniform material conversion. • Complete circulation flow without demolition and formation of stagnant zones, for the discharge of biogas and even distribution of freshly introduced substrates. • Uniform ground flow. • Avoidance of blockages on the agitator. The design of the agitators is individual for each plant; there are no uniform concepts, as the ingredients and the process control vary depending on the plant. The technical requirements for the agitator can be subdivided according to the process. Biogas from wastewater plants can be classified as technically rather simple, since the solids content ranges between 3% - 6% (Gomez, 2016). The solids content in biogas plants is considerably higher, up to 22% (Krieg et al., 2016). The technical effort to realise an effective agitation system is higher. In the following, some frequently used agitators are discussed in more detail. Submersible mixer (propeller): This type of agitator is very common and has been used in biogas plants for a long time. Submersible propeller agitators are used in fully mixed fermenters as well as in digestate stores. In the fermenter, this type of agitator is often combined with other types of agitators (Mohammadrezaei et al., 2018). It is also possible that several agitators are operated in one tank. The propeller and the electric motor form a unit that is completely immersed in the medium. Therefore, the entire agitator unit must be watertight and corrosion-resistant. Attached to a vertically arranged guide rod, the height of the agitator can be adjusted. The direction of action of the propeller can be changed by means of a crank, and the agitator unit can also sometimes be swivelled up and down by 30°. This makes them easily adjustable in position and is good against floating covers and sinking layers (Brehmer et al., 2016). The fermentation substrate cools the motor in the submersible mixer. In thermophilic fermenters, one has to verify that the cooling capacity is sufficient (Bauer et al., 2019). Submersible mixers are counted among the fast-rotating agitators, they are equipped with small agitator blades (Ø 400 - 1000 mm) and have propeller speeds of up to 600 rpm. Due to the high speed, submersible mixers generate a large directional thrust, which is suitable for mixing low-viscosity fermentation substrates. The agitators have an output of between 0.25 and 35 kW. For fermenters between 24 and 28 m in diameter, common sizes are between 12 and 18 kW (Persson et al., 2019). With viscous media, such agitators only function to a limited extent; an effective mixing effect is only achieved with long agitation times. Viscous media damage or wear the agitator blades very quickly, which is why they should not be used in this application. For higher viscosity, modified submersible mixers are available, which have larger agitator blades (Ø >1000 mm) and reduced rotational speed. There are also special designs of submersible mixers that are driven hydraulically and not electrically, and these are operated by hydraulic units, which are installed outside the fermenter (Bauer et al., 2019). Submersible mixers are well suited for fermentation tanks with variable filling heights. However, the agitator blades must be completely submerged during operation to prevent imbalances. An advantage is that submersible mixers can also work at very low fill levels. Fast-running submersible mixers are better suited for stirring floating layers than slow-running submersible mixers, as they have a higher suction and pushing effect. The disadvantage is the relatively high energy demand, and that moisture can penetrate the motor. 40 CHAPTER 1 — STATE OF THE ART Rod mixer (Figure 16 & 17): Like submersible mixers, rod mixers have long been used in biogas plants. They are used in fully mixed fermenters with a constant liquid level and are often combined with other agitators (Mohammadrezaei et al., 2018). For low-viscosity media, the combination of submersible mixers and high-speed rod mixers has proven successful. For viscous fermentation substrate, paddle or long axis agitators are combined with slow-running large blade rod mixers. Propellers are attached to a drive shaft of the agitator, the number of which can vary. In the field of biogas production, rod mixers are driven by electric motors located outside the fermentation tank. Compared to submersible mixers, they require less maintenance and have a longer lifetime, which reduces costs. Like submersible mixers, rod mixers are fast agitators. The diameter of the agitator blades is relatively small (<700 mm) and the speed of the propellers is high (up to 600 rpm). There are also other types of hand blenders with slower speeds (<100 rpm) and larger blades (Ø > 1200 mm). These slow-running stick mixers are used in the fermentation of substrates like corn or silage, which leads to an increase in dry matter content and viscosity (Kissel et al., 2014). By changing the direction of rotation, the Figure 15: High-speed submersible mixer with cable guide on the suspension cable (Kissel et al., 2014). Figure 16: High speed rod mixer with smaller propeller (Kissel et al., 2014). mode of action of the agitator can be changed from pushing to pulling. The shape of the agitator blade, the orientation of the agitator and the direction of rotation must be adjusted. The rod mixers are mounted in the upper area of the fermenters, either guided through the fermenter wall or, in the case of a concrete ceiling, through the fermenter ceiling. To be able to counteract the formation of floating layers, the rod mixers are designed to swivel hydraulically, and their in- 41 CHAPTER 1 — STATE OF THE ART clination can be varied (Bauer et al., 2019). There are multiple scenarios on how rod mixers are applied. For example, there is scenario, were sinking layer are avoided by mounting the mixer close to the floor. For this specific case, short agitator shafts are used, which are not swivelling. Compared to the submersible mixers, however, swivelling stick mixers are limited in their adjustment possibilities and cannot, for example, work tangentially to the container circumference. Long-axis agitators (figure 18): This type of agitator was specially developed for the use in biogas plants. It is well suited for fermentation substrates with a high viscosity and for mixing viscous media. Due to their low speeds, they are not suitable for stirring floating blankets at variable filling levels and for thin-bodied fermentation mixtures. Long-axis agitators are combined with slow-running large vane mixers or large vane submersible motors to achieve a good agitating result (Krieg et al., 2001). Due to their large blade diameters (>1.5 m), long axis agitators manage with low speeds (<40 rpm) for effective mixing. Long-axis agitators are mounted on the tank bottom as well as on the digester wall/ceiling. On tanks with a concrete ceiling, they are guided through the ceiling. In the case of fermenters with an attached gas storage and a weather protection foil, they are guided through the Figure 17: Walland ceiling-mounted rod mixer with hydraulic (left) and mechanical (right) (Kissel et al., 2014). Figure 18: Walland ceiling-mounted rod mixer with hydraulic (left) and mechanical (right) (Kissel et al., 2014). 48 CHAPTER 1 — STATE OF THE ART In the power-to-gas process, electrical energy is temporarily stored in the form of gaseous energy carriers (Zapf, 2017). Excess electricity from renewable energies is used for this purpose and is available when production is greater than demand. The gaseous energy carrier can be ammonia, hydrogen or methane. In the context of biogas technology, the production of hydrogen and methane from hydrogen is interesting. Hydrogen is obtained from water by electrolysis using electrical energy. • 2 H2O → 2 H2 + O2 Hydrogen formation: There are various technical options for realizing this. The best researched are alkaline electrolysis (AEL) (Brauns and Turek, 2020), electrolysis on polymer electrolyte membranes (PEM) (Carmo et al., 2013), and solid oxide electrolysis (SOEC) (Wang et al., 2019). • Alkaline electrolysis: Alkaline electrolysis is the most technically mature variant of hydrogen production by electrolysis. It is carried out at high pH values with KOH or NaOH as electrolyte. Commercial plants producing hydrogen by alkaline electrolysis have been available for many years (Zoulias et al., 2004). These plants can reach outputs of up to several MW (Götz et al., 2016). • Polymer electrolyte membranes electrolysis: This variant of electrolysis is characterised by the use of a solid membrane that only allows protons to pass. On an industrial scale, the method is less widespread than alkaline electrolysis, but there are also industrial applications with outputs above one MW (Ayers et al., 2010). • Solid oxide electrolysis (SOEC): This process is already well researched, but not yet established on an industrial scale. The process is characterised using a solid body as an electrolyte. At high temperatures around 900°C, this becomes conductive for oxygen ions. The high temperature also causes a drop in the necessary voltage and thus in the electrical energy requirement (Wang et al., 2019). Hydrogen conversion: Hydrogen can be used directly in many ways, however, the aim of power-to-gas processes is to store electrical energy in chemical form. The storage of hydrogen is problematic because of the high diffusion rate in most materials and the high corrosiveness towards steel (Li et al., 2020). More practical is the conversion and storage of methane. Methanation can take place chemically or biologically and essentially follows the following reaction equation (Götz et al., 2014). • CO2 + 4 H2 → CH4 + 2 H2O Chemical H2-methanation: In chemical methanation, carbon monoxide is first formed as an intermediate, which then reacts with hydrogen to form methane and water in a modification of the Fischer-Tropsch synthesis (Wei et al., 2011). The reaction is carried out at high temperatures and pressure on metal catalysts. Chemical methanation has been established as a large-scale process for fuel production for decades. The required carbon dioxide can be obtained from exhaust gases of combustion engines, from biogas (only after adsorption of H2S) or from off-gas from biomethane production plants. Biological H2-methanation: Hydrogen can also be converted to methane by microorganisms (biological methanation). The process is known as hydrogenotrophic methanogenesis from biogas formation and therefore also takes place in biogas plants. A big difference is that in biogas plants the hydrogen is formed in situ in the aqueous phase and immediately consumed again. In biological methanation, the hydrogen is present as a gas and must first be dissolved in the aqueous phase. Due to the poor solubility of hydrogen at normal pressure in water, this is a technical obstacle (Jensen et al., 2021). Since hydrogenotrophic methanogenesis is part of the formation process of biogas, the idea of feeding hydrogen from 49 CHAPTER 1 — STATE OF THE ART electrolysis into a biogas plant is obvious. It allows the usage of the existing infrastructure and reduces investment costs. Alternatively, the process can also take place in a stand-alone reactor and detached from a biogas plant (Thema et al., 2019). In a stand-alone methanation reactor, both CO2 and biogas (approx. 50% CO2) can be used as a carbon source. The process is not yet technically mature, but a general advantage is that a stand-alone reactor can be tailored specifically to the needs of hydrogenotrophic methanogenesis and the corresponding archaea. The transport of hydrogen into the aqueous phase can be specifically improved. Various reactor types are being developed for this purpose (Shiga et al., 1998). As already mentioned, the introduction of hydrogen directly into a tank of the biogas plant is also possible. Here, the entry of hydrogen into the liquid phase is a critical point (Jensen et al., 2021). It would be conceivable to inject the gas into the liquid or to mix the gaseous and liquid phases. However, both methods lead to an increase in energy demand and thus to a reduction in energy efficiency. An interesting approach is also the direct implementation of an electrolysis cell in the aqueous phase of a biogas reactor. Here, hydrogen could be formed and consumed in situ, which would reduce the handling of gaseous hydrogen to a minimum and avoid the problem of mass transport. However, there is still a lot of research to be done on this (Marchese et al., 2020). No matter which technique is used for biological methanation, the product is biogas with an increased methane and hydrogen content. Higher hydrogen contents (>5%) are a considerable problem for conventional biogas plants because the membranes of the gas storage tanks are too permeable for hydrogen. There is then a risk of explosive gas mixtures of hydrogen and air forming in the vicinity of the tanks. The utilisation of biogas containing hydrogen is also not easy, as it would cause malfunctions in combustion engines as well as in gas treatment at high concentrations. If the biogas is to be upgraded to biomethane and fed into the natural gas grid, the limits for hydrogen in natural gas must also be observed (European Association for the Streamlining of Energy Exchange-gas, 2008). The hydrogen must therefore be dosed in such a way that it can be consumed continuously. In Germany, the gas storage tanks of a biogas plant must be able to temporarily store at least the gas from 6 hours of gas production. Then the additional methane (produced during the day from surplus solar electricity) could be consumed at night for electricity and/or heat production. Another technical hurdle is the changing methane content of the final gas. Since power-to-gas only really makes sense with surplus electricity, it must be assumed that the methane content in the biogas fluctuates greatly over the course of the day, depending on whether additional hydrogen is introduced or not. However, both the gas control system of the unit power plants, and the control system of the gas treatment plants are set to relatively fixed methane contents. Special units would have to be used here to record the methane content and automatically readjust it. Projects implemented in Europe: In recent years, about 100 Power to X plants with an output (based on the electrical output of the electrolysis) of 5 - 600,000 kW have been planned and built in Europe (Wulf et al., 2018; Wulf et al., 2020). More than 30 of these are power-to-gas plants, about half of which are biological and half chemical methanation plants. The largest plant for biological methanation was built in Germany in 2019 (pfi Germany, 2017). This has an electrolysis capacity of 1800 kW. The methanation is carried out biologically in a column. The resulting biogas is desulphurised, dried and fed into the natural gas grid. The largest chemical methanation plant is planned in France with an electrolysis capacity of 600,000 kW (Wulf et al., 2018). 1.3.6 Two stage digestion For decades, the operation of biogas plants was mainly linked to the operation of agriculture and animal farming. The focus was and is therefore often on investment costs and reliable and simple operation. A sin- 50 CHAPTER 1 — STATE OF THE ART gle-stage process in a tank that only has to be stirred, fed and heated is much easier for a farmer to handle than two process stages that have to be operated in coordination with each other. Therefore, it is not surprising that single-stage biogas plants are much more common than two-stage plants. However, two-stage plants offer advantages that could contribute to an improved utilization of the energy potential of the feedstock in the future (Theuerl et al. 2019). In two-stage biogas plants, acidification and methane formation take place in different reactors (hydrolysis stage and methane stage). By spatial separation, the digestion conditions can be optimized for the respective microorganisms (Köllmeier, 2014). The interaction of the two processes can also be improved by selective dosing of the substrate flow from the hydrolysis stage to the methane stage, which improves the performance of the overall process. Gas yield can be increased, residues can be reduced, less substrate needs to be fed, and plants can be made smaller due to better space-time yield (Hahn et al., 2012). Hydrolysis of substrates is usually faster compared to methanogenesis (depending on the complexity and degradability of the substrate). Hydrolysis tanks can therefore usually be built smaller than a digester for the same input quantity, since the necessary residence time is shorter. An advantage of separate hydrolysis is the possibility to convert substrates that are more difficult to degrade, for example those containing cellulose or even lignocellulose (Čater et al., 2014). But also solid substrates like potato peels which need more time for microbial digestion due to their particle size (Parawira et al., 2005). Separate hydrolysis is also interesting for particularly easily degradable substrates such as sugar-containing residues from the food industry. In single-stage processes, the use of such impurities quickly leads to an accumulation of organic acids once methane formation is inhibited (Ziemiński and Kowalska-Wentel, 2017). This then often causes a lowering of the pH value, which then permanently inhibits methane formation. This danger can be reduced by using the already hydrolyzed material as an input. In contrast, separate hydrolysis does not make sense when already acidified substrates are used, such as silage from grasses or corn. Also, for plants using cattle slurry (partly also for manure and slurry from other animal species) as substrate, a two-stage operation is not reasonable. Cattle manure contains a lot of water and relatively few carbohydrates, the cells have already been broken down by the stomachs of the cattle, methanogenic archaea from the stomachs of the cattle are already present in the substrate and, in addition, manure often has a high buffer capacity for organic acids (Torrellas et al. 2018). A two-stage design is also interesting if the substrates contain impurities that are not to enter the entire plant but only a specially equipped area. In this case, percolation of the substrate with a liquid phase is often used. The solid components or impurities are retained and only the degradable compounds are to be dissolved from the substrate by the percolate. Such plants are usually realized in the “garage design” in which individual chambers are fed discontinuously and emptied again (Qian et al., 2016). Hydrolysis stage: The designs for hydrolysis vessels are selected according to the substrates. Although round stirred tank reactors are also common here, special designs are also often encountered. The spatial separation required for separate hydrolysis is usually achieved by two separate reactors. Often, special crushing equipment or other mechanical pretreatment or collection systems are integrated. Since hydrolysis, just like methanogenesis, only takes place in the absence of oxygen, a hydrolysis vessel must also be gas-tight. In addition, gas is also produced during this process, which must be collected and recycled. The underlying biochemical process produces mainly carbon dioxide and hydrogen. Methane is produced only in very small quantities. The resulting gas can be used in various ways. The simplest is to discharge it into the methane stage (Weiland, 2010). There, the mixture of hydrogen and carbon dioxide can be further metabolized to methane. The bacteria used 51 CHAPTER 1 — STATE OF THE ART in hydrolysis are more robust compared to the metanogenic archaea (Köllmeier et al., 2012), which allows for example the use of higher loading rates, which in turn improves the turnover rate of substrates (Hansen et al., 2021). In general, a lower pH is observed in hydrolysis stages than in methane stages (Dobre et al., 2014). This places special demands on the materials used. For example, concrete must be protected from corrosion by special protective coatings. In any case, it makes sense to monitor the pH value of a hydrolysis tank (Hahn et al., 2012). Methane stage: In two-stage plants, the methane stages are fed with the output of the hydrolysis stage. Since the formation of organic acids has already been completed, the required amount of substrate can now be precisely metered. In essence, their operation is similar to that of the classic fermenters. Differences can arise, however, due to the fact that solids have already been broken down in a separate hydrolysis stage, or sometimes these solids are specifically retained (cf. percolation). In these cases, the stirring technology can be run with less power. If the gas from the hydrolysis is to be further metabolized in the methane stage, it must be brought into contact with the liquid phase by a suitable technology. 1.4 Utilisation of biogas 1.4.1 Intermediate storage in the gas storage facility Flexibilization: Usually, biogas is produced constantly and with continuous feeding. Biogas production should be kept at a constant level in the long term. To compensate for small differences between gas production and gas utilisation during continuous electricity generation, biogas plants have gas storage facilities. In the case of demand-oriented feed-in (flexible operation), biogas storage has an additional significance. Discontinuous utilisation of the biogas is made possible by regularly emptying and filling of the gas storage tanks. In a flexible mode of operation, the storage system holds the biogas to be able to run at increased output during attractive hours (Fachagentur Nachwachsende Rohstoffe, 2018; www. biogas-netzeinspeisung.at, accessed 07.10.2021). The dimensioning of the gas storage depends on the operating strategy of the biogas plant. With constant gas output, the necessary size of the gas storage capacity increases with the degree of flexibilization. With increasing plant output, the necessary storage capacity grows. On one hand, the storage size determines the maximum idle time and, on the other hand, the maximum running time of the CHP unit. The capacity of the biogas storage should generally be more than 12 hours, because the peak price periods that occur in the typical daily rhythm have approximately this time interval (daily flexibility) (Fachagentur Nachwachsende Rohstoffe, 2018). With a larger storage volume, the costs of interval operation for the CHP can be reduced because it has to be started less frequently (www.biogas-netzeinspeisung.at, accessed 07.10.2021). In principle, the construction costs for flexible plants are higher than for non-flexible ones. Whether this makes economic sense depends mainly on whether demand-responsive feed-in is a prerequisite for government subsidies for electricity remuneration. Gas storage: Gas storage tanks must be pressure-resistant, gas-tight, resistant to media, UV, temperature, and weather (Weiland, 2010). Before commissioning, they must be checked for tightness. Gas storage tanks are equipped with mechanical or hydraulic over pressure and under pressure safety devices to ensure that the internal pressure remains within the prescribed limits. Container-bound storage systems: This refers to a round container that is covered with a gas-tight dome-shaped roof. A general advantage of container-based storage systems is the space saving compared to external gas storage systems, as the containers 52 CHAPTER 1 — STATE OF THE ART are already available. If the fermenter or the secondary fermenter itself is used as a gas storage, foil roofs are usually used. In the case of a concrete cover and a constant gas volume, the gas space cover merely provides a gas-tight seal of the fermenter against the environment. With variable gas volumes, the gas storage serves to compensate for fluctuating gas production. For the subsequent section, the gas storage tank provides a constant gas volume flow and gas pressure. The tank covers can be designed as a concrete cover, made of stainless steel, from a single-membrane storage tank or double-membrane storage tank. Only the last two tank covers mentioned are suitable for flexibilization. A comparison of the different covers can be found in table 9. When using foils, these are attached gas-tight to the upper edge of the container. A support frame is built onto the tank on which the foil can rest when the gas storage tank is completely empty. Roofs consisting of only one foil must be expandable and must be able to expand depending on the filling level of the gas storage tank. Supporting air roofs (see figure 26) or double membrane storage tanks consist of two, non-expandable foils. The upper film is a weather protection film that is applied over the actual storage film. The weather protection foil serves to protect against environmental influences (especially wind gusts) and to absorb acting loads. Air is blown into the space between the two foils. The supporting air blown into the gap ensures the stability of the outer film and causes a relatively constant pressure on the inner membrane. The upper weather protection foil is always in a taut, stretched state, while the storage foil adjusts variably to the amount of biogas to be stored. With this system, the gas pressure is kept as stable as possible (Weiland, 2010). In principle, the container cover is independent of the type of material used in the fermenter. Container-bound gas storage systems are always pressure less or low-pressure storage systems; the pressure range is between 0 and 5 mbar for simple foil storage systems and 0 - 50 mbar for double membrane storage systems. The sealing of the foil roofs is achieved by means of a so-called Figure 26: Biogas plants with load-bearing roofs (AEV Energy GmbH) Seeger seal or with clamping rails (Persson et al., 2019). Gas storage membranes have a certain diffusion rate of 1‰ - 5‰, concrete and steel covers are emission-free. Storage foils are mostly made of PE or fabric-reinforced PVC. Some odour emission is released by diffusion, which does not require any further measures during normal operation. The odour emission is reduced in double-membrane storage tanks (Schmidt-Eriksen, 2011). External storage systems: The gas storage tank is spatially decoupled from the fermenter/digestate storage. They are availa- 53 CHAPTER 1 — STATE OF THE ART ble as low-pressure, medium-pressure and high-pressure storage tanks. The advantage is that less gas is lost when the fermenter is opened. The disadvantage is the increased space requirement (Khan et al., 2017). External low-pressure storage tanks can be designed in the form of foil cushions or supporting air storage tanks on concrete foundations. To protect them from the weather, they can be covered with an additional foil or housed in a building. The advantage of this system is the low cost compared to other systems, and the usable volume of foil cushions is relatively large. The disadvantage is the short life of the foil if it is exposed to the weather. The basic principle is the same as for internal double-membrane storage. An outer foil is supported by supporting air, which protects the movable storage membrane inside (Persson et al., 2019). An overview of the types of storage tanks in the low-pressure range is given in table 10; an example of an external double-diaphragm storage tank is shown in figure 27. 1.4.2 Conveying technology for biogas In normal operation, only biogas is transported in the gas-carrying system. This must comply with special safety regulations, as the escape of gas or the entry of air can form explosive mixtures. In case of malfunctions, Table 9: Overview of container covers and container-bound gas storage tanks (Postel et al., 2008). Type Description Application area Advantages Disadvantages Concrete ceiling Attached steel concrete plate Only for concrete tanks Stable, can be walked on, partly driven over, suitable for agitators Constant gas volume not suitable as "lung" function Enamelled steel or stainless steel Steel construction segments, foil as inner membrane Concrete tanks, steel tanks Lighter than concrete, better to install Gas volume somewhat larger, but still inflexible Singlemembrane storage tank (single shell) » Foil cover made of a single membrane » Mast supported All standing tanks widespread in older and small plants » Larger storage volume » Low cost » Possibility of optical process control » Constant gas volume, susceptible to weather, wind and snow » Foils can tear under negative pressure Doublemembrane storage tank (double-shell) » Inner foil as gas-tight membrane, outer foil as protective cover » Mast supported » Supporting air roof For all standing containers, is standard » Larger storage volume, more weather-resistant than single-skin » Possibility of optical process control » Permanently low prepressure supplied » More expensive due to increased film requirements » Foils can tear under negative pressure » Energy requirement for supporting air 54 CHAPTER 1 — STATE OF THE ART the liquid level in the fermenter may rise above the opening edge of the gas outlet, thus digestate or foam may enter the gas line. Gas pipes must be inspected and maintained regularly; corrosion increases the risk of gas leaks and explosions. Raw biogas also contains water vapour, hydrogen sulphide, hydrogen and ammonia, which are reactive gases that can attack metallic pipelines. Water collected in the condensate separator can corrode the pipes. The pipelines must therefore be resistant to media and corrosion; they are made of stainless steel, polyethylene (PE-HD) or PVC-U. In general, stainless-steel pipelines are often used above ground; plastic pipelines must be protected from mechanical and thermal damage and need more supports than stainless steel pipes when laid above ground. The connections are flanged, welded, glued or screwed. All fittings and pipelines must be protected against frost. The pipes are always laid with a gradient, this enables targeted condensate collection. It must be possible to drain condensate from all gas pipes, all fittings must be easily accessible and easy to maintain. When laying pipes in the ground, ensure good compaction, the laying must be stressfree, if necessary, compensators or U-bends must be planned. 1.4.3 Gas treatment In addition to methane (CH4) and carbon dioxide (CO2), general raw biogas also contains considerable amounts of hydrogen sulphide (H2S) and ammonia. The combination of hydrogen sulphide and the water vapour contained in the biogas results in acid formation (Fachagentur Nachwachsende Rohstoffe, 2013), which leads to corrosion. Most modern CHP units are equipped with oxidation catalysts; sulphur atoms can occupy the active centres of the catalysts even in moderate concentrations and thus render the catalysts ineffective. Water vapour in larger quantities is adsorbed by the activated carbon and inhibits the absorption of sulphur. In the case of biogas plants, desulphurisation and drying of the biogas is usually carried out. The manufacturers of CHP units set minimum requirements for the quality of the Table 10: Overview of external gas accumulators in the pressure less and low-pressure range (Postel et al., 2008). Type Description Application area Advantages Disadvantages Foil pillow feeder » In fixed old buildings (silo, barn) » In new building, e.g., lightweight hall » On concrete ceiling of a digester » Exposed or under roofing » Farms that have buildings available » Special cases, e.g., if digester is under-ground » Tanks with concrete ceiling » Smaller storage tanks » Inexpensive with existing building » Easily accessi-ble » Space-saving » Easily accessible » Volume (shape/ quantity) limited » If necessary additional space required » Relatively small volume » Exposed to weather and sun Complete system Double membrane accumulator Accumulator can be positioned on terrain Flexible and modular installation Additional space requirement 55 CHAPTER 1 — STATE OF THE ART fuel gases used; these should be complied with in order to be able to operate the CHP units effectively and to protect them from damage (Fachagentur Nachwachsende Rohstoffe, 2018). Desulphurisation: Possible processes (for comparison see table 11) can be divided into biological, chemical and physical separation processes. In addition to the gas composition, the flow rate is a decisive parameter for the desulphurisation equipment. The flow rate can fluctuate; it is increased when fresh substrate is fed in and during operation of the agitators. Power peaks of the flow rate can therefore occur by more than 50% above the average value. Therefore, the plants must be appropriately dimensioned to be able to absorb the peaks. Several processes can be switched in succession (Okoro and Sun, 2019). Biological desulphurisation in the fermenter: Biological desulphurisation is often carried out directly in the fermenter. By means of the bacterium Sulfobacter oxydans, hydrogen sulphide is converted with oxygen into elemental sulphur (Ramos et al., 2013). 2 H2S + O2 ⟶ 2 S + 2 H2O The sulphur accumulates on the surface of the tanks and eventually falls back into the substrate. The elemental sulphur can therefore be discharged from the reactor via the fermentation residue. The bacteria are already present and do not have to be added separately. The oxygen is introduced into the fermenter by injecting defined amounts of air (max. 5% of the biogas formed in the same period). This process is very cost-effective, no additional chemicals are required. The technology is low-maintenance and lowfailure. Sulphur can then be discharged as fertiliser. The introduction of oxygen may impair the process and methane oxidation is possible. It is difficult to react to different gas production rates. Another disadvantage of this process is the possible formation of sulphuric acid. Bacteria can develop (in the presence of atmospheric oxygen) that oxidise the resulting hydrogen sulphide to sulphuric acid. This leads to corrosion damage on surfaces susceptible to this (concrete and metal materials). The use of biological desulphurisation has proven itself in technology; its effectiveness depends on the sulphur content. It is hardly possible to reduce the hydrogen sulphide concentration to the level required for combustion in a CHP with oxidation catalyst, but the process improves the gas quality in a simple way to such an extent that downstream methods can work more economically. It is therefore mostly used in combination with other processes (Okoro and Sun, 2019). Biological desulphurisation in external reactors - trickling filter process Biological desulphurisation is also possible outside the plant. Separate containers with biological desulphurisation columns can Figure 27: Example of freestanding double diaphragm accumulator (AEV Energy GmbH). Figure 28: Two tanks with piping and pressurerelief device (AEV Energy GmbH). 56 CHAPTER 1 — STATE OF THE ART be used for this. In the droplet process, hydrogen sulphide is absorbed with the help of a scrubbing medium. Degradation rates of 99% can be achieved, which can lead to a residual gas concentration of less than 50 ppm sulphur. The technology is available for all biogas plant dimensions and is basically suitable for all biogas plants. This process is rather unsuitable for feeding into the natural gas grid due to the high air input. The advantage is that the air input takes place outside the fermenter, so the process is not impaired by the oxygen input. Targeted automatic regulation of hydrogen sulphide decomposition (through nutrient, air supply and temperature management) is possible. No use of chemicals is necessary. The disadvantage is that an additional, costly unit is needed and the high air input in the biogas. Another disadvantage is that maintenance is very expensive (Okoro and Sun, 2019). Sulphide precipitation: Sulphide precipitation is a chemical process which, like biological processes, is used for coarse desulphurisation. With sulphide precipitation, hydrogen sulphide values between 100 and 150 ppm are achieved. By adding ferric chloride or ferric hydroxide, the sulphur is chemically bound in the substrate, thus preventing its release as hydrogen sulphide. • Fe2+ + S2- ⟶ FeS • 2 Fe3+ + 3 S2- ⟶ 2 FeS + S Figure 29: Left figure: Gas control for air injection (Fachagentur Nachwachsende Rohstoffe, 2013). Right figure: Sulphur in the fermenter (Biomin, accessed 18.10.2021). The iron salts are dosed via the preliminary pit or the solids dosing unit or directly into the fermenter. The hydrogen sulphide formed is still bound in the liquid phase of the fermenter as sparingly soluble iron sulphide. The iron sulphide remains as a solid in the digestate and can be discharged from the system with the solids discharge. The exact dosing quantity depends on the respective system; it is usually between 100 - 220 giron/tsubstrate. The dosing quantity is strongly dependent on the substrate used. The binding of H2S directly in the fermenter can have a positive effect on the microbiological processes taking place (Fachagentur Nachwachsende Rohstoffe, 2013). Sulphide precipitation is also often used in combination with biological desulphurisation (Okoro and Sun, 2019). Adsorption on activated carbon: For the upgrading of biogas to biomethane and its injection into the natural gas grid and for the operation of CHPs with oxidation catalysts, very low H2S concentrations are necessary in the biogas. To achieve this, fine desulphurisation is necessary. Activated carbon filters are very often used for this purpose. This process uses adsorption on activated carbon based on the catalytic oxidation of the hydrogen sulphide on the activated carbon surface. The hydrogen sulphide is adsorptively bound in the pore system and then catalyt- 57 CHAPTER 1 — STATE OF THE ART ically oxidised. Impregnation or doping of the activated carbon to improve the reaction rates and the loading capacities is possible. Potassium iodide and potassium carbonate are used as impregnating agents. During operation, the activated carbon filters are loaded once with carbon. This is used until the loading limit is reached. When the activated carbon loses its effect can be determined by measuring the hydrogen sulphide content after desulphurisation. The loaded activated carbon is then either regenerated or disposed of in a waste incineration plant, for example. In order to increase the service life of activated carbon filters, a combination with coarse desulphurisation is recommended, which makes the process more economical (Okoro and Sun, 2019). Drying: To optimise combustion in CHP units and meet the requirements of subsequent purification stages, water vapour must be removed from the raw gas. The amount of water vapour in the biogas depends on the gas temperature. In the fermenter, the relative humidity is close to 100% and the biogas is saturated with water vapour. The processes used to dry the biogas are condensation drying, adsorption drying and absorption drying (Okoro and Sun, 2019). Condensation drying: To separate the condensate, cooling takes place within the gas pipe. The operating principle of this process is based on cooling the biogas below the dew point. In small systems, the cooling of the gas as it passes through buried pipes can already remove sufficient water. For this purpose, the gas pipes are laid at an incline, and at the lowest point of the pipe the condensate is collected at a condensate separator. For the gas to cool down, it must have a certain residence time in the pipeline. In addition to the water vapour, other undesirable constituents such as water-soluble gases and aerosols are removed from the biogas. The separator must be emptied regularly, so it must be easily accessible for maintenance work. In larger plants, the flow velocity is often too high for all the water to be separated in this way. In this case, additional gas cooling systems are used to reduce the dew point. Dew points of 3 - 5°C are thus possible. The efficiency can be further increased by prior compression (Fachagentur Nachwachsende Rohstoffe, 2013). After gas cooling, the absolute humidity is lower; the relative humidity is adjusted by reheating the gas. This value is important for the operation of activated carbon filters and combustion engines. Condensation drying is usually sufficient for the needs of a biogas plant. Figure 30: External biological desulphurisation, on the left bio-trickling bed reactor, on the right bio-moisture reactor [S&H GmbH & Co. Umweltengineering KG] (Fachagentur Nachwachsende Rohstoffe, 2013). Figure 31: Activated carbon filter biogas with base and bypasses (BioBG, accessed 25.10.2021). 64 CHAPTER 1 — STATE OF THE ART the digestate is separated before storage to reduce the amount to be stored. For practical reasons, digestate storage facilities for solid and liquid digestate are located directly on the premises of a biogas plant. The storage facilities are integrated into the operational processes of a biogas plant via pumps, pipelines, or other conveying equipment. In some cases, they are also built outside biogas plants. In newer plants, a gas-tight cover of the tanks is mandatory to prevent methane emissions. In practice, it sometimes makes sense to combine gas-tight storage tanks for liquid digestate with secondary fermenters. In this case, the digestate storage tanks are only partially emptied and thus used for gas production and digestate storage at the same time. Solid digestate: These are produced during solid matter fermentation and as a separated (solid) component of the fermentation product of wet fermentation. Solid digestates are stackable and pourable. Depending on the intended use, the digestates are stored in different ways. Storage can take place in the open air or in halls as well as in containers. It is important that the storage facilities meet the requirements of the fermentation product. Percolation liquid and press water must not penetrate the soil. Therefore, the floors of the storage facilities are made of concrete or asphalt. Escaping liquids are collected and returned to the fermenter or the digestate store, for example. Silage camps (see chapter 1.2.2 Delivery and storage of substrates) are used, for example (Weiland, 2010). An outdoor ground plate is constructed in concrete or mastic asphalt, it is liquid-tight towards the bottom. They are built with a slope for the drainage of liquids. In some cases, they are equipped with backfill walls and a roof. The advantage is that large storage volumes can be realised relatively cheaply. A disadvantage is the loss of nitrogen during long storage periods. If the floor slab is not covered, nutrients are washed out and very high liquid quantities may have to be collected. A storage hall with a concrete floor or mastic asphalt floor is sealed at the bottom. Like the floor slab, it must be able to capture seepage water. The advantage is that it is independent of the weather, the disadvantage is that ventilation is required, and the increased constructional and technical effort compared to outdoor storage. A container, usually made of steel, is another storage option. In contrast to the other storage applications, this one is transportable. If it is not roofed, nutrients may be washed out (Persson et al., 2019). Liquid digestate: Liquid digestate is stored in earth basins (lagoons) and in cylindrical or rectangular containers. Earth basins are usually rectangular and are embedded in the ground. To prevent emissions, they are covered with a plastic film. Characteristic of this type of storage are the relatively low construction costs for a large volume. More frequently used are containers made of concrete or steel. These can be constructed above or below ground. As described in chapter 1.3.3, fermentation residue stores can be equipped with agitators. These serve to homogenise the digestates before removal (Paolini et al., 2018). Earth basins are soil pits lined with plastic film, usually rectangular in shape. They are designed with two layers for leakage detection. Agitators can be used here. They are very inexpensive to realise and quick to construct. Construction with large volumes is possible, and maintenance costs are very low. NH3 emissions can be reduced by using an emission protection foil. A disadvantage is the possible outgassing of CH4, NH3 and N2O, a loss of nitrogen or fertiliser value and, in the case of open earth basins, odour emissions and precipitation can occur. Earth basins can now also be covered with gas storage tanks. Figure 35: Open construction of digestate storage tank, surface of digestate is forming a dry crust (Vanek, 2011). 65 CHAPTER 1 — STATE OF THE ART Storage tanks (figure 35) are usually cylindrical or rectangular, they are available as high or low tanks, and the digestates contained can be homogenised by means of an agitator. They are available in open or closed design, mostly unheated, but also with heating and insulation. The advantage is that they can be covered, so there is no precipitation, which also has a positive effect on any odour emissions. The quality of the fertiliser can be maintained. In a gas-tight design, the containers represent an additional gas reservoir. Open tank systems have the same disadvantages as open earth tanks. Fermentation residue storage usually results in the formation of a floating layer (figure 35). This is desirable in most cases of open construction, as it reduces odour emissions. Due to the low dry matter content of the liquid digestates, there is no danger of sediments settling on a large scale. There is also no risk of disturbing the gas discharge, as the digestates usually only have a very small gas potential. The stirring technique can therefore be kept much simpler with digestates than with fermenters. 1.5.2 Solid/liquid - separation of digestates Solids separation: The basic process of digestate treatment is solids separation. Solid’s separation makes it possible to reduce the storage volume for liquid fermentation residues and to reduce the formation of sinking and floating layers. In addition, the solids separation enables a separation of the nutrients in the fermentation substrate, the soluble mineral nitrogen remains in the liquid phase, while organically bound nitrogen and phosphorus largely remain in the solid phase. The separated liquid phase can be spread or further processed, the separated solids can be composted or dried. The properties of the digestate and the configuration of the separator determine the separation efficiency of the processes used. The higher the dry matter content in the digestate, the higher the possible volume reduction (Lukehurst et al., 2011). Screw presses (figure 36): Screw presses are used as the first link in the mechanical processes for solids separation or on their own. In the presses, the digestate is pressed against a screen basket by means of a screw (Lindner, 2020). Typical pressure levels of presses are around 30 to 50 bar. In addition to the technical condition of the press, a certain amount of structural material is necessary in the digestate substrate. This forms a plug, which considerably improves the pressing result. Due to the high-pressure levels and the mineral components present in the substrate, screw presses are subject to high levels of abrasion. The screw press is usually installed directly downstream of the fermentation process; the high temperatures and the pH value usually causes the release of ammonia. This can be compensated for by exhaust air collection and treatment. Good separation efficiencies can be achieved with presses of this type; dry matter contents of up to 40% are possible (Guilayn et al., 2019). Decanter / Belt filter presses (figure 37): Decanters as well as belt filter presses are used for the separation of digestates from wet fermentation plants, as well as for the further dewatering of the liquid digestate after the use of a press screw. When decanter centrifuges are used, it is important for a good separation result that the solids have a large density difference to the liquid phase. To improve the dewatering behaviour, flocculants can be added to accelerate the sedimentation or flotation of the suspending particles. Decanter centrifuges achieve a degree of separation of 15% solid phase and 85% liquid phase. Decanter cakes have a high density, in this state they are not compostable and must first be made compostable by mixing. Belt filter presses are continuous pressure filters with circulating screen cloths as filter media, in which the process sequence takes place in two stages (Lindner, 2020). 66 CHAPTER 1 — STATE OF THE ART The first stage is a gravity filtration, the second stage is a squeezing out of the sludge between two screens, which are guided around several rollers. The degree of separation is up to 51%. A disturbing factor is the large amount of rinsing water required. Both the decanter centrifuges and the belt filter presses are connected downstream of a screw press during digestate preparation. The aim is to process the liquid phase for downstream processes (see chapter 1.5.3 Processing and utilisation of digestates) (Lindner, 2020). 1.5.3 Processing and utilisation of digestates Often, digestates are not processed further, but used directly as fertiliser. In plants that cultivate energy crops on their own land, it is essential to return the nutrients to their own land. In other cases, for example, when resFigure 36: Screw presses for dewatering biowaste digestate (Raussen und Kern, 2016). idues from other farms are used for biogas production, there is no internal use for them. Due to the high content of macroand micronutrients and the large quantity, it is not possible to discharge the liquid phase into the sewage network. In this case, processing the digestate can help to increase economic efficiency. Under certain circumstances, the digestate can be sold as fertiliser; in any case, drying helps to save weight and thus transport costs. This is also a possibility to use waste heat from the CHP. The following drying processes correspond to the state of the art for the treatment of solid digestate. They differ greatly in their prevalence and functional reliability (see table 12). The processes for drying the solid phase are borrowed from other areas of application and have been tested; the degree of adaptation is to be regarded as low. In other cases, nutrients must be removed from the liquid digestate in order not to overfertilize the soils. In these cases, separation of the nutrients and sale to other regions would be desirable. The processes for treating the liquid phase do not yet correspond to the state of the art, a high need for development is seen here. The membrane process is the most advanced, it is established on the market and is operated in several reference plants. There is still potential in this process, which can reduce energy consumption and wear. Evaporation and stripping processes are not yet far advanced in terms of largescale continuous operation. In the future, stricter fertiliser regulations will greatly increase the problem locally. Figure 38 shows 67 CHAPTER 1 — STATE OF THE ART an overview of different digestate treatment processes. Evaporation of liquid digestates: This method is a multi-stage process in which the liquid is first heated and then the temperature is gradually increased to the boiling point under negative pressure. Evaporation under vacuum requires reduced electrical energy compared to evaporation without vacuum. Temperatures of 80°C are already sufficient in a vacuum (Lindner, 2020). The pH value is lowered by adding acid, this avoids ammonia losses. The amount of fermentation residue is reduced by 70%. In this process, the fermentation residue is sanitised due to the high temperatures. Up to 4 times higher solids concentrations can be achieved, which significantly reduces storage and transport costs (Lukehurst et al., 2010). Evaporation requires upstream dewatering of the liquid digestate by means of a decanter or belt filter press. Another disadvantage of this process is the high amount of thermal energy required. This process only makes sense if hygienization is necessary, e.g., when using slaughterhouse waste and a lot of excess heat is available (Nag et al., 2021). Figure 37: Decanter (l), belt filter press (r) - for further dewatering of press water from pressed biowaste digestate (Raussen und Kern, 2016). Figure 38: Classification of treatment processes by type (Fachagentur Nachwachsende Rohstoffe, 2010). Treatment method Physical Chemical Biological Solids preparation Possibly with flocculant Stripping Possibly with precipitation Drying Composting Nitrification/ dentrification Evaporation Membrane technology Elimination of nutrients 68 CHAPTER 1 — STATE OF THE ART Drying of solid digestates: For drying, established technology from other areas is used. For this process step, for example, drum, belt, or push-turn dryers can be used. In most processes, heat is transferred by hot air which flows over or through the material to be dried (Lukehurst et al., 2010). A belt dryer (see figure 39) consists of a drying chamber and several conveyor belts. On these, the free-flowing material to be conveyed is usually transported on several conveyor belts running in opposite directions. The belts are permeable to air and are made of wire mesh or perforated steel plates. Hot air flows through the conveyor belts and the material is dried. The material is mixed and homogenised by transferring it to different belts. The temperature level of the belt dryers is between 80 - 120°C, for which dissipated heat from the CHP can be used. The exhaust air must be treated (Lindner, 2020). A drum dryer (see figure 39) consists of a slightly inclined rotary tube with an integrated hot air blower (Lindner, 2020). Mechanically dewatered digestate is fed onto the raised end of the rotary pipe and passes through the rotating pipe on a long spiral track. For this purpose, multiple-pass drum dryers are used. Hot air at more than >200°C flows through the drum in co-current or counter-current. For economic reasons, it is advisable to use the waste gas heat of a CHP (Lindner, 2020). The ammonium contained in the solid phase passes into the drying air as ammonia during drying (Fachagentur Nachwachsende Rohstoffe, 2013). Exhaust air treatment is necessary to prevent ammonia emissions and odour emissions. Solid digestate composting: Composting is an aerobic process for treating organic waste. The aim of this process is to stabilise the organic components, kill pathogenic germs and weed seeds and reduce odour emissions. For composting, oxygen must be added to the digestate. The digestate is low-structure material, for successful composting it needs to have high-structure material such as bark mulch added to it and the material needs to be turned over. Only 55°C is reached during composting, not 75°C, which is necessary for hygienization. The reduced self-heating is caused by the anaerobic decomposition of carbon within the biogas plant; therefore, the treated material only reaches low temperatures during composting (Nag et al., 2021). Membrane technology for liquid digestate: This process has its origins in wastewater treatment and is used there to treat water with a high organic load. In the field of biogas plants, it can be used to process the liquid digestate to such an extent that it can be discharged into the sewage network. The full treatment process has been adapted for biogas plants and is already in use. A unique feature is that no heat is required for this type Figure 39: Belt dryer for fermentation residues of the biowaste fermentation Leonberg plant (left) Drum dryer for digestate (right) (Raussen and Kern, 2016). 69 CHAPTER 1 — STATE OF THE ART of digestate treatment. It can thus be used in plants that do not have any surplus heat (for example, satellite CHPs). In this filtration process, decreasing pore size in combination with reverse osmosis produces a permeate from a liquid that can be discharged into the wastewater network and a concentrate enriched with nutrients. Phosphorus is retained in the ultrafiltration and is present in the retentate, while ammonium and potassium accumulate in the concentrate. The permeate is largely free of nutrients and can be discharged as wastewater. In this process, the dry matter content must be very low ≤3%, which requires solid-liquid separation by means of a decanter (Fachagentur Nachwachsende Rohstoffe, 2013). Since the costs for this process are very high on a large scale, it only makes sense if another use of the digestates is ruled out. Stripping of liquid digestate: Stripping is a process in which substances to be separated are removed from liquids by passing gases through the liquid and the ingredients pass into the gas phase. The aim is usually to reduce the nitrogen content in the liquid digestate to such an extent that it can be used as fertiliser. To support the process, the temperature and the pH value can be increased. In the case of steam stripping, the temperature is increased, and the required gas volume flow decreases with increasing temperature. Desorption then takes place, the ammonia in the gas phase is converted into a disposable or recyclable product. The desorption of ammonia can be done by condensation, washing with acids or by reaction with an aqueous solution of gypsum. Ammonium sulphate is the end-product of desorption (Palakodeti et al., 2021). Table 12 gives an overview of possible processes for digestate treatment. 1.6 Environmental and plant protection 1.6.1 Avoidance of gaseous emissions and dusts Gaseous emissions and aerosols: When operating CHP units, the combustion engines release gases such as nitrogen oxides, sulphur oxides and formaldehyde in addition to carbon dioxide. Depending on the type of engine used, its combustion settings and its condition, the quantity and composition of the gases released varies. The quality of the burnt biogas also has an influence on the assessment of engine emissions. The gases are released via the exhaust stack of the CHP unit (Persson et al., 2019). Operation-related emission sources can be represented by substrate and digestate storage, open pits or containers, and substrate preparation. Emitters are, for example, methane, ammonia, hydrogen sulphide and nitrous oxide. The amount of emitted gases depends on the construction and process-technological design of the biogas plant. To avoid emissions of gases and dusts, closed systems should be used instead of open systems. Gases may be released if components are leaking, if safety devices are activated or in the event of malfunctions (Persson et al., 2019). Aerosols are two-substance mixtures of suspended matter and a carrier gas. In biogas plants, there are various sources for the formation of aerosols. Examples include the dumping of dry substrates into storage containers, the improper handling of powdered operating materials, dust swirling up from vehicles or soot particles on the exhaust stack of a CHP unit (ASUE, 2014). 1.6.2 Avoidance of liquid emissions Liquid emissions: The gases ammonia, carbon dioxide, carbon monoxide, hydrogen sulphide and nitrogen oxides can form aerosols and dissociate in water to form acids or alkalis. This brings the water hazard and possibly corrosive effects to the fore. Other liquid substances can escape due to incorrect storage, leaking components or incorrect handling. These substances can be silage leachate, liq- 70 CHAPTER 1 — STATE OF THE ART uid manure or digestate as well as operating fluids (Liebetrau et al., 2013). To detect the undesired escape of liquid phases, digesters must be equipped with leakage detection; pipes can be double-walled and equipped with leakage detection. All installations must be carried out professionally and checked for leaks before commissioning and regularly during operation. A wall can be built to collect escaping liquids. The concrete measures required can be derived from local laws and regulations and are determined during the approval process. 1.6.3 Emission reduction measures For substrate delivery, the substrates should be delivered in closed containers; this applies to odour-intensive and dusty substrates. The roads should be paved, and the biogas plant operator must ensure cleanliness on the premises. When storing the substrate, the silage must be covered; this reduces the seeping juices and dry matter losses. A film can be used for the cover. Dusty and odour-intensive substrates are to be covered and possibly stored in storage halls. Collection containers for the leachate must be installed and, if necessary, fed directly into the fermenter. Slurry systems are to be covered with an odour-mitigating film. The state of the art for substrate introduction is to cover the preliminary pit and the introduction opening. The filling level of all types of gas storage tanks must be monitored. Before the gas storage tank releases gas via the pressure relief valve due to overfilling, the gas utilisation system (CHP or gas emergency flare) must be activated. Safety-relevant components such as the supply of supporting air in the case of load-bearing roofs or the compressed air clamping in the case of single-skin foil roofs are designed redundantly and integrated into the emergency power supply. The emergency power supply must be designed in such a way that the supply is guaranteed after 20 minutes at the latest. This can be achieved by automatically starting generators or 24-hour standby service. In the case of load-bearing roofs, the supporting air must be checked regularly for methane content and the entire system Table 12: Comparative evaluation of digestate treatment processes (Fachagentur Nachwachsende ++ = very good, + = good, o = average, - = poor Separation Drying Membrane technology Evaporation Stripping Operational reliability ++ +/0 + 0 0 Dissemination status ++ + + 0 0 Costs + +/0 0/- 0 +/0 Product usability Solid phase +/0 0 0 0 Liquid (nutrient rich) 0 0 + + ++ Liquid (nutrient poor) 0 + 0 0 71 CHAPTER 1 — STATE OF THE ART should be checked annually for leaks using a methane-sensitive, optical gas camera. For additional gas consumption units, the following applies: before the overpressure safety devices are activated, another gas consumption unit (flare) must automatically go into operation. The maximum biogas flow must be assumed for the entire biogas plant. The over pressure and under pressure safety devices may only be activated after the biogas flare has been operated and in the event of a malfunction. All liquid-carrying pipes in gas-carrying containers must always be submerged to prevent biogas from flowing in. For this purpose, the filling levels must be monitored. This also applies to the condensate line from the gas system (Liebetrau et al., 2013). In the field of biogas utilisation for CHP units, gas boilers, gas turbines, internal engine measures are taken to reduce emissions and exhaust gas purification devices such as oxidation catalysts are installed downstream. For biogas upgrading, post-treatment of the exhaust gas is necessary for all processes (except for amine scrubbing); this can be, for example, thermal post-combustion or catalytic post-combustion. For separation during digestate processing, these should be enclosed and have an exhaust air cleaning system. Ideally, the digestate should be processed immediately, and the solid fraction should be compacted and covered until it is spread. Drying should also be enclosed and have exhaust air purification. If necessary, the digestate storage facilities must be covered gas-tight (strongly dependent on the retention time and the substrate used) and have a connection to the gas utilisation system. A hydraulic retention time of 150 days in the gas-tight system should be observed for substrates that require a long retention time, such as straw. When transporting fermentation residues, the transport should take place in closed containers (Beil et al., 2021). 1.6.4 Noise protection Noise emissions during the operation of biogas plants are mainly caused by vehicles and the operation of the CHP. One way to minimise noise emissions is to use an automated substrate feed system. This can minimise the frequency of use of wheel loaders or forklifts. The emissions of the CHP unit can be reduced by a soundproof cover and by installing it in closed rooms, halls, or containers. Silencers are installed at the supply and exhaust air openings and at the exhaust pipe. The use of low-noise air coolers as well as the avoidance of structure-borne noise transmission via the exhaust air stack, cooler or motor by means of sound-decoupled mountings are technical measures to reduce noise emissions. Often, sound predictions must be made before approval (Liebetrau et al., 2013) and sound level measurements are carried out on the finished installation to confirm compliance with the limit values. 1.6.5 Explosion protection Biogas can form an explosive gas mixture in combination with air. Depending on the state variables methane/carbon dioxide content, temperature, pressure, and humidity, they can shift the explosion limits. Above the limits, there is no longer a risk of explosion, but fires can be caused by open fire, switching functions of electrical equipment or lightning. It must therefore be assumed that there is an increased risk of explosion and fire in the vicinity of fermentation tanks and gas storage tanks. Different areas of the plant are therefore legally divided into different “potentially explosive atmospheres”. Special labelling, precautionary and safety measures apply to these Ex-zones. The explosion zones of the individual components of biogas are shown in table 13. It is mandatory to draw up a risk assessment documenting where explosive atmospheres may form and describing appropriate countermeasures (Cividino et al., 2014). Zone 0 In the hazardous area of zone 0, an explosive atmosphere occurs continuously over 72 CHAPTER 1 — STATE OF THE ART a long period of time or predominantly over time (Fachagentur Nachwachsende Rohstoffe, 2013). These zones are generally not found in biogas plants (Friedl and Keckstein, 2022). Zone 1 In these areas, an explosive atmosphere may occasionally occur during normal operation of a biogas plant. These areas are usually in the immediate vicinity of entry points to the gas storage tank or on the gas-carrying side of the fermentation tanks and in the vicinity of blow-off devices, overpressure protection or gas flares (Fachagentur Nachwachsende Rohstoffe, 2013). In the case of free ventilation, these areas must be provided with safety measures within a radius of 1 m. Only equipment approved for Zone 0 and Zone 1 and explosion-protected equipment may be used here. If Zone 1 occurs in a closed room, Zone 1 extends to the entire room (Friedl and Keckstein, 2022). Zone 2 In these zones, an explosive gas-air mixture is not expected to form during normal operation. If such a mixture does occur, it is likely to be very rare and not of long duration (e.g., during maintenance work or in the event of a malfunction). Zone 2 can be found, among other places, at the entry openings and inside the fermenter, and in the case of gas storage tanks, the immediate vicinity of the ventilation openings. In these areas, the measures of zone 2 must be implemented within a radius of 1 to 3 m (Friedl and Keckstein, 2022). For zones 0 to 2, no ignition sources may occur within the zones. Sources of ignition are, for example, hot surfaces, naked flames or mechanically or electrically generated sparks. Such areas must be provided with warning and information signs. There are also divisions of explosion protection zones for dusts, but these are of less relevance. They usually play a role if solid, dust-like operating aids are used improperly. This can lead to the formation of explosive atmospheres for a short time. Precautions must be taken to ensure safe storage and handling. For example, packaging and containers with operating materials must be tightly closed and the rooms must be adequately ventilated (Fachagentur Nachwachsende Rohstoffe, 2013). 1.7 Potential for improvement 1.7.1 Technical potential for improvement There is potential for technical improvement Table 13: Properties of biogas components (Fachagentur Nachwachsende Rohstoffe, 2013). CH4CO2H2S CO H Density kg/m30,.2 1.98 1.54 1.25 0.09 Density relative to air - 0.55 4.53 1.19 0.97 0.07 Ignition temperature ºC 600 - 270 605 585 Explosion range Vol.% 4.4 -16.5 - 4.3 - 45.5 10.9 - 75.6 4 - 77 Workplace exposure limit (MAC value) ppm n. s. 5000 10 30 n. s. 73 CHAPTER 1 — STATE OF THE ART in many areas of biogas production (Theuerl et al., 2019). This includes above all the development of special aggregates adapted to the conditions of a biogas plant. There is a need for development, among other things, of separators for low-structure materials such as liquid manure, for drying plants or for ways to remove nutrients from the digestate in a targeted manner. Above all, there is a lack of specialised methods for digestate processing. Most of the methods used so far have been borrowed from other areas of application and therefore do not yet function optimally. However, due to the tightening of fertiliser guidelines, there will be more demand for targeted digestate treatment in the future. There is also development potential in the development of novel processes for the fermentation of special input materials such as chicken manure or residues from the food industry. So far, these substrates can usually only be used for biogas production when mixed with other substrates. 1.7.2 Biological improvement potential There is potential for improvement in biology mainly in the adaptability of microorganisms to certain milieu conditions to increase process stability. Increasing the tolerance to pH-value fluctuations and to high ammonium concentrations would be particularly useful here. This would allow the increased use of poultry droppings. This can be obtained cheaply as a residual material from poultry farming, but quickly causes inhibitions in the fermenter due to its high nitrogen content. It would also be economically favourable to improve methane formation at lower temperatures. This would reduce the necessary heating capacity and more heat could be sold (Wu et al., 2019). 1.7.3 Potential for organisational improvement There is potential for organisational improvement mainly in the legal framework for the construction of biogas plants. 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Nature, 401, 266269. • Ziemiński K, Kowalska-Wentel M. 2017. Effect of different sugar beet pulp pretreatments on biogas production efficiency.” Appl Biochem Biotechnol. 181, 12111227. • Zinder SH, Anguish T, Cardwell SC. 1984. Effects of temperature on methanogenesis in a thermophilic (58°C) anaerobic digestor. Applied and environmental microbiology, 47, 808-813. • Zoulias E, Varkaraki E, Lymberopoulos N, Christodoulou CN, Karagiorgis GN. 2004. A review on water electrolysis. Tcjst, 4, 41-71. 82 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION 02 Technological description Abstract: Waste disposal and green waste: In Germany, there are almost 700 plants to produce biogas from waste, and many more throughout Europe. Depending on the composition, the substrate properties of waste vary greatly. Technical challenges often exist in the processing of the waste. Special legal requirements also must be taken into account. Advantages, on the other hand, include low-cost purchasing and, in some cases, very high gas yields. Agriculture and Livestock: Due to the widespread use of biogas plants that are operated with animal residues and energy crops, many findings on the technical requirements have been documented. In plants for the utilization of energy crops, care must be taken to ensure that the DM content and space load are not selected too high; the retention time should be about 60 days. If mainly animal residues are used, too high DM contents and overloading are usually not to be feared. The retention time can be chosen to be shorter than in other plants, often about 40 days are sufficient. Authors from the AEV Energy GmbH: Benjamin Rocktäschel, Astrid Palen, Phillipp Kyas. 83 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION Optimizing the space load, retention time, and methane concentration Monitoring and adjusting parameters like pH and temperature Controlling the dry matter content, viscosity and avoid foam formation Using of sensors, power feed-in systems, emergency power supplies... 2.1 The goals of fermentation The goal of fermentation is usually the production of biogas for the generation and sale of electricity and heat. Even in plants where the biogas is upgraded to biomethane for injection into the natural gas grid, the production of biogas is the primary goal. Accordingly, the substrates are selected and used in such a way that the desired amount of biogas can be produced at the lowest possible cost. Downtime due to process disturbances and fluctuating gas production is to be avoided, and stable gas production is to be aimed for. The quality of the produced gas is also important, for use in combustion engines (CHP) the biogas must have a methane content of at least 45%. Also, the hydrogen sulphide content should not be too high, otherwise the biogas plant will be damaged. 8 4 C H A P T E R 2 — T E C H N O L O GI C A L D E S C RI P TI O N A n i m p ort a nt si d e e ff e ct of t h e utili z ati o n of c attl e or pi g m a n ur e i m pl a nt s i s t h e a v oi d - a n c e of m et h a n e e mi s si o n s t h at w o ul d r e s ult fr o m st or a g e a n d dir e ct s pr e a di n g ( M a s s é et al. , 2 0 0 8). H o w e v er, i n t h e c a s e of pl a nt s f or t h e a n a er o - bi c tr e at m e nt of r e si d u al m at eri al s a n d w a st e, t h e f o c u s i s s o m eti m e s a ct u all y o n w a st e di sp o s al. I n t h e s e c a s e s, t h e pr o d u cti o n of bi o - g a s i s o nl y a w el c o m e si d e e ff e ct t h at m a k e s t h e pr o c e s s m or e e c o n o mi c al. 2. 2 T e c h ni c al p a r a m et e r s of o p e r ati o n m a n a g e m e nt T h e f oll o wi n g p ar a m et er s c a n b e u s e d t o o bt ai n d et ail e d i nf or m ati o n a b o ut t h e pr o - c e s s e s t a ki n g pl a c e i n a bi o g a s pl a nt. T h e s e m u st b e r e c or d e d, c oll e ct e d, a n d e v al u at e d o n a pl a nt s p e ci fi c b a si s. T h e y s er v e t o a v oi d pr o c e s s di st ur b a n c e s, t o r e c o g ni z e tr e n d s, t o i n cr e a s e pl a nt s af et y a n d t o o pti mi z e pl a nt o p er ati o n. A di sti n cti o n c a n b e m a d e b e - t w e e n c o ntr ol p ar a m et er s f or e arl y d et e cti o n of pr o c e s s di st ur b a n c e s (f or e x a m pl e, v ol atil e or g a ni c a ci d s, F O S/ T A C r ati o, r e d o x p ot e nti al, h y dr o g e n i n t h e g a s a n d li q ui d p h a s e s) a n d c o ntr ol p ar a m et er s f or a s s e s si n g t h e pr o c e s s st at e (f or e x a m pl e, s u b str at e c o m p o siti o n a n d q u a ntit y, g a s pr o d u cti o n a n d c o m p o si - ti o n, t o xi cit y, f er m e nt ati o n t e m p er at ur e, D M c o nt e nt a n d vi s c o sit y of t h e f er m e nt ati o n mi xt ur e, a n d p H v al u e ( D r o s g, 2 0 1 3). 2. 2. 1 S p a c e l o a d T h e s p a c e l o a d ( fi g u r e 4 0 ) i n di c at e s t h e a m o u nt of or g a ni c D M ( o D M) i n kil o gr a m s t h at c a n b e f e d t o t h e di g e st er p er w or ki n g v ol u m e p er u nit of ti m e ( F a c h a g e nt u r N a c h w a c h s e n d e R o h st o ff e, 2 0 1 3) . F or m ul a 1: Or g a ni c l o a di n g r at e B R » = Q u a ntit y of s u b st r at e s u p pli e d p er u nit of ti m e [ k g/ d] » c = O r g a ni c m att e r c o n c e n t r ati o n [ % o D M] » VR= R e a ct or v ol u m e [ m 3] T h e m a s s of t h e f e d s u b str at e s i s n or m all y r e c or d e d b y t h e f e e d er vi a b uilt-i n w ei g hi n g el e m e nt s i n t h e c a s e of s oli d s u b str at e s, or vi a fl o w m et er s u p str e a m of t h e p u m p s i n t h e c a s e of li q ui d s u b str at e s. T h e D M c o nt e nt i n t h e s u b str at e m u st b e c o nti n u o u sl y d et er mi n e d b y r a n d o m s a m pli n g, t hi s al s o s er v e s q u alit y a s s ur a n c e p ur p o s e s. M o bil e m e a s uri n g d e vi c e s ar e u s u all y a v ail a bl e f or t hi s p ur p o s e at bi o g a s pl a nt s. T h er e ar e diff er e nt b al a n c e li mit s f or t h e r o o m l o a d. O n e l e v el i s t h e g a s-ti g ht, i n s ul at e d f er m e nt ati o n t a n k, a n ot h er i s t h e t ot al s y st e m (t h e s u m of t h e w or ki n g v ol u m e s of all l e v el s) a n d wit h or wit h o ut t h e i n cl u si o n of m at eri al r e cir c u - l ati o n. D e p e n di n g o n t h e r ef er e n c e v ari abl e s, di ff er e nt s p a c e l o a d s r e s ult. T o b e a bl e t o c o m p ar e di ff er e nt bi o g a s pl a nt s, it i s r e co m m e n d e d t o s p e cif y t h e s p a c e l o a d f or t h e t ot al s y st e m wit h o ut m at eri al r e cir c ul ati o n ( D r o s g, 2 0 1 3). W hi c h s p a c e l o a d i s c h o s e n f or t h e pl a nt d e p e n d s o n t h e c orr e s p o n di n g c o n diti o n s. I n g e n er al, a n o p er at or will c h o o s e t h e s p a c e l o a d a s hi g h a s p o s si bl e t o o p er at e hi s pl a nt e ff e cti v el y. Li mit s ar e s et b y t h e e xi sti n g pl a nt t e c h n ol o g y ( p erf or m a n c e of p u m p s, a git at or s a n d ot h er a g gr e g at e s), t h e m a xi m u m p er mitt e d bi o g a s pr o d u cti o n ( u s u all y d et er mi n e d wit h t h e p er mit, t o g et her wit h t h e m a xi m u m a m o u nt of s u b st a n c e s u s e d), t h e ti m e t h e s u b str at e s n e e d e d i n t h e f er m e nt er t o b e d e gr a d e d (r et e nti o n ti m e) a n d t h e l o a d c a p a cit y of t h e mi cr o or g a n - i s m s. A s a r e s ult of t h e m at eri al’ s tr a n s p ort t hr o u g h t h e f er m e nt er, t h e s e ar e di s c h ar g e d wit h t h e f er m e nt ati o n r e si d u e. If m or e mi - cr o or g a ni s m s ar e di s c h ar g e d t h a n c a n b e i n t h e s a m e p eri o d of ti m e, t h eir n u m b er d ecr e a s e s a n d wit h it t h e bi ol o gi c al a cti vit y i n t h e f er m e nt er. Si n c e t h e m et h a n e-f or mi n g ar c h a e a i n t h e mi cr o bi ol o gi c al c o m m u nit y 8 5 C H A P T E R 2 — T E C H N O L O GI C A L D E S C RI P TI O N of a f er m e nt er ar e u s u all y t h e or g a ni s m s t h at gr o w t h e sl o w e st, m et h a n e f or m ati o n t h e n d e cli n e s. D u e t o t h e l a c k of d e gr a d ati o n of t h e a ci di c d e gr a d ati o n pr o d u ct s of t h e pr e c e di n g bi ol o gi c al pr o c e s s e s, t h e p H t h e n b e gi n s t o d e cr e a s e, f urt h er i n hi biti n g t h e a cti vit y of t h e or g a ni s m s. T hi s i s r ef err e d t o a s tilti n g of t h e f er m e nt er. If t h e s u b str at e s u p pl y ( a n d t h u s t h e di s c h ar g e of mi cr o or - g a ni s m s fr o m t h e f er m e nt er) i s n ot st o p p e d i m m e di at el y, t h e pr o c e s s i s irr e v er si bl e, a n d t h e f er m e nt er m u st b e r e st art e d o v er a l o n g p eri o d of ti m e. H o w f ar t h e r o o m l o a d c a n b e i n cr e a s e d wit h o ut ti p pi n g i s a m att er of e x p eri e n c e. H er e, o p er at or s c a n a p pr o a c h a n o pti m al v al u e i n s m all st e p s o v er a l o n g p eri o d of ti m e ( s e v er al m o nt h s). T h e r o o m l o a d c a n al s o b e s p e ci ff e d i n di m e n si o n s ot h er t h a n k g o D M m - 3 d1. F or e x a m pl e, t h e m a s s of v ol atil e f att y a ci d s ( V S) c a n b e u s e d i n st e a d of t h e D M c o nt e nt k g V S m3 d1. T h e a dj u st m e nt of t h e s p a c e l o a d i s a dj u st e d at t h e e n d vi a t h e a d d e d s u b str at e q u a ntit y p er ti m e ( S c hl e g el et al. , 2 0 0 8). 2. 2. 2 R et e nti o n ti m e T h e h y dr a uli c r et e nti o n ti m e i s t h e p eri o d t h at a s u b str at e r e m ai n s i n t h e di g e st er o n a c al c ul at e d a v er a g e a n d i s dir e ctl y d e p e n d e nt o n t h e s p a c e l o a d (J o ut o v s k y et al., 2 0 0 4). F or m ul a 2: H y dr a uli c r et e nti o n ti m e » τ = H R T = H y d r a uli c r et e nti o n ti m e » VR = R e a ct or v ol u m e [ m3 ] » V = V ol u m e of s u b str at e f e d p er u nit ti m e [ m 3/ h] T h e c o m p o siti o n of t h e s u b str at e s h a s a hi g h i m p a ct o n t h e r e q uir e d h y dr a uli c r et e nti o n ti m e. Li k e wi s e, t h e t e m p er at ur e h a s a si g nifi c a nt i n ff u e n c e o n t h e H R T. T h e d e p e n d e n c e b et w e e n t e m p er at ur e a n d r e si d e n c e ti m e c a n b e s e e n i n fi g u r e 4 1 . At hi g h er t e m p erat ur e s t h e r e si d e n c e ti m e d e cr e a s e s, at l o w - er t e m p er at ur e s it i n cr e a s e s. I n t h e i niti al p h a s e, t h e r el ati v e g a s yi el d i n cr e a s e s s h ar pl y a n d fi att e n s wit h i n cr e a si n g r e si d e n c e ti m e. T h e r e si d e n c e ti m e i s s el e ct e d s o t h at t h e g a s yi el d i s a s hi g h a s p o s si bl e. T h e r el ati o n s hi p b et w e e n s p a c e l o a di n g a n d r e si d e n c e ti m e i s s h o w n i n ff g u r e 4 1 . F or a c o n st a nt s u b str at e c o mp o siti o n, a s t h e s p a c e l o a d i n cr e a s e s, m or e i n p ut i s a d d e d t o t h e f er m e nt er a n d t h e r e si d e n c e ti m e d e cr e a s e s. T h e r e a ct or c o nt e nt s ar e c o n st a ntl y e x c h a n g e d, a n d t h e r e si d e n c e Fi g u r e 4 0: C orr el ati o n b et w e e n or g a ni c l o a di n g r at e a n d h y dr a uli c r et e nti o n ti m e f or v ari o u s s u b str at e 160 140 120 100 8 0 4 0 2 0 0 1. 0 1. 5 2. 0 2. 5 3. 0 3. 5 4. 0 4. 5 5. 0 6 0 Or g a ni c l o a di n g r at e [ k g V S / ( m 3d)] R et e nti o n ti m e [ d] 1 5 0 k g V S / m 3 1 0 0 k g V S / m 3 5 0 k g V S / m 3 8 6 C H A P T E R 2 — T E C H N O L O GI C A L D E S C RI P TI O N ti m e s h o ul d b e s el e ct e d s o t h at n o m or e micr o or g a ni s m s l e a v e t h e f er m e nt er vi a t h e e ff u e nt m a s s t h a n c a n r e gr o w i n it. If t h e r e si d e n c e ti m e i s t o o s h ort, t h e mi cr o or g a ni s m s c a n n ot d e c o m p o s e t h e i ntr o d u c e d s u bstr at e, w hi c h r e d u c e s t h e g a s yi el d. F or t hi s r e a s o n, t h e r e si d e n c e ti m e m u st b e a d a pte d t o t h e s u b str at e u s e d. If t h e a m o u nt of s u b str at e a d d e d a n d it s c o m p o siti o n ar e k n o w n, t h e r e a ct or v ol u m e c a n b e c al c ul at - e d f or a gi v e n r e si d e n c e ti m e ( F a c h a g e nt u r N a c h w a c h s e n d e R o h st o ff e, 2 0 1 3) . 2. 2. 3 Pr o d u cti vit y, yi el d a n d d e gr e e of d e gr a d ati o n If t h e g a s pr o d u cti o n i s r el at e d t o t h e f erm e nt er v ol u m e, t hi s c orr e s p o n d s t o t h e m et h a n e pr o d u cti vit y. It i s d e fi n e d a s t h e q u oti e nt of t h e d ail y g a s pr o d u cti o n a n d t h e r e a ct or v ol u m e, a n d t h u s pr o vi d e s i nf or m ati o n a b o ut t h e e fi e cti v e n e s s ( S c hl e g el et al. , 2 0 0 8) . T h e pr o d u cti vit y c a n r ef er t o t h e bi og a s ( P bi o g a s ) a s w ell a s t o t h e m et h a n e ( PC H 4 ). F or m ul a 3: M et h a n e pr o d u cti vity » VR = R e a ct or v ol u m e [ m3 ] » VC H 4 = M et h a n e pr o d u cti o n [ N m 3 / d ] If t h e g a s pr o d u cti o n i s r el at e d t o t h e i n p ut s u b st a n c e s, t hi s r e s ult s i n t h e yi el d. It i s d e - ff n e d a s t h e q u oti e nt of t h e a m o u nt of g a s pr o d u c e d, a n d t h e or g a ni c m att er a d d e d ( F a c h a g e nt u r N a c h w a c h s e n d e R o h st o fl e, 2 0 1 3) . It pr o vi d e s i nf or m ati o n o n t h e e ffci e n c y of m et h a n e pr o d u cti o n fr o m t h e s u b - str at e s f e e d i nt o t h e di g e st er. A s a n i n di vi d u - al p ar a m et er, t h e yi el d i s n ot v er y m e a ni n gf ul si n c e t h e e ff e cti v e l o a d of t h e f er m e nt er h a s n o i n ff u e n c e i n t hi s r e s p e ct. F or a b ett er u n - d er st a n di n g of t h e pr o c e s s, t h e yi el d s h o ul d t h er ef or e al w a y s b e st at e d t o g et h er wit h t h e s p a c e l o a d ( S c hl e g el et al. , 2 0 0 8). F or m ul a 4: M et h a n e yi el d » VC H 4 = M et h a n e p r o d u cti o n [ N m 3 * d] » o T S = A d d e d o r g a ni c dr y m att e r [t / d] T h e d e gr e e of d e gr a d ati o n η o D M d e s cri b e s t h e e ffi ci e n c y of utili z ati o n of t h e s u b str at e s u s e d ( S c hl e g el et al. , 2 0 0 8). Fi g ur e 4 1: R el ati v e bi o g a s yi el d s, d e p e n di n g o n t e m p er at ur e a n d r et e nti o n ti m e ( A d a pt e d fr o m O ut, 2 0 2 0). D a y s [ d] 100 100 120 140 9 0 8 0 8 0 7 0 5 0 4 0 4 0 3 0 2 0 2 0 1 0 0 0 6 0 6 0 5 0º C 3 0º C 2 0º C Bi o g a s ( c u m ul ati v e) M et h a n e ( c u m ul ati v e) PC H 4 =VR [ N m 3 * m3 * d - 1] VC H 4 87 C H A P T E R 2 — T E C H N O L O GI C A L D E S C RI P TI O N F or m ul a 5: Bi o m ass d e gr a d ati o n r at e » mi n= M a s s of f r e s h m a s s a d d e d [t] » o D M o u t = or g a ni c dr y m att er c o nt e nt of t h e f er m e nt e r e ff u e nt [ k g/t] » m o u t = M a s s of t h e f er m e nt ati o n r e si d u e [t] 2. 2. 4 G a s c o m p o siti o n, g a s v ol u m e m e a s ur e m e nt a n d g a s a n al y si s T h e g a s c o m p o siti o n of bi o g a s c a n b e t a k e n fr o m t a bl e 1 4. Li k e t h e g a s pr o d u cti o n, t h e g a s c o m p o siti o n d e p e n d s v er y m u c h o n t h e l o a d of t h e di g e st er a n d t h e c o m p o siti o n of t h e s u b str at e s u s e d. If t h e g a s pr o d u cti o n or m et h a n e c o nt e nt d e cr e a s e s wit h a c o n st a nt f e e d, t hi s i n di c at e s a di st ur b a n c e or i n hi biti o n of t h e pr o c e s s e s. Bi o g a s q u a ntit y m e a sur e m e nt a n d bi o g a s c o m p o siti o n m e a s ur e - m e nt ar e m a n d at or y o n bi o g a s pl a nt s a n d ar e b e st s uit e d f or pr o c e s s c o ntr ol, a s t h e y ar e m e a s ur e d c o nti n u o u sl y a n d r e a ct r el a - ti v el y q ui c kl y. G a s pr o d u cti o n at bi o g a s pl a nt s i s n ot al w a y s m e a s ur e d dir e ctl y b y a ff o w m et er. It c a n al s o b e d et er mi n e d i n dir e ctl y b y t h e filli n g l e v el of t h e g a s st or a g e t a n k s a n d t h e o ut p ut a n d r u n ni n g ti m e of t h e C H P u nit s. Si n c e t h eir g a s c o n s u m pti o n i s k n o w n, t hi s i s s u fi ci e nt t o i nf er t h e a m o u nt of g a s pr o d u c e d. T h e fflli n g l e v el of t h e g a s st or a g e t a n k s i s i m p ort a nt t o b e a bl e t o c o ntr ol t h e o p er ati n g ti m e s of t h e C H P u nit s. I n d o u bl e di a p hr a g m r o of s, t h e l e v el i s oft e n d et er mi n e d b y m e a n s of a t e n si o ni n g c a bl e wit h a r o d a n d a n gl e s e n s or ( L o s si a n d P üt z, 2 0 2 1). A n ot h er m et h o d i s t o u s e a t e n si o n c a bl e wit h a p ull e y, m e a suri n g w ei g ht a n d pr o xi mit y s e n s or s ( L o s si a n d P üt z, 2 0 2 1) . T h e s e m e c h a ni c al s y st e m s h a v e pr o v e n t h e m s el v e s i n pr a cti c e a n d of - f er t h e a d v a nt a g e of eli mi n ati n g i g niti o n s o ur c e s wit hi n t h e r o of. Ot h er s y st e m s f or d et e cti n g t h e h ei g ht of t h e g a s st or a g e m e m - br a n e ar e b a s e d o n ultr a s o u n d or di st a n c e m e a s ur e m e nt wit h l a s er s. I n pr a cti c e, t h e s e s y st e m s h a v e pr o v e n t o b e m or e v ul n er a bl e a n d l e s s r eli a bl e. I n s o m e c a s e s, t h e g a s v ol - u m e fl o w i s m e a s ur e d b ef or e e nt eri n g t h e g a s c o ntr ol s e cti o n i n t h e C H P u nit. T o o bt ai n g o o d m e a s ur e m e nt r e s ult s, t h e g a s s h o ul d alr e a d y b e dri e d. T h er m al pr o b e s ar e u s e d f or t h e m e a s ur e m e nt. M e c h a ni c al v ol u m etri c ff o w m et er s ar e l e s s s uit a bl e d u e t o t h e m oi st ur e i n t h e g a s. T h e g a s a n al y si s i s c ar - ri e d o ut eit h er wit h m o bil e g a s m e a s uri n g i n str u m e nt s f or i n di vi d u al m o nit ori n g of t h e c o nt ai n er s, or it i s c arri e d o ut i n a c e ntr al r e - c or di n g b ef or e e nt eri n g t h e g a s c o ntr ol li n e. T h e st ati o n ar y g a s m e a s uri n g i n str u m e nt s u s u all y r e c or d t h e p ar a m et er s C H 4, C O2, O2 a n d H 2S. T h e m et h a n e c o n c e ntr ati o n i s t h e m o st i m - p ort a nt c o m p o n e nt of bi o g a s f or t h e e v al u a - ti o n of t h e o p er ati o n m a n a g e m e nt. It r e pr es e nt s t h e c o m b u sti bl e fr a cti o n of bi o g a s a n d dir e ctl y i n ff u e n c e s t h e c al orifi c v al u e. T h e hi g h er t h e pr o p orti o n s of c ar b o n di o xi d e a n d nitr o g e n, t h e m or e t h e c al ori ff c v al u e of t h e g a s i s r e d u c e d. T h e g a s c o m p o siti o n a n d t h u s t h e m et h a n e c o nt e nt i s m ai nl y d e p e n dT a bl e 1 4: C o m p o siti o n of bi o g a s ( F a c h a g e nt ur N a c h w a c h s e n d e R o h st o ffi e, 2 0 1 3). G a s C o n c e ntr ati o n M et h a n e ( C H 4) 5 0 - 7 5 V ol.- % C ar b o n di o xi d e ( C O 2) 2 5 - 4 5 V ol.- % W at er ( H 2O) 2 - 7 V ol.- % ( 2 04 0 ° C) H y dr o g e n s ul p hi d e ( H2S) 2 0 - 2 0, 0 0 0 p p m Nitr o g e n ( N 2) < 2 V ol.- % O x y g e n ( O 2) < 2 V ol.- % H y dr o g e n ( H 2) < 1 V ol.- % 88 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION ent on the substrate used; the composition can only be influenced to a limited extent by controlling the process. The methane concentration depends on the process parameters such as the fermentation temperature, the load condition of the reactor and the hydraulic retention time as well as on process disturbances and procedures such as biological desulfurization (Weiland, 2010). 2.2.5 Fermentation temperature Fermentation is an exothermic process, nevertheless, to maintain a defined temperature level, the fermentation tanks must be equipped with heating. The temperature must be measured during the fermentation of the substrates. A rapid change of the temperature results in a continuous disturbance of the process biology, due to which the temperature is not suitable as a variable for process control. The temperature inside the fermenter has a strong effect on the environmental conditions. With an increasing temperature, the balance between ammonia and ammonium shifts towards ammonia. This has an inhibitory effect on the acetoclastic methane bacteria and thus has negative consequences, especially when processing protein-rich substrates. Therefore, the temperature of the anaerobic degradation processes must be monitored. Temperatures below 40°C slow down the biology within the fermenters, the retention time increases, at temperatures above 50°C fewer bacterial strains are present, and the overall process becomes more prone to failure (Song et al., 2004). Temperature sensors are placed at different heights in the fermenter. 2.2.6 Dry matter content and viscosity of the fermentation mixture The DM content and viscosity are important parameters with regard to the mixing, pumpability and gas discharge of the digested sludge. If the gas bubbles in the digester cannot escape, this results in foam formation. The DM value does not correlate directly with viscosity, yet it is monitored in many plants (Krieg and Fischer, 2001). Direct measurement of the viscosity of the fermentation substrates is very difficult, but it can be measured indirectly via the power input of the agitators. This is possible because the power input of the agitators depends, among other things, on the viscosity of the mixture to be agitated. Normally, the agitators are controlled via a fixed speed or via a predefined time interval. Automatic control of the agitators is state of the art. Nevertheless, manual intervention is also necessary, for example if the viscosity is too high, it must be mixed with fermentation substrate with a very low DM content, with process water from the biogas plant or fresh water. The fermenter is fitted with sight glasses, which are used for visual inspection of the fermenter contents. Associated with this is the problem of foam formation. This is the result of reduced surface tension, which is caused by surface-active substances (Koch et al., 2017). The exact causes of foam formation in a biogas plant are often not known. It occurs more frequently, for example, when spoiled silage is used. It is possible that surface-active intermediates or bacterial groups in interaction with increased gas production are the cause. Foam can clog the gas lines and increase the pressure in the fermenter, which can trigger the overpressure protection or even destroy the roof (Fachagentur Nachwachsende Rohstoffe, 2013). The state of the art in digesters is the monitoring of the filling level and, to some extent, the formation of foam. Pressure sensors, conductivity sensors and occasionally infrared sensors are used for this purpose. It is controversial whether measurement of foam formation is useful; these can create a false sense of security. In the event of a biological overreaction occurring and the foam sensor responding, only a few minutes remain before foam enters the gas sampling system. In extreme cases, even the foil roof can tear open. If there is no foam sensor, charging processes that lead to an overdrive 8 9 C H A P T E R 2 — T E C H N O L O GI C A L D E S C RI P TI O N of t h e bi o g a s pl a nt m u st b e a v oi d e d ( L o s si a n d P üt z, 2 0 2 1). 2. 2. 7 F O S/ T A C T h e F O S/ T A C d et er mi n ati o n i s a titr ati o n t e st f or d et er mi ni n g t h e q u oti e nt s of a ci d c o n c e ntr ati o n a n d b u ff er c a p a cit y i n t h e f er m e nt ati o n r e si d u e ( Lili et al. , 2 0 1 1). T h e F O S/ T A C v al u e t h er ef or e pr o vi d e s k n o wl e d g e a b o ut t h e d e gr a d ati o n p erf or m a n c e of t h e f er m e nter. F O S st a n d s f or V ol atil e Or g a ni c A ci d s, u nit [ m g/l a c eti c a ci d e q ui v al e nt s] a n d T A C f or T ot al I n or g a ni c C ar b o n at e ( al k ali n e b u ff er c a - p a cit y, u nit [ m g C a C O 3/l]. T h e v al u e i s a m e a sur e of t h e a ci di fi c ati o n ri s k of a pl a nt. Wit h t h e h el p of t h e F O S/ T A C, pr o c e s s di st ur b a n c e s c a n b e d et e ct e d at a n e arl y st a g e a n d a r e - a cti o n c a n b e t a k e n if n e c e s s ar y ( R ei n h ol d, a c c e s s e d 0 4. 1 1. 2 0 2 1) . T h e d et er mi n ati o n i s c arri e d o ut i n e xt er n al l a b or at ori e s a n d n ot i n t h e bi o g a s pl a nt. H o w e v er, a c o ntr a ct wit h a n a p pr o pri at e l a b or at or y f or s a m pli n g a n d c o n - tr ol i s n ot u n u s u al. A s t h e p er c e nt a g e of or g a ni c a ci d s i n a di - g e st er i n cr e a s e s, t h e m et h a n epr o d u ci n g b a ct eri a ar e i n hi bit e d. Wit h d e cr e a si n g p H, t h e i n hi biti o n e fi e ct i n cr e a s e s ( at p H > 7. 4). H o w e v er, t h e p H v al u e i s o nl y s uit a bl e f or a c - c ur at e pr o c e s s a n al y si s t o a li mit e d e xt e nt, si n c e t h e a ci d s i n t h e f er m e nt er ar e i niti al - l y b u ff er e d. I n c o ntr a st t o t h e p H v al u e, t h e F O S/ T A C v al u e d et e ct s t h e pr e s e n c e of t h e b u fl er ( P o st el et al. , 2 0 0 9). T h e d et er mi n ati o n c a n b e d o n e b y m a n u al ti - tr ati o n or b y a titr at or. A m e a s ur e m e nt i s c arri e d o ut i n t h e ffr st st e p b y dr a wi n g a r e pr es e nt ati v e f er m e nt ati o n s u b str at e s a m pl e. I n a f urt h er st e p, t h e s a m pl e i s fr e e d fr o m c o ar s e c o m p o n e nt s ( e. g., b y m e a n s of a fflt er or a c e ntrif u g e). A d e ff n e d a m o u nt of s u b str at e i s t a k e n a n d i m m er s e d i n a n el e ctr o d e. T h e s a m pl e i s pl a c e d o n a m a g n eti c stirr er a n d p er m a n e ntl y h o m o g e ni z e d. N o w titr ati o n wit h 0. 1 N H 2S O 4 f oll o w s u ntil a p H v al u e of 5 i s r e a c h e d. T h e a ci d c o n s u m e d i s r e c or d e d. T hi s st e p i s r e p e at e d u ntil a p H v al u e of 4. 4 i s r e a c h e d. T h e v al u e c a n t h e n b e c al c ul at e d a c c or di n g t o f or m ul a 8 ( R ei n h ol d, a c c e s s e d 04.11.2021). F or m ul a 6: D et er mi n ati o n F O S F or m ul a 7: D et er mi n ati o n T A C F or m ul a 8: D et er mi n ati o n F A S/ T A C R ati o » F P 1= mi x e d e n d p oi nt u p t o p H 5. 0 » F P 2= mi x e d e n d p oi nt u p t o p H 4. 4 » 1. 6 6 = f r e e or g a ni c a ci d s » 0. 1 5 = f r e e or g a ni c a ci d s » 5 0 0 = f r e e o r g a ni c a ci d s » 2 5 0 = f r e e o r g a ni c a ci d s » fH 2 S O 4 = tit e r f r o m c ( H 2S O 4 ) = 0. 0 5 m ol/ L A F O S/ T A C r ati o of 0. 3 t o 0. 4 i s c o n si d er e d n or m al i n pr a cti c e, b ut t hi s v al u e i s ffi u ct u - ati n g i n pr a cti c e b e c a u s e it i s pl a nts p e ci fi c; t h er e i s a str o n g d e p e n d e n c e o n t h e s u bstr at e c o m p o siti o n. I n E n er g y cr o p s pl a nt s, t h e v al u e i s b et w e e n 0. 4 a n d 0. 6 wit h st a bl e pr o c e s s c o ntr ol ( Si n gliti c o et al. , 2 0 1 7). T h e s m all er t h e a m o u nt of a ci d r e q uir e d u n - til t h e p H v al u e of 5. 0 i s r e a c h e d, t h e l o w er t h e b u ff er c a p a cit y of t h e s u b str at e. If t h er e ar e f e w or g a ni c a ci d s i n f er m e nt er, t h e m or e a ci d a d diti o n i s n e e d e d t o r e a c h a p H of 5. 0. T h e hi g h er t h e b a si c b u ff er, t h e m or e a ci d i s n e e d e d t o g et t o a p H of 5. 0. A hi g h st arti n g p H a b o v e 7. 9 i s i n di c ati v e of i n cr e a s e d N H 4N. A hi g h F O S v al u e m e a n s t h at t h er e ar e m or e or g a ni c a ci d s i n t h e di g e st er, t h e hi g h er t h e F O S v al u e, t h e m or e a ci d i s n e e d e d t o g et fr o m a p H st arti n g at 5. 0 t o a t ar g et p H of 2 0 * (( F P 2 - F P * 1, 6 6 - 0, 1 5) * f H S O * 5 0 0 F O S = s a m pl e siz e F O S F O S / T A C = T A C 96 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION other factors. Additives are substances added to the fermentation process with the aim of positively influencing it. All substances or working materials added to the fermenter to promote the microbiological degradation processes, which are not substrates, are auxiliary substances. The fermentation aid itself has no biogas formation potential or this is negligible. Fermentation aids can be of organic or inorganic composition (including algae preparations, trace elements for supplying the microorganisms, enzymes for hydrolysis) (Messtechnik für die Biogasanlage…, accessed 10.11.2021). Two applications can be defined for the use of fermentation aids, on the one hand the one-time addition in the event of a malfunction and on the other hand the prophylactic-preventive application. Practical experience has shown that instabilities in the fermentation process can occur if liquid manure and solid manure are not used. This can be caused by a lack of micronutrients as well as the presence of inhibitors. In practice, this is counteracted by using fermentation aids. There are different reasons for using them, e.g., the specific methane yield is too low [m3/kgoDM], failure to achieve higher digester load [kgoDM/ (m3 * d)], reduction of hydrogen sulphide, high viscosities, and insufficient mixing, foaming and floating layer formation. The examples listed are indicative of a process upset. Prompt laboratory analysis allows the operator to intervene quickly and efficiently. The symptoms can be traced back, for example, to an accumulation of volatile organic acids, a shift in the acid spectrum towards higher-value fatty acids, an increasing FOS/TAC value, an increased concentration of ammonia. Also, causal factors can be an insufficient degradation efficiency as well as an increasing solids content in the fermenter (Messtechnik für die Biogasanlage…, accessed 10.11.2021). These examples illustrate that, in addition to the efficiency of the biogas plant (gas yield with short retention time), the ongoing operating costs are also a reason for using auxiliary materials. These can be used to reduce the energy required for the agitators, and the cost of desulfurization can also be reduced. If the benefit of fermentation aids exceeds the expense, their use is recommended. Table 18 shows typical fermentation aids with examples and the areas of application. A clear subdivision of the individual substances is difficult in that they often have several modes of action at the same time. Legal regulations apply to the use of auxiliary substances; too high an addition of these can lead to the fermentation residue no longer being allowed to be discharged, for example (Landwirtschaft, B. B, 2018). Trace elements: The microorganisms involved in anaerobic fermentation require nutrients, the so-called bulk elements, to maintain metabolism and for their own reproduction. These bulk elements are: Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O), Phosphorus (P) and Sulphur (S) (12). Also, sufficient sodium (Na), potassium (K), calcium (Ca), iron (Fe) and magnesium (Mg) must be present (Drosg, 2013). To maintain their metabolism and to produce enzymes, microorganisms need so-called trace elements. These are involved in enzymatic conversions, in the formation of co-factors, in redox reactions and other processes in which living organisms are involved. Trace elements in the biogas reactor include nickel (Ni), cobalt (Co), molybdenum (Mo) and selenium (Se). But also, other metals, such as copper (Cu), zinc (Zn), manganese (Mn), tungsten (W), or vanadium (V) and nonmetals such as boron (B) (Drosg, 2013). If a plant is operated with slurry or manure, enough trace elements are available to the biogas plant, but a deficiency can still not be excluded. Biogas plants that use only corn silage have long-term trace nutrient deficiencies. These deficiency situations occur with mono fermentation of energy crops. Trace elements usually come from the substrate, but are often not available in required amounts, so the additional supply of trace elements is the rule in practice. Biogas plants with a very high slurry input, good management and low space loading can do completely without the supplementation of micronutrients. Trace element supply is a function of the biocenosis in the digester and is determined by technical, physical and chemical conditions. Poorly soluble precipi- 97 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION tated sulphides in the fermenter, for example, can reduce the concentration of trace elements by several orders of magnitude. Agitators can release trace elements, which leads to an increase in the concentration of trace elements. The trace element contents added must be determined by an external laboratory 97 service provider and must be repeatedly validated if the process is stable. Proper use of trace elements can accelerate fermentation, results in a more stable process, and greater space loads and biogas yields can be achieved. Overdosing can have an inhibitory effect on the fermentation process; too many elements have a toxic effect on the biocenosis (Drosg, 2013). Enzymes: These are proteins that are involved in degradation processes as biological catalysts. In normal operation, these enzymes are produced by the microorganisms; with an optimal fermenter operation, the new enzyme formation is ensured. According to the manufacturer, the addition of enzymes improves the degradability of plant components, which should be broken down more quickly. This is said to improve the productivity of the gas process. By adding enzymes, a faster and more intensive digestion of the biomass is to be achieved, the viscosity is to be reduced, which lowers the stirring energy costs. Furthermore, a faster degradation of cellulose and hemicellulose is to be achieved, floating layers in the fermenter are dissolved. Massive piles of solids from grain, hay, grass silage and solid manure are to be better dissolved and liquefied, floating layers in the final storage are to be avoided with enzymes. Practical studies had shown that there can be significant effects with substrates rich in water and through the application of enzymes. Enzymes do not have a protein structure and are therefore subject to bacterial degradation; for a lasting effect, appropriate preparations must be continuously added to the process. (Bochmann et al., 2007). Additives to reduce the hydrogen sulphide concentration: Gaseous hydrogen sulphide is formed in the biogas process from organic compounds of sulphur, which are reduced to hydrogen sulphide by microorganisms. Increased levels of organic sulphur components are present in canola and rapeseed products and in protein-rich substrates such as grain, food waste, pig manure, and poultry manure. Hydrogen sulphide affects many microorganisms and can inhibit methane formation in the digester. Sulphide precipitation binds trace elements in the fermenter, which are thus no longer available to the microorganisms. By using the auxiliary substances iron (II) and iron (III) Table 18: Typical fermentation aids with examples and areas of application (Fachagentur Nachwachsende Rohstoffe, 2010b). Type of fermentation additive Examples Definition Trace elements Iron, cobalt, nickel, zink, etc. Trace elements are chemical elements, that are necessary for the optimal growth of microorganisms. Ion exchanger Zeolites, clay minerals Reduce the concentration of potentially inhibitory/ toxic fermenter ingredients Microorganisms Hydrolytic cultures Completion of the existing biocenosis organisms that optimize the process (speed, stability) or faster adaptation to new substrate compositions or changed enable boundary conditions 98 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION salts such as FeCl2, FeSO4, FeCl3 and Fe(OH)3, a targeted precipitation of sulphide is possible so that it does not enter the biogas as hydrogen sulphide (Meegoda et al., 2018). Additives for the reduction of the ammonia concentration: Ammonia occurs in low concentrations in biogas. It is formed during the breakdown of proteins and nitrogenous compounds such as urea or uric acid in animal excrement. To reduce the ammonia concentration, mineral substances are added as additives. These bind excess ammonium ions. Molecular sieves and clay minerals, which can fix ammonia by ionic forces, are also used for the sorption of ammonia. In this process, cations such as potassium, calcium or magnesium are replaced from the lattice structure and bound to ammonium as in an ion exchanger. The ammonium remains bound to the solids and is thus removed from the system. If these additives are used incorrectly or too frequently, sink layers may form, the effectively usable fermenter volume decreases as a result, and there may also be an abrasive effect on the agitators (Drosg, 2013). Minerals and buffers (pH stabilizers): Methane formation requires a stable pH value. The pH value is influenced by the base and carbonate concentration or the lime-carbonic acid balance, as well as by the ratio of ammonium to ammonia. If excessive amounts of carbonic acids are formed in the fermentation process, acidification of the fermenter contents may occur. To counteract this, buffering substances such as sodium hydrogen carbonate or combination preparations with trace elements and carbonates can be used. These are intended to safeguard the pH value against over acidification (Drosg, 2013). Floating layer remover: Floating layers (see figure 45) can form on the surface of the liquid in fermenters, secondary fermenters or in the digestate store. Floating layers are foamy, dense to solid deposits that may result from lack of mixing, incomplete decomposition of the organic mass, high space loading or hydraulic short circuits. Using enzymes and algae extracts, etc., longfibered substrates can be better degraded. The hydrolytic degradation should be faster, and the viscosity should be reduced, which leads to an improvement of the stirring performance. As a result, floating layers can be dissolved more easily (Drosg, 2013). Defoamer: One of the most common malfunctions in biogas plants is the formation of foam. It can lead to blockage/clogging of the pipelines and thus to severe damage to the plant. Pipelines can become fouled, sensors can be disturbed, and gas lines can become clogged. In the worst case, tank leaks can occur. Foam is an important indication that the operation of the biogas plant is faulty. Foams occur when substrates with a high protein content, such as grain, potatoes or dry poultry manure, are fermented. Fats can be released, which are hydrophilic and collect at the liquid surface. There they stabilize formed foam by binding hydrophobic membranes around the air bubbles. Increased acid formation within the fermenter can also contribute to foam formation. Natural oils such as rapeseed oil and silicone oils as well as poly alcohols are used as defoamers. These affect the surface tension of the liquid and change the properties of the bubble formation. In the case of foam formation, the contents of the fermenter are insufficiently stirred and the feedstock and nutrient exchange at the surfaces of the fermentation mixture is reduced. This results in a lack of nutrient supply to the microorganisms, and their cell density and growth rate are negatively affected. The physical gas discharge from the liquid medium is also impaired, which can lead to inhibition or process breakdown (Drosg, 2013). Figure 45: Large foam bubbles in the fermenter formed by the use of sugar beet (Kliche and Lebuhn, 2017). 99 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION Microorganisms: The addition of microorganisms is intended to accelerate biogas production, increase degradation and improve gas yield. The load capacity of the biogas plant, or more precisely the space load, is to be increased with high-performance bacteria (Drosg, 2013). The addition of special externally bred species also has its drawbacks. These have to show a high performance and face the selection pressure within the biocenosis. However, they can displace existing species in the process. If the species do not grow naturally on a sufficient scale, they must be added again and again (Messtechnik für die Biogasanlage…, accessed 10.11.2021). Capillary carbon: Plant carbon is not to be understood here as a fertilizer, it is a carrier substance and serves the periodic storage of essential nutrients. It provides habitats for microorganisms. Two types of charcoal can be distinguished, one is charcoal with a large surface area, which favours the growth of bacteria. On the other hand, charcoal with a high capillary density, which particularly ensures effective material flows and substrate supply (Franke, accessed 10.11.2021). Capillary coal consists to a large extent of pure carbon and thus forms an ideal habitat for microorganisms, which can live and multiply there. The pH value of the charcoal is between 8 - 8.7 and thus provides an ideal growth habitat for methane-producing archaea. The structure of the coal ensures pressures in the pores and capillaries, which guarantee the substrate supply for the microorganisms. Salts can be introduced into the pore structure and serve as nutrients. In the acetogenic phase, the carbon suppresses the sulphate-reducing bacteria and promotes the denitrifying bacteria. This promotes the formation of acetic acid, which generates a higher methane yield (Franke, accessed 10.11.2021). 2.4 Automation and control technology 2.4.1 Sensors In practice, the process of biogas production is focused on a few essential parameters in order to reduce the effort of evaluation as well as the acquisition and maintenance costs. Some values such as filling levels, gas composition or power consumption and power generation are recorded online and evaluated without loss of time. Other parameters such as the FOS/TAC value, DM and oDM values are recorded discontinuously, and in some cases determined and evaluated in external laboratories. Values determined off-line and in external laboratories are prone to errors; inaccuracies occur due to sample collection and transport, making precise and timely process control difficult.(Anleitung zur Ermittlung des FOS/TACs, accessed 03.11.2021). Sensors must often be used in areas where explosive gas mixtures may occur. These sensors must then be suitable for this and must also be installed accordingly. In addition, the sensors must be resistant to high humidity and corrosion. Measuring devices with moving parts in the biogas stream are susceptible to malfunctions due to contamination. Special requirements also apply to all sensors that must trigger safety shut-downs. For example, level sensors that must shut down the filling of the tank when a maximum level is reached. Depending on local regulations, these probes must be connected by cable and trigger shutdowns via relays. Other sensors can communicate directly with the 100 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION plant control system via BUS systems. Some important process control parameters and their measurement are explained below. Input quantity and substrate composition: For solids, it is advisable to weigh them, for example by means of scales embedded in the floor on which, for example, loaded machines are weighed before and after unloading. Pressure sensors are used for feeding systems such as the solids feeder. Flow measuring devices can be used on the pipelines for the introduction of liquid substrates, and the volume added can also be determined using level measuring devices. Inductive and capacitive sensors are mainly used for flow measurements, ultrasonic and thermal conductivity sensors are also used less frequently. DM measurements in the fermenter with microwave spectroscopy (figure 46): Mean-while, there are compact and robust microwave-based measuring systems that have been developed specifically for determining the DM content in biogas plants. These can record the individual or total DM content of the substrates involved. There are also versions that are specifically designed for digester installation. Usually mounted in a guard, the sensor measures through a Plexiglas front and there is no direct contact with the fermentation substrate. Microwave radiation passes through a dielectric window of its shroud and detects a representative sample volume. Solids can be detected in water because their relative dielectric constant is much lower (about 2 to 10) than that of water (about 80). Reflective microwave arrays are used for DM measurements. An electromagnetic wave with very low energy is radiated into the material from an antenna and reflected. The reflected wave is detected again, and the reflected portion depends on the dielectric properties of the material under investigation. This provides information on the water or dry substance content (Henkelmann et al., 2010). The method is not very susceptible to interference, changes in pH values or conductivity have no effect, the microwave DM sensors are available in a wide variety of designs, can be retrofitted and have long-term stability. Mobile DM measurement: This measurement is performed gravimetrically. The measuring device has a heated balance for this purpose. A sample of the material to be measured is weighed in the moist state and then automatically dried until the weight is constant. The water content is calculated from the difference in mass. This method cannot be operated continuously but has the advantage that the device is inexpensive and easy to operate and can also measure substrates and fermentation residues. These values are also important for the operation management. Level measurements: To detect the amount of energy present in the gas storage tank, the pressure, temperature, gas composition and storage volume must be known. A variFigure 46: Microwave DM sensor MWDM PP tube Figure 47: Image of a wey rope sensor for detecting the gas level. Source AEV Energy, biogas plant Ansprung (Germany). 101 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION ety of sensor types exist for level detection in the fermenter, these can be hydrostatic pressure sensors, also distance measurements to the surface can be determined by ultrasound or radar. Way rope sensors (figure 47) are sensors for length, displacement, and position determination. Mechanical level gauges are often used in double diaphragm accumulators to determine the height of the inner gas diaphragm. The filling level of the gas accumulator can be determined by the distance between the gas accumulator diaphragm and the weather protection diaphragm. Drawwire sensors attached to the gas storage membrane are used for this purpose. The cable is guided through the storage membrane and ends in a measuring tube via deflection pulleys. If the diaphragm lifts, the change in the sensor’s travel is registered. Magnets are installed at the end of the cable, and these switch via installed reed contacts in the measuring tube. The output signal, which is proportional to the displacement, is converted into a level. This technology is simple but relatively robust and reliable. Rod probes, measure the electrical conductivity between metal rods. Depending on the number of bars installed, different measuring points can be detected. They are often used for leakage detection and shutdown in case of overfilling. Leakage probes are used to detect leaks in tanks containing liquids hazardous to water. On contact with an electrically conductive liquid, the integrated electronics react, and a permanently emitted signal is interrupted. This can result in an acoustic or optical signal. Overfill sensors are mostly self-sufficient and not coupled in a bus system. Rod probes can only determine discrete levels. Other systems are used for continuous measurement. Pressure probes can continuously and reliably measure liquid levels. For this purpose, the probe is installed near the bottom of the tank. This technology is widely used to determine the levels in tanks of biogas plants. Radar sensors are also suitable for continuous level measurement. The advantage of radar sensors is their maintenance-free operation due to non-contact measuring methods and the fact that they provide exact measurement results independent of the process conditions. The instrument emits a continuous radar signal via its antenna. The signal is reflected by the medium and received by the antenna as an echo. The frequency difference between the transmitted signal and the received signal is proportional to the distance. This depends on the level and can therefore be determined. Radar probes can be used more flexibly than pressure probes. They can also detect foam formation. Ultrasonic measurements based on the transit time principle offer another non-contact method. A sensor emits ultrasonic pulses, they are reflected by the surface and detected by the sensor. The required transit time is a measurement of the distance travelled in the empty part of the tank (Arbeitsgemeinschaft Landtechnik, accessed 08.11.2021). This value must be subtracted from the total height of the tank, from this the level can be calculated. Radar uses electromagnetic waves, while ultrasound uses Figure 48: Radar measuring device for level measurement (Micropilot FMR52 from Endress+Hauser) (Messtechnik für die Biogasanlage…, acces- 102 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION Table 19: Overview of advantages and disadvantages of different sensors for gas volume measurement (Drosg, 2013). Sensor type + - Ultrasonic flow meters » Good results at low pressure » No moving parts » Very reliable even at changing process conditions » Long straight measuring distance needed (15 times the diameter) Fluidistor oscillator » No moving parts » High accuracy » Low cost » Easy handling, exchange and cleaning » Complex calculation to norm cubic meters » Error of 1.5% » Sensitive to vibrations in the biogas caused by e.g., piston compressors Turbine flow meters » Robust technology » Deposits can become problematic » Moving parts Vortex flow meter » No moving parts » High durability » Resistant to corrosion » Low pressure loss » Sensitive to disturbances in flow » Long straight measuringdistance needed (30 times the diameter) Dynamic pressure probes » Long durability » Dirty gas has little influence » Pressure fluctuations have no negative effect on accuracy » Works better at higher gas pressure » Large calibration effort » Error of 1.5-5% » For calculation of Nm³ the density (gas composition) is needed » Long measuring distance needed Thermal flow meters » Easy handling » Good for mobile applications » Direct measurement of Nm³/ mass » Exact Measurement also at pressure fluctuations » No dirty biogas measurement possible » Measurement error of 3-5% (increases rapidly if gas is dirty) » Extremely sensitive to humidity » Long straight measuringdistance needed » Calibration once a year Diaphragm gas meters / bellows gas meters » Simple and cheap » Direct volume measurement » Robust technology » Corrosion, fouling or deterioration of gas meter by biogas components and particles » Increased utilization time decreases accuracy of measurements » External calibration and maintenance 103 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION mechanical waves that propagate at the speed of sound. Ultrasonic level sensors are insensitive to changes in density and viscosity. Foam, turbulence, vapor and haze can affect measurements. Other optical sensors such as laser measurement have problems with steam and particles and are therefore not generally recommended. Biogas production: A change in gas composition or gas volume can be an indicator of process imbalance. Accurate measurement of biogas volume is a technical challenge because biogas is composed of different gases, is saturated with water vapor, and may also contain particles. In addition, biogas is produced at low pressure. Gas metering systems should be located so that they are easy to remove and clean. There are many different types of sensors available for gas meters, these and their typical advantages and disadvantages are listed in Table 19 (Arbeitsgemeinschaft Landtechnik, accessed 08.11.2021). Flow sensors are used for both gas flow and substrate flow. Special requirements apply if the determined values must be used for billing purposes. When biomethane is fed into the natural gas grid, the meters are provided by the grid operator. Gas composition: In many biogas plants, online measuring devices are installed to record the gas composition, but portable measuring devices are also used. Figure 49 shows a stationary analysis station from Binder Engineering GmbH. It detects CH4 and CO2, which are measured with infrared or thermal conductivity sensors. In the vast majority of cases, H2S and O2 are determined by electrochemical sensors. Oxygen is also measured by paramagnetic sensors. H2S is measured less frequently than other parameters, this increases the lifetime of the sensors. Measurement of biogas composition is not necessary for modern CHP units, they are self-regulating, however, biogas composition is a useful parameter to monitor the processes in a biogas plant. For example, a decrease in methane content may indicate an overload, and an increase in H2S may indicate process instability. The table shows different measurement methods or gas sensors for the analysis of biogas. Methane, carbon dioxide and oxygen are indicated in vol.-%, hydrogen sulphide is usually indicated in ppm. Table 20: Suitable gas sensors for the analysis of biogas (Franke, accessed 10.11.2021). Figure 49: Flexible modular gas analysis station (COMBIMASS® GA-s hybrid premium; Binder Engi-neering GmbH) (Binder Engineering GmbH, acMeasurement method CH4CO2O2H2S Thermal conductivity sensor X X Spectrometer valence electrons (Near infrared) X X Spectrometer valence electrons (UV light) X Spectrometer Molecular Vibrations X X Electrochemical gas sensors X X Paramagnetic sensor X 104 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION Temperature: Biogas digesters require a stable process temperature; this is necessary for high performance of the microorganisms. Pt100 T probes are mainly used to measure the temperature. They must cover a temperature range of 20 - 60°C to be able to measure in the psychrophilic, mesophilic, and thermophilic fermentation environment. Due to measurement inaccuracies, it is recommended to use several temperature sensors in different local areas. Pt-100 measurement sensors are very accurate temperature sensors, all parts are made of stainless steel, they are characterized by their robust mechanical design and low cost. Electricity meter/power feed: An electricity meter is a measuring device that records the energy supplied and the instantaneous power of a consumer. The measured values are given in kilowatt-hours for energy and in kW for power. An electricity meter in a biogas plant evaluates not only the kilowatt-hours delivered to the grid operator, but also the own consumption and the most important electrical consumers of a biogas plant are the CHP, agitators, solids input, solids and slurry pumps, drives, the support air blowers, and pumps. The average own power consumption is between 6 - 12% of the generated electrical energy. The energy consumption of aggregates can be measured at various points and is evaluated by the plant control system. 2.4.2 Power feed-in Grid connection point and transfer station: The electrical energy generated in the CHP unit can be fed into the low-voltage grid (usually 400 V) or medium-voltage grid (7 - 25 kV (after conversion in the transformer)). The power is fed into the grid at the grid connection point, where the operator’s power grid merges with that of the power supplier. At the point of connection to the grid, the energy fed in and taken out must be measured and protective devices must be installed for disconnection in the event of a fault. These installations form the transfer station . Telecontrol technology, control specifications of the grid provider: In most cases, the transfer station must be included in the communication of the plant control system since the grid provider must be able to throttle the power of the plant in case of emergency or to take it off the grid completely. In simple cases, the transfer station contains a radio receiver for this purpose, which acts on the CHP unit through a direct wired connection and regulates it in stages (30%, 60%, 100%). The connection to the CHP unit is then made via a switching contact without the system controller being interposed. This ensures that the control from the grid provider always has priority over other controls. In new buildings, the grid provider usually communicates with the plant control system and the CHP unit via a BUS system. The advantage is that the grid provider receives important information for grid operation, such as available power or available runtime (for flexibly operating plants). Disadvantage is a considerable financial expenditure for the installation of this remote-control technology. The grid provider determines which method is used. Regardless of the grid provider’s ability to intervene in power generation via remote access, general compliance with various characteristic curves is often required by the grid provider. Such characteristics can, for example, control the reactive power fed into the grid as a function of the power or the grid voltage. Depending on the control concept, such characteristic curves can be stored directly in the CHP controls or in the plant control system. Protection concept: During the planning phase of the power feed-in, a protection concept must also be drawn up. It regulates the separation of the generating unit (CHP) and the power grid from the grid of the generating plant in the event of a fault (overcurrent, undervoltage and overvolt- 105 CHAPTER 2 — TECHNOLOGICAL DESCRIPTION age, frequency protection, shutdown if the contractually regulated power is exceeded, shutdown if the reactive power is too high, etc.). Basically, the protection takes place in two stages. The higher-level protection disconnects the entire plant from the power grid (this usually also means a power failure on the entire plant). The subordinate protection disconnects only the generating unit from the mains. The protection functions must be parameterized in such a way that the subordinate protection trips first. 2.4.3 Plant control The plant control system handles the communication between the individual plant components, aggregates, and their control. It essentially consists of electrical components such as frequency converters for speed-controlled motors, relays for switching and the programmable logic controller (PLC) on which the programming is stored. Usually, the PLC has a visualization of the plant on which the most important parameters can be read, and many values can be set. An example for a plant control is shown in figure 50. The basis of the control is the programming of the PLC. Here, the running times of the stirrers, the feeding quantities of the tanks, the running times, and the schedule of the CHP and much more are set. In the event of malfunctions, for example the failure of a pump, the PLC must register this and issue an error message. Usually, this error message is also forwarded as a text or voice message to stored telephone numbers. In addition to the control tasks required to maintain operation, the system controller must also perform safety shutdowns. A special feature here is that these switches do not Figure 50: Example of a plant visualization (AEV Energy). 112 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS 3.1 The Economic Biogas Framework in Europe The production of biogas and biomethane has been growing constantly worldwide as well as in Europe, and incentives for the use of this renewable energy have been gaining importance within public policies. According to the Renewable Energy Directive, by 2030, 32% of all energy used in the European Union (EU) must come from renewable sources, including biomass, bioliquids and biogas. In addition, European countries must establish a national action plan with the objective of defining the share of energy from renewable sources as well as establishing procedures for the reform of planning, pricing scheme and access to electricity networks, promoting energy from renewable sources (Biogas3, 2016). More generally, incentives in the biogas sector seek to implement a sustainable bioeconomy that represents the renewable segment of the circular economy, capable of transforming organic waste into valuable resources and creating innovations and incentives to help reduce waste and improve organic waste treatment (Communication, 2018). As a product, biogas constitutes an economy generator centre in Europe, especially because it is considered a consumer of a significant supply chain, presenting concrete economic results in the form of energies, carbon credits and energy efficiency, which constitute economic revenues in the biogas economy. The demands of this economy are prerequisites for biogas generation to be implemented, such as projects, environmental licensing, regulation, technical training, and others. In addition, the biogas sector moves a supply chain which is necessary for a biogas plant to be installed and operated, such as engines, generators, controls, biodigesters, filters, pipes and several other parts, components, and processes of industrial origin, which drive commerce and specialized services. The economic results that come from the biogas economy are considered direct, such as electric, thermal, automotive energy, and digestate applied for self-consumption and for the sale of surplus. Indirectly, these are supplemented, for example, by obtaining emission credits for the reduction of greenhouse gas emissions and environmental protection through the reduction of organic pollution and energy efficiency (Fagerström et al., 2018). However, according to current trends, biogas production still depends on subsidies to attract investors and establish substantial scale. In the EU, there is still no specific policy on biogas, biogas solutions are addressed in various policy documents and directives related to renewable energy and bioenergy such as the Waste Directive (Directive 2018/851; Directive 2008/98/EC), EU Bioeconomy Strategy (Communication, 2018), Renewable Energy Directive (Directive 2018/2001; Directive 2009/28/EC) and Landfill Directive (Directive 1999/31/EC), but there were no major initiatives aimed directly at the development of biogas. Currently, some new strategies are emerging in which biogas solutions play a central role. As an example, the EU Methane Strategy (Communication, 2020) emphasizes the importance of biogas production to reduce methane emissions from the agriculture, waste and energy sectors, and the Energy System Integration Strategy (Communication, 2020) pronounces the mobilization of residual resources for energy production and replacement of fossil gas with renewable gases. However, despite the various incentive schemes to stimulate the economic development of the biogas sector like certification systems, feed-in tariffs, and investment support, the implementation process depends on several factors, including national market conditions, energy prices, tax regimes, economic policy, technical, institutional, sociocultural, and environmental factors (Nevzorova and Kutcherov, 2019). The advantage is that biogas solutions offer many benefits in different sectors of the economy and can contribute to a sustainable, circular economy. However, the many functions and externalities of biogas solutions also make the assessment more complex. (Gustafsson and Anderberg, 2022). 113 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS 3.2 Exemplary biogas frameworks of selected countries 3.2.1 Finland Finland is a prominent country in terms of the implementation of renewable energies (Lyytimäki et al., 2018), and also has a growing development in the biogas sector. However, due to the high costs regarding the conversion of biogas into biomethane, the country has a much lower biomethane production compared to biogas. Thus, in order to support biogas and biomethane production, Finland has developed new tariff and tax systems in which both biogas and biomethane are exempt from excise duties in all end-use applications (EBA 2021; Winquist et al., 2021). Nevertheless, the taxation of biomethane as a vehicular fuel is now under discussion. Currently, the country has a biogas and biomethane production potential of 10 TWh, with a theoretical potential of 25 TWh (Biokaasu2030) and with the purpose of implementing improvements and increasing production. The Finnish biogas sector set a target of reaching an annual production of 4 TWh by 2030, which was confirmed by the government in September 2021 (EBA, 2021). Due to the different biogas incentive strategies used by the government, the number of biogas plants increased considerably, reaching more than 38 plants between 2011 and 2021 (EBA, 2021). Presently, Finland has a total of 113 plants that use biomass from different sources such as landfills (34 plants), urban solid waste (26 plants), agriculture (25 plants), sewage treatment sludge (19 plants), and industrial biogas (9 plants) (figure 51). In addition, the urban organic waste and sewage sludge plants are responsible for presenting a greater amount of biogas production (EBA, 2021). However, despite the biomass from the agricultural sector presenting the highest generation potential, only a small part is treated in biogas plants (Huttunen et al., 2018). In 2020, Finland reached a production of 878 GWh of biogas, and it is estimated that approximately 60% of the biogas produced is used as thermal energy for heating or sold directly as raw biogas, while the other part is consumed in cogeneration. (EBA, 2021). As well as the biogas sector, despite slower growth, the biomethane sector presents a promising development due to the support policies that have been implemented in reFigure 51: Development of biogas production (GWh) (left); and development of the number of biogas plants (right), (Adapted from EBA, 2021). 1000 750 500 250 02018 2019 2011 Biogas production (GWh) Number of biogas plants 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2020 125 100 75 50 25 0 Sewage Other Organic municipal solid waste Land llAgricultural Industrial 114 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS cent years by the Finnish government, and mainly due to Finland’s national biogas action plan published in 2020, which describes all the measures and actions that will support the sector until 2024 (EBA, 2021). This can be observed between 2011 and 2021, when the number of biomethane plants increased in the country from 1 to 22, reaching a production of 109 GWh in 2020, with most biomethane plants using solid urban waste as a source of biomass (EBA, 2021). However, although the number increase of biomethane plants, currently, only 40% of plants are connected to the biogas grid. This is because only the southern part of the country has a gas network enabling direct injection into it. In general, due to the great political incentive to promote biogas for the use of transport fuel, the production of biomethane is growing. Thus, from 2022 it will be part of the national biofuel delivery obligation, giving a stable perspective to increase the production and use of biomethane by 2030 (EBA, 2021). Influence of the biogas sector on GHG emissions: According to the National Inventory Report, the total greenhouse gas emissions in 2020 was 47.8 million tonnes of carbon dioxide equivalents (Mt CO2 eq.). However, compared to the base year (1990) and 2019, emissions decreased by 33% (23.4 Mt) and 9% (5 Mt) respectively. Regarding the different sectors responsible for gas emissions in Finland, it is clear that the energy sector is considered the most significant source among them (table 21) with a variation in CO2 emissions according to the economic trend, the supply structure of energy, and weather conditions (Finland. 2020 National Inventory Report (NIR) | UNFCCC). However, over the years, due to some changes related to the level of electricity imported annually, the consumption of energy based on fossil fuels and the growth in the use of renewable energy, greenhouse gas emissions have decreased considerably (IEA, 2021). Table 21: Finnish greenhouse gas emissions and removals (Mt CO2 equivalent) (NIR, 2020). Sector Base year 1990 1995 2000 2005 2010 2011 2012 2013 2014 2015 Mt CO2 eq. Energy 53.4 53.4 55.3 53.7 53.7 60.2 52.8 47.5 48.1 44.3 40.6 Industrial processes and product use 5.3 5.3 4.9 5.2 5.6 4.8 4.7 4.6 4.4 4.2 4.4 Agriculture 7.5 7.5 6.7 6.6 6.5 6.7 6.5 6.4 6.5 6.6 6.6 Waste 4.7 4.7 4.6 3.8 2.8 2.6 2.5 2.4 2.3 2.2 2.1 Indirect CO2 emissions 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 TOTAL 71.2 71.2 71.8 70.2 69.9 75.7 67.9 62.4 62.8 58.6 55.0 115 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS In 2020, emissions from the energy sector fell by 12% to 34.3 Mt CO2 eq. from the previous year. Emissions were 36% below the 1990 level and 51% below the 2003 level (Finland. 2020 National Inventory Report (NIR) | UNFCCC) This way, renewable energy consumption has been growing steadily in the country as well as the development of the biogas sector, representing 44.6% of the total consumption of final energy. In 1990, the share of renewable energy was only 18%, showing that the increase in the use of renewable energy was the main reason for the decrease in gas emissions, despite the growth in total energy consumption (NIR, 2021). 3.2.2 Spain Spain started to support renewable energy in 1997, through the “General Electricity Law 54/1997”; however, the country still has a very strong dependence on energy imports, dominated by oil and Natural Gas (IEA, 2019). With the implementation of a national policy mechanism that provides payments and long-term contracts to renewable electricity producers called the Feed-in Tariff (FIT), biogas production has increased considerably in the country (EBA, 2021; Del Rio, 2008). Unfortunately, according to the EBA 2021 report, the records referring to the number increase of biogas plants do not reflect reality, as many of the plants that already existed in the past were not registered due to lack of data. However, the growth in the number of biogas plants in Spain is believed to have largely occurred between 2000 and 2004, mainly with the use of sewage and sanitary landfills for biogas production. In addition, after the enactment of Royal Decree-Law 1/2012, which eliminated all incentives aimed at generating electricity from renewable sources, the commissioning of new plants decreased dramatically (EBA, 2021; Flórez and Isabel, 2017). Figure 52 shows biogas production and the reported number of biogas plants in Spain during the last decade. Furthermore, the Royal Decree Law 413/2014 establishment, which regulates the calculation of feed-in tariffs (FIT) for the electricity production from biogas together with the hydrocarbons tax applied to biogas (0.65 €/ GJ), helped to further reduce activities in the biogas sector (Romero-Rubio and de Andrés Díaz, 2015; Ministerio de Industria, Sector 2016 2017 2018 2019 2020 Mt CO2 eq. Energy 43.3 40.9 42.1 38.9 34.3 Industrial processes and product use 4.7 4.6 4.6 4.4 4.1 Agriculture 6.7 6.6 6.5 6.6 6.6 Waste 2.0 1.9 1.8 1.8 1.7 Indirect CO2 emissions 0.1 0.1 0.1 0.1 0.1 TOTAL 57.9 55.1 56.2 52.8 47.8 116 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS Energía y Turismo, 2014). Currently, Spain has no incentive for new projects related to biogas production. However, there are still projects regarding to the biogas generation, which are not encouraged by the potential for selling biogas, but rather they are driven mainly by the needs of waste treatment and/or by the prospect of providing energy for private consumption (EBA, 2021). Today, Spain has 210 active biogas plants, the majority coming from sewage treatment plants, followed by agriculture, landfills and others. With this number of biogas plants, the country has a total biogas production capacity of 836 MW, corresponding to 318 MW of installed electrical capacity (EBA, 2021). Spain also has three biomethane plants, with two of them coming into operation in 2021 (EBA, 2021). In addition, due to the incentive associated with private initiatives (R&D) and partly funded by the EU (LIFE, H2O2 and CEF2), several pilot-scale projects are growing. Currently, the Spanish Ministry of Ecological Transition and Demographic Challenge (MITTERD) has approved the first Law on Climate Change and Energy Transition of Spain in the first half of 2021, where it commits the country to reduce emissions by 23% until 2030, comparing to the levels of 1990 (BOE, 2021). With this achievement, the government has the possibility to approve mechanisms to support renewable gas and its injection into the gas grid. The law also approves the guarantees registration of origin (GOs) for renewable gases injected into the natural gas network (BOE, 2021). This way, with more support and encouragement of the government, Spain can exceed 100 TWh per year if every type of raw material available is exploited, increasing biomethane production (EBA, 2021). Influence of the biogas sector on GHG emissions: According to the NIR report (2021), total greenhouse gas emissions (GHG) in Spain estimated for 2019 were 314,528.5 kilotons of CO2 equivalent (CO2eq). This represents a reduction of -5.6% in relation to the estimated emissions for the year 2018. Moreover, it constitutes +8.5% in relation to the base year 1990 and -28.9% in relation to the year 2005. In Spain, the gases that showed the highest emission rates in 2019 were CO2 (80%) and CH4 (12.2%) (tables 22 and 23). However, comparing the different sectors, all of them suffered a drop in GHG emissions, the main being related to electricity generation (-27.7%), to the commercial and residential sector (-8,6%), and the industrial sector (-1.2%) (NIR, 2021). In general, the evolution of CO2 and CH4 emissions in Spain over time responds to a four phase pattern fundamentally linked to variations in economic growth, population, and energy consumption in Spain since 1990 (IEA, 2021; NIR, 2021). However, the Figure 52: Development of biogas production (GWh) (left); and development of the number of biogas plants (right) in Spain (Adapted from EBA, 2021). 2011 2011 10,000 7,500 5,000 2,500 0 2012 2013 2014 2015 2016 2017 2018 2019 2020 2012 2013 2014 2015 2016 2017 2018 2019 2020 250 200 150 100 50 0 Number of biogas plants Biogas production (GWh) Sewage OtherTotal Land llAgricultural Unknown 117 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS decrease in emissions was mainly marked by the reduction in emissions from electricity production (-27.7%), due to the greater production of renewable energies, such as wind, photovoltaic, solar thermal and biomass, which increased +9.4%, +19.0% and +16.8% respectively, and the decrease in the use of coal in electricity production (-66.0%) (IEA, 2021; NIR, 2021). The energy sector accumulates a total GHG reduction of -6.6% (NIR, 2021). This represents a significant reduction in GHG emissions compared to the increase in the renewable energy use, showing that with incentives through public policies, and the appropriate treatment of organic solid waste significantly influences the biogas sector growth and the preservation of the planet. 3.2.3 Netherlands In the biogas market, the Netherlands is one of the countries that has stood out significantly throughout the world due to the implementation and upgrading of biogas plants to large-scale biomethane plants, presenting strategies well established by the government in partnership with the private sector (IEA, 2021; (Winquist et al., 2021). According to Kwant, 2003, the Netherlands had its first support for encouraging the renewable energies use in 1995 with the Green Funds certification. However, the biogas sector only showed significant growth in 2006, 3 years after the introduction of the “Environmental Quality of Electricity Production” Feed-in-Premium (FIP) which helped to support the implementation of new plants (EBA, 2021). In addition, with the goal of stimulating the production and cogeneration of renewable energy, the SDE (Stimuling Duurzame Energie) was created. This was later updated to the SDE+ and SDE++ which are currently being used to encourage sustainable growth and the circular economy through subsidies from the government (Netherlands, 2022; IEA, 2021). As a result, there was an increase in the number of biogas plants (260 in 2020) and biomethane plants (60 in 2020), showing the importance of implementing efficient laws driven by the government (EBA, 2021). Thus, the Netherlands produced 2,439 GWh of biogas in 2020, of which approximately 927 GWh of electricity was generated (figure 53) (EBA, 2021). Table 22: CO2 emissions: absolute values, temporal variation and ratios (NIR, 2021). Table 23: CH4 emissions: absolute values, temporal variation and ratios (NIR, 2021). 1990 2005 2010 2015 2018 2019 CO2 (kt CO2-eq) 231.194 369.681 283.873 271.694 269.713 251.498 Variation % vs. 1990 100% 159.9% 122.8% 117.5% 116.7% 108.8% CO2 / INV (CO2-eq) 79.7% 83.6% 79.3% 80.6% 80.9% 80.0% 1990 2005 2010 2015 2018 2019 CH4 (kt CO2-eq) 36.647 40.924 39.462 38.177 38.566 38.493 Variation % vs. 1990 100% 111.7% 107.7% 104.2% 105.2% 105.0% CH4 / INV (CO2-eq) 12.6% 9.3% 11.0% 11.3% 11.6% 12.2% 118 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS Due to government incentives for biogas production, the biomethane market has also grown significantly, making the Dutch market in this sector one of the pioneers in Europe (Winquist et al., 2021). The Netherlands currently has its own national renewable gas registry operated by Vertogas, the green subsidiary of Nerderlandse Gasunie, since 2009, and registration and certification via Vertogas is mandatory for renewable gas producers in the country (Verto-gas). Because of this incentive, the number of biomethane plants grew from 21 in 2016 to 60 plants in 2020, with agricultural substrates as the most used feedstock, followed by industrial waste. Furthermore, in 2020 the Netherlands reached a production of 2,166 GWh of biomethane (EBA, 2021). Today, it is known that out of the 60 biomethane plants in the Netherlands, 53 are connected to the gas grid, 4 plants produce Bio-CNG on site and have no connection to the grid, one produces Bio-LNG, and the others do not have record (IEA, 2021; EBA, 2021). However, it was pointed out in 2020 that the production of 402 GWh of biomethane which were used in transport representing practically 1/5 of the biomethane produced in the same year. Due to the large investment in the production of vehicular biofuel, seven new Bio-LNG plants are planned for the period 2021-2024 with a total capacity of 1.5 TWh per year (IEA, 2021; EBA, 2021). Influence of the biogas sector on GHG emissions: Another important factor related to the growth influence in the Dutch biogas sector is the significant reduction in greenhouse gases (GHG). According to NRI (2021), the energy sector is responsible for 83% of total GHG emissions in the Netherlands, being considered the most important source of GHG emissions. However, in 2019, total GHG emissions in the Netherlands were estimated at 180.7 Tg CO2-eq. This is 18% lower than the 220.5 Tg CO2-eq. reported for the base year in 1990 (figure 54). In the period 1990-2019, emissions mainly of carbon dioxide (CO2) and methane (CH4) decreased by 5.6%, and 45.9%, as well as nitrous oxide (N2O) and fluorinated gases (F-Gases) 54.9% and 75.9%, respectively. However, despite the use of fossil fuels has been decreasing and the amount of energy from renewable sources has been increasing, natural gas (44%) and oil (36%) are still the most important energy sources in the Netherlands (IEA, 2021). Renewable energies accounted for 181 PJs in 2019 (8.7% of total energy use in the Netherlands). Figure 53: Biogas production in the Netherlands: development of biogas production (GWh) (left); and development of the number of biogas plants (right) (Adapted from EBA, 2021). 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 3,500 3,000 2,500 2,000 1,500 1,000 500 00 50 100 150 200 250 300 Number of biogas plants Biogas production (GWh) Sewage OtherTotal Land ll Organic municipal solid wasteAgricultural Unknown 119 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS 3.2.4 Belgium Before 2001, the biogas sector in Belgium was stagnant and consisted mainly of plants operated in landfills or sewage treatment located in the region of Brussels, Wallonia and Flanders (EBA, 2021). However, after the introduction of the Green Certificate Scheme project by royal decree, the biogas sector grew considerably (Marchal et al., 2007). The Green Certificate determined the electricity share from renewable sources delivered to users connected to the distribution network (from 2% in 2002 to 6% in 2006), and a fine of 75 euros per certificate for non-compliance with the quota corresponding to 1 MWh (EU Comission, 2018). Between 2015 and 2018, due to technical reasons, several small biogas plants had to be closed in the Flanders region, but with the effort of the sector to solve this problem, the growth rate of plants grew again in 2019 (figure 55) (EBA, 2021). Currently, Flanders and Wallonia are the regions that most biogas produce in Belgium, with 134 and 55 plants in operation. In 2020, the country reached a production of 2700 GWh resulting in the generation of 1075 GWh of electricity (EBA, 2021). In addition to biogas plants, interest in biomethane production increased in the country after the installation of the first biomethane plant in the Flanders region, which helped to encourage policy changes across Belgium (IEA, 2021). In Wallonia, the system to support the use of green energy has changed to include the biomethane production, for which the company can receive a government subsidy by reducing the use of fossil fuels. Furthermore, Wallonia has taken on the “Plan wallon Energie Climat”, which is the Wallonia regional climate plan to help achieve its 2030 targets for biogas development. A voluntary registration system for renewable gases was also created by the association of gas distributors in Belgium (Gas. be) to help with the incentive in the sector (Biogaz). In September 2021, Belgium had the registration of the first Bio-CNG produced in Wallonia, however a second plant is being built in Flanders (EBA, 2021). Currently, six biomethane plants are in operation in Belgium, where two are in the Flanders, and four in the Wallonia region. Of these six plants, three are operating with agricultural substrates, one producing biomethane from sewage sludge, one using organic municipal solid waste and an industrial biomethane plant. It is estimated that today Belgium reaches about 17% of its biogas and biomethane production potential (EBA 2021). Influence of the biogas sector on GHG emissions: As well as the other countries, the biggest contribution to GHG emissions in Belgium comes from the energy sector. In 2019, the sector contributed 74% to total GHG Figure 54: Greenhouse gases: emission levels and trend, 1990-2019 (Adapted from NIR, 2021) 120 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS emissions. Mainly, CO2, CH4 and N2O emissions come from the energy sector. However, since 1990, emissions from this sector have declined by around 17% (NIR, 2021). Another largest source of greenhouse gases is agriculture with 8%, where emissions from this sector arise mainly from CH4 and N2O. However, since 1990, emissions have fallen by 19%. In 2019, the waste sector contributed around 1.2% where emissions arise from CO2, CH4 and N2O and originate from waste incineration, solid waste disposal on land and wastewater treatment. Emissions from this sector have steadily declined and are 69% below the 1990 level since 2019. According to the National Inventory Report from Belgium, the total net emissions have decreased by 18.8% since 1990. This way, it is evident that the emissions decrease in the energy, agricultural and waste sectors in Belgium is due in part to technological improvements, the shift from solid fuels like coal to gaseous fuels (natural gas) and the use of renewable energies, such as the biomass for biogas and biomethane production, as well as the proper treatment of urban solid waste using anaerobic digestion. 3.2.6 Greece Greece began to explore energy using biogas in the early 1980s. The main raw material was animal waste and food-processing industries waste. However, due to a lack of adequate legislation, financial incentives and public awareness, the projects ended up being forgotten (Markou et al., 2017). From 2000 to 2010 the country started to produce biogas again, which was dominated by plants that used sewage and sanitary landfills as a source of substrate. Between 2011 and 2020 there was an increase in biogas production from 543 GWh to 718 GWh, most of which came from Athens and Thessaloniki (EBA, 2021; IEA, 2021). With the introduction of Feed-in Tariffs (FiTs) in its Renewable Energy Act in 2010, fees for landfill-based biogas plants were up to 120 €/MWh, while for agriculture-based biogas plants they were up to 220 €/MWh, which resulted in an increase in agricultural biogas production (EBA, 2021; Markou et al., 2017). Later, with the increase of the maximum tariff rates (FiT) to 129 €/MWh (landfill biogas) and 225 €/MWh (agricultural-based biogas), Greece further increased the number of plants, mainly biogas plants using agricultural substrate, reaching a production of 392 GWh in 2020 related to this type of plant (Markou et al., 2017). Due to these changes, that year the country increased by 42 plants with a total production of 1,126 GWh (figure 56) of biogas, from which 428 GWh of elecFigure 55: Biogas production in the Belgium: development of biogas production (GWh) (left); and development of the number of biogas plants (right) (Adapted from EBA, 2021). 3,000 2,500 2,000 1,500 1,000 500 0 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 50 0 100 150 200 250 Biogas production (GWh) Number of biogas plants Sewage Other Organic municipal solid waste Land llAgricultural Industrial 121 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS tricity were generated (EBA, 2021). However, biomethane production has lagged due to lack of incentive through public policies. It is estimated that there will be no significant growth within the next 2 years in this area. In order to have and improvement in this sector, the government must encourage through tariffs (FIT), for example, with generous prices for production as well as facilitating the injection of biomethane into the gas network (Rutz, 2021). Influence of the biogas sector on GHG emissions: According to the National Inventory Report from Greece, in 2019 GHG emissions totaled 85.63 Mt CO2-eq, showing a reduction of 17.10% compared to 1990 levels. Carbon dioxide emissions represented 76.77% of total GHG emissions in 2019 and decreased by approximately 21.20% compared to 1990. Methane emissions accounted for 11.70% of total GHG emissions in 2019 and decreased by 9.29% compared to 1990. The energy sector represented 71.50% of total GHG emissions and decreased approximately 20.51% in 2019 compared to 1990 levels. This is due to the improvement in living standards, due to economic growth, the significant growth of the services sector and the introduction of natural gas into the Greek energy system for the period 1990 to 2007. Emissions from the waste sector have decreased by around 0.52% since 1990. Improved living standards have resulted in an increase in waste generation and therefore in emissions since 1990. However, the increase in recycling along with exploration of the biogas produced limits the increase in methane emissions (NIR, 2021). In this way, it is possible to realize the importance of the biogas sector development for the reduction of negative environmental impacts and the economic improvement of the country. 3.2.7 Germany Germany is considered the leading country both in Europe and in the world regarding production of biogas and biomethane (EBA, 2021). With around 11,000 biogas plants and 242 biomethane plants currently installed, it is responsible for more than half of the total primary energy produced from biogas. This fact is largely explained by the large state incentive about plantations for the energy production from biomass (WBA, 2021; Zlokower, 2019; EBA, 2021). With several laws encouraging the biogas and biomethane production, Germany was Figure 56: Biogas production in the Greece: development of biogas production (GWh) (left); and development of the number of biogas plants (right) (Adapted from EBA, 2021) 2020 Sewage Land ll Agricultural Biogas production (GWh) Number of biogas plants Other 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2011 2012 2013 2014 2015 2016 2017 2018 2019 0 250 500 750 1000 1250 80 60 40 20 0 128 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS The increased uptake of heat by the oceans led to its warming which in turn led to the expansion of the ocean water and the increase in the sea level. This is enhanced by the high transfer to the ocean of water currently stored on land, particularly from glaciers and ice sheets. There is also evidence that climate change led to changes in extremes. For example, it is very likely that the number of cold days and nights has decreased, and the number of warm days and nights has increased globally between 1951 and 2010. It is also likely that since 1950 the number of heavy precipitation events over land has increased in more regions than it has decreased. Finally, the global biogeochemical cycles have also suffered from changes due to effects related to climate system drivers. We will take about methane cycle in following headings. Opportunities for biogas industry: Even though the combustion of CH4 molecules produces the same number of CO2 molecules (CH4 + 2 O2 → CO2 + 2 H2O), the overall effect on global warming is smaller because methane effect as GHG is 28 times higher than CO2 (Hijazi et al., 2016). Besides, the carbon source of biogas or biomethane belongs to a fast domain of the carbon cycle because it is generated from biomass that obtained the C atoms and compounds from the atmosphere (plants) or the biosphere (animals). So, contrary to fossil fuels, there is no net increase in CO2 molecules in the atmosphere because the emitted CO2 molecules are balanced to molecules that were transferred from the atmosphere to the biosphere. 3.3.3 Artificial mineral fertilizers According to the Food and Agriculture Organization of the UN, the cultivated area worldwide was 4.8 billion hectares (ha) in 2019 (FAO, 2021). This agricultural land is used in two thirds as meadows and pastures, and one third as cropland. Although agricultural land decreased since 2000, on average there is a steady increase by 0.1% since 1961, with a peak in the 1990s. In spite of this, cropland area per capita decreased globally between 2000 and 2019 as population increased faster than cropland. This can be explained by an increase of 53% in primary crop production from 2000 to 2019 (9,4 billion tonnes in 2019). However, this was not due to increased global workforce employed in agriculture. Indeed, this figure decreased from 1050 million people in 2000 to 874 million in 2020. On contrary, the increased production is explained by an increased yield due to developments in irrigation, increase in pesticide use (+36% from 2000 to 2019) and increase in fertilizer use (FAO, 2021). The same report states that the use of inorganic fertilizers worldwide in 2019 was 190 million tons of nutrients, of which 57% corresponds to nitrogen (N), while the rest corresponds to phosphorus (P) and potassium (K). This number represents an increase of 40% (54 million tons) of the fertilizer use since 2000, which is explained by the increase in fertilizer use per crop area, from 91 kg/ha in 2000 to 122 kg/ha in 2019, of which the N:P:K ratio is 70:28:24. Europe accounts for the 12% of the total fertilizer use. The main nutrient is N, representing 60% of the European fertilizers. Fertilizer use per area in Europe is 80kg/ hectare, which represents a 12% increase since 2000 (FAO, 2021). The nitrogen budget of the soil represents the difference between the amount of N added with synthetic and organic fertilizers and the amount of N taken away by livestock and crop production during their biomass growth. While a negative budget indicates that more N is being taken from the soil than added with fertilizers, which impacts yield and the health of the soils, a very positive budget indicates that excess fertilizers are being used, which leads to environmental and human health issues. Nitrogen cycle: While the fourth element in the biosphere is nitrogen, dinitrogen gas (N2) is the main compound in the atmosphere. All 129 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS N-containing species except N2 are called reactive nitrogen species (Nr). These compounds are the N sources that support cellular metabolism and growth (Stein and Klotz, 2016). The N cycle is composed by Nr compounds together with N2, spanning nine different oxidation states of N atoms, which shows the complexity of this biochemical cycle. Roughly, there are three types of chemical reactions in the N cycle: N fixation, denitrification, and nitrification. N fixation is the reduction of N2 gas to ammonia (NH3) or ammonium (NH4+). These species are the only ones that can be assimilated by cells, which is when N interacts with C atoms making N organic compounds. Nitrifications are the reactions that oxidize NH3 or NH4+ to nitrite (NO2-) or nitrate (NO3-). Denitrification are the reactions that transforms NO2to less oxidized compounds, such as nitric oxide NO and nitrous oxide N2O, and finally to the dinitrate gas N2 (Stein and Klotz, 2016). In the primordial atmosphere, abiotic reactions dominated the nitrogen cycle and provided oxidized forms of N (NO2NO3-) and also reduced forms of N (NH3 NH4+), the lasts being essential for biological assimilation (Stein and Klotz, 2016). Before the industrial age, the interchange between N2 in the atmosphere to the rest of the Nr species was dominated by terrestrial and marine microorganisms via biological fixation of N2 to ammonia, and in a much smaller extent, by NOx production by lighting events (Fowler et al., 2013; Stocker et al., 2013). Since the industrial era and specially since the 20th century´s Green Revolution, three anthropogenic sources of Nr have greatly increased Nr creation: the Haber-Bosch process to create NH3 from atmospheric N2 and H2 for fertilizer and for industrial inputs; the cultivation of legumes and other crops capable of biological N fixation; and the combustion of fossil fuels, which converts atmospheric Nr from fossil fuels and N2 into nitrogen oxides (NOx) emitted into the atmosphere and redeposited on the land surface or in the oceans (Stocker et al., 2013; Stein and Klotz, 2016). In 2010, global nitrogen fixation from atmospheric N2 to terrestrial and marine ecosystems was 413 Tg N as estimated by Fowler et al. (2013). Of this Nr, 210 Tg N were fixed by anthropogenic causes, either intentionally or unintentionally. The main anthropogenic causes were fertilizer production through Haber-Bosch process (120 TgN) and agricultural fixation by legumes and grasses (60 TgN), with 30 TgN produced unintentionally by combustion. The main natural cause was the marine biological fixation of 140 TgN, followed by terrestrial biological fixation of 58 TgN and 5 TgN fixed by lightning (Fowler et al., 2013). The Nr that is denitrified to N2 is between 100-280 TgN/yr depending on different authors. The remaining N (difference between fixed and denitrified) is: emitted as NO or N2O from soils to the atmosphere (5 and 13 TgN/ yr respect.) or as N2O from the oceans to the atmosphere (5.5 TgN/yr); emitted as NH3 from terrestrial ecosystems and oceans to the atmosphere (60 and 9 TgN/yr respect.); wet and dry deposited as NOx to terrestrial surfaces and oceans (70 and 30 TgN/yr); and buried in oceans (20 TgN/yr) (Fowler et al., 2013). The concept of nitrogen cascade illustrates how an atom of Nr circulates along the biogeochemical pathway, with different effects to the environment and human health, until it is denitrified again to nonreactive N2. For example, a N cascade can start with the use of fertilizer, transforming non-reactive N from atmospheric N2 molecules to Nr (NH3) by Haber-Bosch process. From the 120 TgN fixed by Haber-Bosch, 80% is used as agricultural fertilizer and 20% as feedstock for industrial processes. Half the Nr fertilizer applied to agroecosystems is incorporated into crops harvested for human and livestock consumption. The other half is transferred to the atmosphere as NH3, NO, N2O or N2, or deposited as NOx to terrestrial surfaces and oceans (Fowler et al., 2013). Environmental effects: As stated above, when Nr creation rates are higher than the rates of removal by denitrification or assimilation by cell metabolism, the remaining Nr is 130 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS available in the atmosphere and in marine and terrestrial ecosystems and is either accumulated or dispersed by hydrologic (leaching and runoff) and atmospheric transport processes. (Galloway et al., 2003; Erisman et al., 2013). The environmental effects of the excess of Nr are varied. Emissions of N2O are the third GHG source in importance, together with CO2 and CH4; the global balance of the effect of Nr on terrestrial radiation, this is in global warming, is 0.24 Wm-2. N2O also degrades the stratospheric ozone layer. NOx species produce O3 (Ozone) and nitrate aerosols increasing smog and the haziness of the troposphere. NH3 leads to the production of nitrate aerosols. Together with S, Nr contributes to the acidification of lakes, rivers, and streams, followed by loss of biodiversity. Nr increases productivity in forests, grasslands, and waters, which can lead to eutrophication and reduced biodiversity. In coastal ecosystems, Nr is considered one of the biggest pollution problems, again leading to eutrophication, hypoxia, and loss of biodiversity. Increased soil Nr changes the rate of decomposition of soil organic matter and therefore affects the emission of CO2; it also affects plant productivity, either increasing it because of the greater availability of Nr, but also decreasing plant availability due to volatile organic compounds and Nox-mediated tropospheric O3. Increased NOx, aerosols, tropospheric O3, and nitrates in drinking water also negatively impact human health provoking respiratory illnesses, cancer and cardiac diseases (Galloway et al., 2003; Leach et al., 2012; Stocker et al., 2013). Opportunities for biogas industry: Nitrous oxide (N2O) emissions from fertilizer application and manure management account for 5% of total Europe GHG emissions (210 MtCO2eq) (Bacenetti et al., 2013). Although Europe is the only region where the N soil budget declined (by 5%) from 2000 and 2018, it is still considerably high, with a budget of 48kg/ ha. This budget was made from an input of 90.7kg/ha of N and an output of 42.9kg/ha. Of these 90.7 kg/ha of input, 51.4 kg/ha correspond to synthetic fertilizers, 24.8 kg/ha correspond to manure, and 0.7 and 4.8 kg/ha correspond to atmospheric deposition and biological fixation respectively (FAO, 2021). The organic agriculture differs from conventional agriculture in that it promotes the avoidance of synthetic fertilizers and pesticides. Fifteen of the top 20 countries with the highest ratio of organic:conventional agriculture is in Europe. This means that the region has emphasized the importance of organic agriculture. However, the percentage of organic agriculture from the total agricultural area is still low with a 3,4% in Europe (FAO, 2021). Digestate as a by-product from biogas production via AD can substitute synthetic fertilizer and be used as an organic fertilizer to boost the adoption of organic agriculture practices in Europe (Hijazi et al., 2016). However, there are many things that must be addressed before adopting digestate as a fertilizer. For example, the digestate from animal manure may contain high concentrations of elements such as copper and zinc (micro-nutrients supplemented in animal feed), and direct application of the digestate to agricultural land may result in phytotoxicity. Under such conditions, the digestate must be diluted with irrigation water before being applied to the field (Sawatdeenarunat et al., 2015). Also, open storage of digested residues is considered a hot spot of emissions in biogas systems. If the CH4 produced during digestate storage is not properly recovered, thus the released CH4 may worsen GHG emissions (Sawatdeenarunat et al., 2015; Hijazi et al., 2016). 3.3.4 Methane emissions Climate warming is a global phenomenon caused mainly by the abundance of wellmixed GHGs in the atmosphere. Among the GHGs, the most important is CO2. However, methane emissions (CH4) have a significant contribution to global warming. Methane, as well as CO2, are compounds that are integrated into the so-called carbon cycle above explained. At normal pressure and temperature conditions, CH4 is a gaseous compound, so most of it is located at the atmosphere. There, the concentration depends on the ratio among sources:sinks. 131 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS Methane cycle: In the methane cycle, there are much fewer sinks than sources. Indeed, 90% of atmospheric CH4 is eliminated by the oxidation with hydroxyl radicals (OH), mostly at the troposphere. The OH radicals are made after the photolysis of O3 in the presence of water vapour and are then eliminated by reaction with CO, CH4 and non-methane volatile organic compounds. The remaining methane is lost by photochemistry in the stratosphere amd perform reaction with atomic chlorine (Cl), atomic fluorine (Fl) and excited atomic oxygen (O(1D)), by photochemistry in the marine boundary layer and by oxidation in soils. This last methane sink is due to methanotrophic bacteria that can oxidize methane in unsaturated oxic soils and consume it as an energy source (Saunois et al., 2020). As stated above, there are many sources of CH4. They can be classified based on the process that creates CH4, or based on if the process is provoked by natural or anthropogenic causes. The processes that can create CH4 can be biogenic, geological, or pyrogenic. Biogenic methane is a result of the AD of organic matter performed by communities of microorganisms. Geological processes that emit CH4 are for example volcanic eruptions, gas reserves, perma-frost or methane hydrates accumulated mainly in the ocean that can emit CH4. Pyrogenic processes emit CH4 because of the incomplete combustion of fossil fuels or biomass (Stocker et al., 2013). Combining both of the classifications, Saunois et al. 2020 describe three anthropogenic categories: (1) agriculture and waste accumulation, (2) fossil fuels, and (3) burning of biomass and biofuels; and eight natural categories: (1) wetlands, (2) other inland water systems, (3) onshore and offshore geological sources, (4) termites, (5) wild animals, (6) oceanic sources, (7) terrestrial permafrost and hydrates, and (8) vegetation. The anthropogenic fossil fuel related methane emissions are due to the exploitation, transportation and usage of coal, oil and natural gas. The composition of natural gas is mostly methane. The extraction, transportation or use of gas can all contribute to methane emissions. The same accounts for shale gas, which emits in a similar amount as natural gas. Also, for example in coal mines, high quantities of CH4 are pumped out to maintain a CH4 percentage of 0,5% inside the mine. While in some countries this CH4 is used as fuel, in many countries it is still emitted to the atmosphere. However, the highest source of anthropogenic CH4 is agricultural and waste accumulation, mainly by livestock production, and followed by rice cultivation, landfills, and wastewater treatment. The livestock emissions are due to domestic ruminants whose digestive system, particularly the rumen, provides methanogenic archaea with stable temperatures (39ºC), optimum pH (6.5 - 7.6) and a constant plant matter flow. These archaea produce methane that is released mostly through the mouth of the animals (87%) or absorbed in the blood system. The rest of the CH4 is emitted through the rectum of ruminants. Manure decomposition is another important source of CH4 when manure is treated in liquid or slurry forms. If manure is deposited as a solid, aerobic conditions produce no CH4, but it produces N2O which has a larger warming impact than CH4. Flooded fields for rice cultivation are another important source of anthropogenic CH4. Finally, landfills and wastewater handling produce CH4 according mostly to the type of waste and to the amount of degradable organic material respectively. In some landfills, however, CH4 is used to generate heat instead of just ventilating it to the air. And for example, Europe counts with laws for the regulation of landfilling. Finally, biomass and biofuel CH4-emissions are 90% produced by anthropogenic causes compared to a 10% of natural fires (Stocker et al., 2013; Saunois et al., 2020). The natural sources of CH4 are even more varied, as stated above. Wetlands are ecosystems where soils or peats are saturated with water, or where surface inundation dominates the soil biogeochemistry. The anaerobic conditions lead to CH4 production. Other inland water systems can also produce CH4, such as lakes, ponds, reservoirs, streams, or rivers. Also, there are important onshore and offshore geological sources of CH4 emissions produced in the earth crust that reach the atmosphere through tectonic 132 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS faults and fractured rocks. Just like livestock, wild ruminants also emit CH4 when they degrade plant biomass in their digestive system through AD. There are even important emissions from insects that generate CH4 in 136 their guts. The most representative are the termites. There are also oceanic sources of CH4, both in coasts and in open ocean, for example, CH4 production from marine sediments, in situ production in the water column, leaks from geological marine seepage and also emission from the destabilization of marine hydrates. Marine CH4 hydrates are ice-like crystals formed under specific temperature and pressure conditions that comprise a potential important source of CH4, but without a significant relevance yet. The permafrost, which is frozen soil below 0ºC, can generate direct and indirect CH4 emissions. The direct ones are due to the release of CH4 contained in thawing permafrost, which was created before the last glacial era and was then trapped as temperatures went down. Indirect emissions rely on methanogenesis induced when the organic matter is released from thawing permafrost. This source is projected to be more important as warming climate thaws more permafrost. Finally, vegetation can produce small amounts of CH4, either through abiotic photochemical processes induced by stress, acting like straws releasing CH4 from soils, or providing suitable environments for methanogenic microorganisms, especially in the stems (Stocker et al., 2013; Saunois et al., 2020). Since beginning of industrial era in 1750 to 2011, atmospheric CH4 levels grew exponentially by a factor of 2.5, from 0.7ppm to 1.8ppm. The 5th intergovernmental panel for climate change stated, with a very high statistical confidence level, that this increase was due to anthropogenic causes. In fact, satellite measurements show that there are higher CH4 concentrations in places where anthropogenic influence is high, for example in urbanized or agricultural areas. This effect can also be detected downwind of urbanized or agricultural areas (Stocker et al., 2013). Actual measurements indicate that anthropogenic causes explain 50-65% of total emissions. And top-down approaches estimates that for the period 2008-2017, there was an atmospheric growth of 18.2 TgCH4/year (Stocker et al., 2013; Saunois et al., 2020). Environmental effects: Although exposure of hydrocarbon mixtures can have some adverse effects on humans, methane emissions are not considered relevant for direct health issues. However, the global warming potential of CH4 as a GHG is estimated to be 28-36 times higher than CO2 in a 100-year lapse. In this way, it is the second major component among anthropogenic GHG (Paolini et al., 2018). The RF associated to CH4 emissions are estimated to 0.97 Wm-2, while for the same period, total GHG and CO2 RF are 3 Wm-2 and 1.68 Wm-2 respectively. This reveals the considerably high effect of CH4 as GHG (Stocker et al., 2013). Almost a quarter of the RF associated to CH4 is not due to a direct consequence of CH4 itself, but due to its reaction with O2 that produces OH radicals and then water. Stratospheric water vapour also acts as a highly potent GHG (Stocker et al., 2013). In Europe, CH4 emissions from enteric fermentation, manure management and rice cultivation produces 5% of total Europe GHG emissions (195 MtCO2eq) (Bacenetti et al., 2013). Opportunities for biogas industry: Biogas production avoids the emission of CH4 to the atmosphere from organic matter going through AD. However, there remains spots of avoidable emissions. For example, the open storage of not-fully digested residues or flaring excess biogas (Hijazi et al., 2016). MICRO4BIOGAS plans to develop new designs of bioreactors for an improved AD of organic matter. 3.3.5 Waste management The European Union defines waste as “an object the holder discards, intends to discard or is required to discard”. There are many types of waste, each one with also many forms to be managed. Some types of waste are agricultural waste, chemical waste, con- 133 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS struction and demolition waste, food waste, green waste, wastewater, sewage, organic municipal waste, etc. Considering municipal waste, annually 2 billion tons of this waste is globally produced. And this is without considering the two other components of organic solid waste, which are agricultural waste and animal waste (Wainaina et al., 2020). The EU countries contribute with approximately 250 millions of tons of municipal solid waste, which leads to an estimate of 482 kg per capita This represents 10% of total waste generated in Europe (Bourguignon, 2018; Commission, 2020). The level of waste generation in EU countries positively correlates with the gross domestic product per capita, which means that the amount of waste depends significantly on economic development. However, the increasing waste production in EU countries does not correlate with an increase in the adoption of reducingor reusing-behaviours. This is different for the recycling behaviour, which correlated with the amount of waste generated, showing to be the only positive-behaviour adopted when people try to manage their waste production. So, as standard of living rises, it also does the level of consumption and the amount of waste, which has to be managed properly (Minelgaitė and Liobikienė, 2019). AD with different residues: Manure as feedstock is a very favourable substrate in terms of environmental impact. Biogas scenarios with manure as input material show lower GHG emissions than their reference systems. The transportation of manure can increase the impact of this feedstock (Hijazi et al., 2016). Animal manure shows more environmental benefits than wastes from food industries and households, with an indirect benefit because of avoided emissions of CH4 and N2O from traditional manure storage. Also, the manure digestion can be even improved for an increased methane yield if animals are fed with higher crude protein content food (Hijazi et al., 2016). Manure is a good substrate to be used in co-digestion treatments. For example, co-digestion of animal manure with lignocellulosic residues can be used to biologically pretreat energy crops and remove the amorphous hemicellulose fraction of the biomass structure (Sawatdeenarunat et al., 2015). The codigestion of energy crops with manure is beneficial for the AD because it increases the yield of biogas, stabilizes the organic matter and decreases the methane emission during storage (Bacenetti et al., 2013). Wastewater treatment is an energy intensive activity. The treatment of municipal waste water accounts for about 3% of global electricity consumption and 5% of global greenhouse gas emission. The biological wastewater treatment demands 20-30 kWh energy per person equivalent per year, but the energy recovery via AD of wastewater sludge is only about 15–18 kWh per person equivalent per year. So, utilizing the sludge for energy recovery, waste water treatment plan (WWTP) can achieve energy self-sufficiency only up to approx. 65%. In practice, a typical WWTP can currently offset 20–30% of the energy consumption and green-house gas emission (Nghiem et al., 2017). Traditionally, WWTPs have been designed with the aim of meeting discharge standards to receiving water bodies while waste produced during treatment is destined for landfill or incineration. An option to make WWTPs more convenient both economically and ecologically is the co-digestion of wastewater sludge with substrates with enough stock availability. For example, wastewater sludge may be co-digested with municipal organic waste. With this, WWTPs could achieve energy-neutrality and reduce the cost of municipal organic waste management while facilitating nutrient recycling. Other possible substrates for this may be food waste from urban areas, organic waste from food processing, market waste, dairy waste, etc. (Nghiem et al., 2017). Co-digestion is also a viable alternative for small WWTPs that require a specific biogas production threshold that can justify the maintenance cost and capital investment of biogas utilization equipment such as combined heat and power unit (Nghiem et al., 2017). It is estimated that in EU countries, 20% of the food produced is lost (Commission, 2020). 134 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS Food waste can be sorted and processed either onsite or delivered in a pre-treated liquid form. In addition to sorting and processing equipment, onsite processing also requires several auxiliary facilities including weighing bridge and even airlock passage to the receiving bay (Nghiem et al., 2017). Environmental effects: The collection of OSW (organic solid waste) is an environmental concern both in urban and rural areas. Technologies for OSW management are important to tackle environmental issues, to develop sustainable practices, and to forge circular economies. Currently, most of the OSW is treated to be converted in organic fertilizers or is deposited in landfills or incinerated (Wainaina et al., 2020). EU countries, just like the rest of the countries in the world, have produced increasing masses of wastes for many decades. The most practical solution for waste management in most of EU countries remains landfilling due to technical, economic, and legal reasons. This is true even with the European Union Directives on waste landfills that has introduced specific goals for reducing the volume of disposed waste, strict requirements of landfilling, and landfill sites (Vaverková, 2019). Landfilling is the worst option in environmental terms: it provokes contamination on soil and water with chemicals that leach from waste, animals can ingest micro plastics, and methane and other pollutants are released to the air. Also, in economic terms, landfilling leads to the loss of valuable resources that could be used for manufacturing other goods (Bourguignon, 2018). Waste disposal options by landfilling or incineration are expensive and not compatible with the concept of a circular economy. Ongoing leachate management and extensive monitoring for potential groundwater and air pollution are required during active landfill operation and even up to 50 years’ post-closure. On the other hand, extensive air pollution control is required for the waste incineration. Incineration of waste materials also results in significant greenhouse gas emission (Nghiem et al., 2017). Sharma et al. (2019) recognizes the positive environmental effects for the recycling (composting, vermicomposting and AD) of agricultural organic wastes and the use of the treated waste (e.g. digestate): improved soil texture and fertility, enhanced crop productivity, GHG´s mitigation and availability of alternatives to agrochemicals. Opportunities for biogas industry: Potentially, 60 million tons of OSW could be recycled by AD and composting technologies in Europe, which could save one million tons of nitrogen and 20 million tons of organic carbon that are currently lost through landfilling organic waste (Mayer et al., 2019). European countries in average recycle only 5% of the total OSW (Commission, 2020). If a higher portion of OSWs could be recycled and reused, it is estimated that approximately 30% of the chemical fertilizer applied to soil could be replaced (Paes et al., 2019). EU-28 nations produced nearly 25.38 million tons of wastes through all the activities including economic and households in 2016. It is estimated that global urban waste collection market can reach to an approx. income of U$S 410 billion. Only 25% of this waste is recycled, which means that there is a big economic opportunity (Wainaina et al., 2020). While policy efforts do not seem to succeed in changing personal habits regarding the reduction of waste, AD of municipal waste may be an important tool for the waste management. This is further relevant considering the close link among economic development and waste production. A study that analysed national and regional approaches linked to circular economy models in several European countries identified waste management strategies in almost every of these approaches, and concluded that waste management appears to be critical in this transition (Vanhamaki et al., 2019). In the final version of this roadmap, we will assess the extent to which AD of organic waste could tackle the currents issues re- 135 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS garding waste management. And we will also assess the efficiency of microbial communities developed by MICRO4BIOGAS for degrading different types of feedstock’s obtained by several sources of waste. 3.3.6 Economical aspects of biogas industry In the previous sections it has been analysed the environmental potential of biogas and digestate production by AD. It has been summarized in which way this technology can provide sustainable alternatives for waste treatment, energy production and land fertilizing. However, the environmental challenges also affect the global economy in several ways. New approaches also arise to meet not only environmental goals, but also increasing the profits and decreasing the loses in terms of money. This include the generation of other platform chemicals out of the process. Two important concepts that emerge as pillars of a sustainable economy are (1) circular economy and (2) bioeconomy/biorefineries. These concepts will be explained in the following sections. However, in summary they relate to the biogas production because AD can be a pivotal piece for the development of circular economies, valorising waste from different sources; and in the way that AD can be considered as a bioeconomy process, obtaining energy and fertilizer without appealing to fossil-derived resources. Circular economy: Circular economy is defined as an economic model where the value of products and materials that they contain are valued for as long as possible. It differentiates from the linear economy based on the “take-make-use-dispose” pattern, and considers waste as another resource that can be reintroduced to the production cycle (Bourguignon, 2018; Vanhamaki et al., 2019). The aim of circular economy is the maximization of resource efficiency and minimization of waste production, with the simultaneous effect of keeping resource consumption within planetary limits and reducing production costs. Obsolete products or waste materials are turned into resources for another purpose, thus closing loops and minimizing waste. By adopting circular economy strategies, several European countries are estimated to be able to reduce their national GHG emissions by 70%, while growing their workforce by 4%, decreasing their dependency from resource and energy import (Nghiem et al., 2017). Two important concepts related to circular economy and waste management are the extended producer responsibility (EPR) and the waste hierarchy (Bourguignon, 2018). The EPR states that producers have the responsibility for collecting used goods as well as sorting and treatment for recycling. The waste hierarchy implies that waste may be treated preferentially by prevention, then by preparing for re-use, then by recycling, then by energy recovery and finally disposal. Bioeconomy and biorefineries: The European Commission defines bioeconomy as “the production of renewable biological resources and the conversion of these resources and waste streams into value added products, such as food, feed, bio-based products and bioenergy”. While some people is strongly optimistic on the potential positive effects of this way of production, others argue that bioeconomy could perpetuate the linear economy model (Stegmann et al., 2020). In this way, the European Commission stated that bioeconomy should be coupled to sustainability and circular economy, coining the concept of circular bioeconomy. In turn, the bio-based circular economy concept refers specifically to the reuse and valorisation of organic wastes and residues (Wainaina et al., 2020). Other options of waste management such as landfilling, and incineration offer very limited possibilities for resource recovery. But in a bio-based circular economy, organic wastes represent a convenient source of resources in terms of energy and nutrients (Nghiem et al., 2017). A biorefinery system can be defined as the processing of non-fossil-based feedstock like biomass or organic waste through biologi- 136 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS cal or chemical unit operations into a wide range of marketable products such as energy, materials, and chemicals. Biorefineries can utilize while valorising a diverse range of renewable feedstock’s such as biomass from forestry, agriculture, aquaculture, and waste (Katakojwala and Mohan, 2021). The classification of biorefineries are based on (1) the type of feedstock (biomass, waste, gas, etc.), (2) the biocatalyst-organism (bacteria, yeast, fungi, algae, etc.), (3) the type of process/technology (fermentation, acidogenesis, methanogenesis, photosynthesis, thermo-chemical, catalysis, etc.), and (4) the targeted product(s) or generation (first, second, third and fourth) (Katakojwala and Mohan, 2021). The major challenges of biorefineries are its acceptance in the existing fossil-based market, the feedstock availability and composition, the volumes to meet market requirement, the resource recovery efficiency, the techno-economic feasibility, and the environmental sustainability. Taking a biorefinery to a commercial scale is an intricate issue because the biobased products face severe competition from petro-chemicals with respect to their market cost, and therefore, consistent processes and competitive products compared to conventional counter parts are needed. Biorefineries shouldn´t only be assessed for their economic competitiveness, but also for their environmental sustainability. That is why life-cycle assessments (LCAs) and techno-economic analysis (TEAs) can help with this (Katakojwala and Mohan, 2021). Although AD is a widely used renewable bioenergy production technology, biogas production alone may not sufficiently justify the capital and operating costs. AD can rather be integrated into a biorefinery process (Sawatdeenarunat et al., 2015). The main by-product obtained with biogas is biomethane. This is made by the upgrading of biogas with different techniques. Once converted in biomethane, this product may be used to feed the gas system or as vehicle fuel. The conversion of biogas to transport fuel has recently been implemented in several EU countries including Germany, Italy, and Sweden (Nghiem et al., 2017). The consortium of microorganisms in the AD of lignocellulosic matter can easily convert the sugar constituents of hemicellulose into CH4. Recalcitrant lignin and cellulosic fibres stays in the digested residue, which can be treated with hydrolytic enzymes to solubilize monomers that can be used as precursors in the production of diverse products ranging from bioenergy/biofuel (i.e., CH4, H2, ethanol and butanol) to organic acids (e.g., succinic acid) and biopolymers (e.g., bioplastic) (Sawatdeenarunat et al., 2015). Successful production of bio-plastic from biogas has been demonstrated at proof-of-concept experimental levels (Nghiem et al., 2017). Regarding to the effluent of lignocellulosic AD, it may be combined with micro-algae production as a nitrogen rich feedstock that may be utilized by algae as a nutrient source. Such produced algae could be utilized for biodiesel production and the residue after lipid harvest could be further fed into the digester for CH4 production (Sawatdeenarunat et al., 2015). Incentives: According to Raboni and Urbini (2014) the incentives granted to the producers and users of biogas and biomethane are in general of the following kinds: • Renewables Obligations that support renewable electricity generation by requiring suppliers to increase the generators production through the purchase of tradable Renewable Obligation Certificates. • Feed-in Tariffs which comprises an extra payment for renewable electricity producers, mainly addressed to support small generation plants. • Renewable Heat Incentive, which provides a guaranteed payment for heat used from biogas combustion and all the biomethane injected into the grid of natural gas. 137 CHAPTER 3 — ECONOMIC AND ECOLOGICAL ASPECTS • Renewable Transport Fuels Obligation which places an obligation on fuel suppliers to source 5% of their transport fuels from renewable sources by 2014. • Various incentives and tax breaks introduced in a few countries to promote the use of biomethane in transport. In the 2010s, there was an expansion of AD plants in Europe mostly thanks to the Renewable Energy Directive I (REDI) with feedin-tariffs scheme in 29 countries (Bacenetti et al., 2016). These incentives aim an economic feasibility of biogas plants at electricity markets (Saracevic et al., 2019). Quantitative analysis of economic impacts: The economic analysis of AD plants can be evaluated using different indexes. For example, the net present value (NPV) is operationalized by calculating the costs of the plants (negative cash flows) and the plant cash inflows (positive cash flows) for each period considered. The payback period is defined as the time span required to recover the cost of a project or an investment. And the internal rate of return is the discount rate that makes the net present value of an investment zero (Lovarelli et al., 2019). However, the technique most widely used to analyse economic yields of AD plants are the techno-economic analysis. These assessments can help to predict an economically competitive production process of bio-based products compared to petro-chemical derived products (Katakojwala and Mohan, 2021). During the techno-economic analysis, three areas of anaerobic digestion process are important: (1) unit operations with collection and transport of feed stocks, (2) to provide the treatment facility for production of biogas, and (3) upgrading the bio-energy for various applications as electricity and liquid methane for household cooking (Saracevic et al., 2019). Opportunities for biogas industry: The reduction of capital and operating costs of AD facilities is promoting the growth of the biogas industry. It is estimated that the cost of production of biogas will reduce 38% by the year 2050 compared to 2015. It the estimated that biogas production worldwide had an average growth rate of 11.2% reaching up to 58.7 billion Nm3 in 2017 (Wainaina et al., 2020). The biogas market is estimated to reach 50 billion in 2026 (Waste Management World 2017). It is estimated that only from the global urban waste collection market, the income through the AD of this waste can be around 410 billion incomes. However, currently only 25% is recycled (Wainaina et al., 2020). 3.3.7 Economical aspects of biogas industry 3.3.7.1 Life cycle assessment (LCA) The AD of organic matter is an activity that has many implications in environmental terms, as stated above. However, as any other economic activity, it is important to analyse thoroughly the possible impacts that it may provoke on the environment, either positive or negative. There are different ways to quantitatively analyse the environmental impact of the AD of organic matter. One of the most used is the so-called Life Cycle Assessment (LCA). LCAs systematically measure the effect of an economic activity based on different environmental parameters. 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This section aims to summarize the EUs decisions, directives, regulations, and other actions in order to provide an ideal overview of the political starting position for biogas and bio-methane in the EU. First, the basics of European policy, especially with regard to climate and revolution, are outlined. The European Green Deal has been at the center of European climate action since 2019. All further additions and procedures to the European Green Deal are presented chronologically below. In addition, sustainability provisions such as the Renewable Energy Directive (REDI & REDII), the Paris Agreement, REPowerEU, the promotion of the bioeconomy, the use of bio-waste for the production of biogas and biomethane are put into context. The individual measures of the European Union or other international associations are arranged chronologically and discussed. However, it must be clarified what decisions, directives, recommendations, opinions and regulations are and what significance they have for the individual countries. This is done in the following section. Author from the Technische Universität Dresden: Pascal Otto. 149 CHAPTER 4 — POLICY OF THE EUROPEAN UNION Furthermore, all important decisions, directives, regulations and measures that concern the European Union or have been adopted by it with regard to biogas and their significance for the biogas sector are presented. In doing so, the relevant document is examined directly for the keywords related to biogas, anaerobic digestion, organic waste and bioeconomy. The occurrence of the keywords is presented as a quotation and the content of what is written is discussed in detail. The last section deals directly with the implementation in the individual countries. The implementation of EU law at national level results in some differences between the selected countries. These are highlighted and discussed in this chapter. The selected countries are countries that strongly promote biogas or bio-methane production and thus achieve the expansion of these plants. The analysis and summary of EU decisions, directives, regulations and other measures regarding biogas production highlights a comprehensive plan to introduce, promote, and exploit anaerobic digestion plants within the European Union. Future structural plan for exploiting AD-plants throughout the European Union. Extensive revision of European regulations on: Waste disposal Circular economy Energy Agriculture 150 CHAPTER 4 — POLICY OF THE EUROPEAN UNION 4.1 European Union policy around biogas and bio-methane 4.1.1 Regulations of the European Union In European law, regulations are legal acts of the European Union and as such form part of secondary Union law. According to Art. 288 (2) of the TFEU, they are of general application and have a direct effect in the Member States. Regulations are addressed either to the European Union itself, to all Member States or to the citizens of all Member States. If the regulation is to affect only selected Member States or their citizens, it is adopted as a decision, i.e. it is directly binding, or as a directive, which must be transposed into national law. Regulations are adopted according to one of the procedures provided for in the Treaties, depending on the subject matter of the regulation. A distinction is made between legislative acts, Commission implementing regulations and delegated regulations. Regulations, which are legislative acts, are usually adopted jointly by the Council of the European Union and the European Parliament in accordance with the ordinary legislative procedure, on a proposal from the European Commission. They are published in the Official Journal of the European Union and are available online in the legal information system EUR-Lex. Regulations are numbered with the word “Regulation”, the year, a serial number and the symbol “EU”. In summary, regulations set the goals and the concrete means to achieve the results. They do not have to be transposed into national law, which is referred to as the “passthrough effect”, and modifications are protected by the “transposition prohibition”. 4.1.2 Directives of the European Union In European law, directives are legal acts of the European Union and as such part of secondary Union law. According to Art. 288(3) of the TFEU, they are binding and not directly applicable, which means that they must first be converted into national law by the Member States. The form in which they are then implemented by the individual member states depends on them, which leaves them a certain amount of leeway. However, there are authorizations and/or obligations that must be complied with and which provide the framework for the directives. Directives, which are legislative acts, are usually adopted jointly by the Council of the European Union and the European Parliament in accordance with the ordinary legislative procedure, on a proposal from the European Commission. They are published in the Official Journal of the European Union and are available online in the legal information system EUR-Lex. Directives are numbered with the word “Directive”, the year, a serial number and the symbol “EU”. In summary, directives set objectives that must be achieved without specifying the means to achieve that result. Member states are required to incorporate the directive’s provisions into their national laws within a specified timeframe. This allows for some flexibility in implementation while ensuring a common standard across the EU. 4.1.3 Decisions of the European Union In European law, decisions are legal acts of the European Union and as such form part of secondary Union law. Alongside regulations and directives, decisions are the third form of EU legislative instrument that have legally binding effects on each individual. Depending on the subject, decisions are adopted through different, treaty-bound procedures. According to Art. 288 TFEU, decisions are binding 151 CHAPTER 4 — POLICY OF THE EUROPEAN UNION in their entirety and may be addressed to specific addressees such as Member States, companies, individuals, or the general public. Decisions are used when a resolution is to be binding but there is no case for the adoption of regulations or directives. This is the case in the following examples Case-by-case decisions, Nominations, Common Foreign and Security Policy, International Agreements, Simplified Amendments to the Treaties. In this way, decisions are used to react to individual problems and abuses, such as mergers of companies, actions that endanger security, non-compliance with EU directives and regulations, the introduction of hazardous substances and the use of genetic engineering. Decisions are binding on those to whom they are addressed, whether an individual, a company, or a member state. They can be used to address specific cases or issues and are often used to ensure compliance with competition rules or to settle disputes. 4.1.4 Further Political Agreements The Treaty on European Union states that “… to ensure the proper functioning and development of the common market, the Commission (…) formulate recommendations or deliver opinions on matters dealt with in this Treaty, if it expressively so provides or if the Commission considers it necessary” (European Union 1993). Recommendations: These are part of the secondary legislation of the European Union and thus legal acts of the Union. Recommendations are usually issued by the European Commission. They are defined in Art. 288 TFEU as non-legally binding acts, although some legal consequences are attached to their adoption. Member States are free to decide whether to implement the recommendations. If they implement a recommendation, it is part of the legal order of the Member State, which means that the resulting rights can only be enforced before the courts of the Member State, not before the European Court of Justice. This makes the recommendation to an instrument of indirect action. Similar to the directive, it aims at the development of legislation in the member states, whereby recommendations are not binding. Opinions: These are part of the secondary legislation of the European Union and thus legal acts of the Union. Opinions are issued by the institutions and other bodies of the Union. They are defined in Art. 288 TFEU as non-legally binding acts, although legal consequences are attached to their delivery. Two important areas in which the submission of opinions is provided for are mentioned below. Procedures for the adoption of legislative acts and infringement procedures. Framework Regulations: These are regulations that set out broad objectives and establish the framework for further detailed measures to be adopted at a later stage. Member states then implement these measures based on the framework provided. White Papers and Green Papers: These are policy documents released by the European Commission. White Papers outline proposals for future legislation, while Green Papers are consultation documents that initiate discussions on particular topics. These papers help gather input from member states, stakeholders, and the public before formal regulations are developed. Impact Assessments: Before introducing new regulations, the EU often conducts impact assessments to evaluate the potential economic, social, and environmental consequences of proposed policies. This helps ensure that regulations are effective and balanced. Public Consultations: The EU frequently seeks input from citizens, businesses, and organizations through public consultations. These consultations help shape regulations by incorporating diverse perspectives and expertise. Cooperation and Negotiation: The EU institutions, including the European Commission, the European Parliament, and the Council of the European Union, work togeth- 152 CHAPTER 4 — POLICY OF THE EUROPEAN UNION er to negotiate, draft, and adopt regulations. This involves discussions, amendments, and compromises to create regulations that reflect the interests of member states and the EU as a whole. Enforcement Mechanisms: The EU also establishes mechanisms to ensure that member states comply with the regulations. This includes the European Court of Justice, which can adjudicate disputes and rule on cases of non-compliance. Action plans: An action plan addresses a wide range of issues, from environmental sustainability and energy transition to digital transformation and economic development. Each plan includes a set of initiatives, targets, and measures designed to drive progress and achieve desired outcomes within the respective areas. 4.1.5 Directive 2008/98/ EC: The Waste Framework Directive (WFD) // November 2008 The Waste Framework Directive (WFD) provides the member states with guidelines for political measures for the transition to a circular economy and in particular for their waste legislation. 1. Waste Hierarchy: The WFD introduces a waste hierarchy that prioritizes waste prevention, followed by preparing for reuse, recycling, other forms of recovery such as energy recovery, and safe disposal as a last resort. 2. Extended Producer Responsibility (EPR): The directive emphasizes the principle of EPR, where producers are held responsible for the environmental impacts of their products throughout their lifecycle, including post-consumer waste management. 3. Waste Minimization and Prevention: The WFD encourages the prevention of waste generation and promotes measures to minimize the environmental impact of waste, including the reduction of hazardous substances in products. 4. Recycling Targets: The directive sets a target for recycling and recovery rates for different types of waste materials, aiming to increase the recycling and reclamation of valuable resources from waste streams. 5. End-of-Waste Criteria: The WFD establishes criteria that define when certain waste materials cease to be considered waste and become secondary raw materials, facilitating their use in various applications. 6. Landfill Restrictions: The directive restricts the disposal of certain types of waste in landfills to prevent environmental contamination and promote more sustainable waste management practices. 7. Reporting and Information: Member states are required to provide regular reports on their waste management activities and progress toward meeting the directive’s goals. Transparency and access to information are important elements of the WFD. 8. Illegal Waste Shipment: The directive addresses the issue of illegal waste shipments between EU member states and non-EU countries, aiming to prevent environmental harm and ensure proper waste treatment. 9. Cooperation and Coordination: The WFD encourages cooperation among member states and coordination of waste management practices to achieve a more consistent and efficient waste prevention and management approach. The aim is to protect the environment and human health by managing waste in the EU. It prioritizes waste prevention, promotes recycling and recovery, establishes responsibilities for various stakeholders, and seeks to minimize the environmental impact of waste generation and disposal, thus ensuring the longterm competitiveness of the EU. This directive is still active (The European Parliament and 153 CHAPTER 4 — POLICY OF THE EUROPEAN UNION Council of the European Union 2008). 4.1.6 Directive 2009/28/EC: Renewable Energy Directive I (REDI) // June 2009 Directive 2009/28/EC, also known as the Renewable Energy Directive I (REDI), was adopted by the European Union in June 2009. The aim of the directive is to promote the use of renewable energy sources and to increase the share of renewable energy in the EU’s energy mix. The main points of the directive include: 1. Renewable energy targets: REDI sets binding targets for each EU Member State to increase the share of renewable energy in its total energy consumption. The overall EU target is to achieve a 20% share of renewable energy by 2020. 2. National action plans: Member States are required to develop national renewable energy action plans setting out how they intend to achieve their individual targets. These plans include measures to promote different renewable energy sources such as wind, solar, biomass and hydropower. 3. Sustainability criteria: The policy sets sustainability criteria for biofuels and bio liquids to ensure that their production and use have a positive impact on reducing greenhouse gas emissions and protecting biodiversity. 4. Guarantees of origin: REDI introduces a system of guarantees of origin to track and verify the origin of renewable energy sources. This helps consumers make informed choices about the sources of their energy consumption. 5. Cross-border cooperation: The Directive promotes cross-border cooperation between Member States to develop joint projects and initiatives in the field of renewable energy production and transmission. 6. Promotion of electricity and heating/cooling from renewable energy: REDI promotes the use of renewable energy in electricity generation and heating and cooling systems by setting targets and providing support mechanisms for these sectors. 7. Integration of renewable energy into the electricity grid: The directive emphasises the need to integrate renewable energy sources into the existing energy infrastructure and grids to ensure stability and reliability. 8. Reporting and monitoring: Member States are required to report regularly on their progress towards their renewable energy targets to promote transparency and accountability. In summary, Directive 2009/28/EC, the Renewable Energy Directive I (REDI), establishes a framework for promoting the use of renewable energy sources in the European Union. It sets binding targets for Member States, outlines sustainability criteria for biofuels and promotes cooperation to achieve a more sustainable and diversified energy mix. This directive was superseded by RED II (The European Parliament and of the Council 2009). 4.1.7 Sustainable Development Goals (SDGs) // January 2016 The Sustainable Development Goals (SDGs, figure 59) are a set of 17 global goals set by the United Nations in September 2015 as part of the 2030 Agenda for Sustainable Development and agreed by 197 countries. These include (1) No Poverty, (2) Zero Hunger, (3) Good Health and Well-being, (4) Quality Education, (5) Gender Equality, (6) Clean Water and Sanitation, (7) Affordable and Clean Energy, (8) Decent Work and Economic Growth, (9) Industry, Innovation and Infrastructure, (10) Reduced Inequality, (11) Sustainable Cities and Communities, (12) Responsible Consumption and Production, (13) Climate Action, (14) Life Below Water, (15) Life On Land, (16) Peace, Justice, and Strong Institutions, (17) Partnerships for the Goals. 256 CHAPTER 6 — MANIPULABILITY OF ANAEROBIC MICROBIOMES and 16S rRNA sequences, it is also a sensible option to include computer models of population dynamics in evaluations of sequence data. However, hardly any research results have been published in relation to biogas production. Two articles were found that presented online tools that allow the implementation of 16S-rRNA gene amplicon sequences in the so-called Lotka-Volterra model (Shaw et al., 2016; Kuntal et al., 2019). The Lotka-Volterra model is a model that predicts predator-prey interactions and is generally known in respect to the “rabbit-wolf graph”. Considering the fact that the tools for applying the Lotka-Volterra model to sequence data have been available since 2016, it is surprising that this tool has so far hardly been used in the biogas sector. To close this gap, Schwan et al., 2020 published an article, which can now be regarded as one of the first articles that apply the Lotka-Volterra model in relation to biogas production. The special feature of this model is that the formula contains a microbial interaction coefficient, which indicates whether microorganisms behave more competitively or more symbiotically. At this point it should be emphasized again that works like that of De Vrieze et al., or Orellana et al. focus on statistical interpretations of empirical process fluctuations. One can ask now whether all these interaction studies based on pure cultures or high-throughput characterizations like the one from De Vrieze et al. allow us now to manipulate anaerobic microbiomes in a more targeted manner? At least they give us a glimpse. In previous chapters of the present thesis, various influencing factors were discussed, which were analyzed with high-throughput data or pure-culture approaches as well. The combined interpretation of all these studies might enable a new function of anaerobic microbiomes: The combination of conductive particles, light, low viscosity, low pH, high concentrations of COD, VFAs, salt and ammonia could give rise to formation of phototrophic, electroactive high-performance microbiomes, although this possibility has not yet been proven. Another approach, which is also somewhat more targeted than the mere variation of process parameters, is the addition of microorganisms enriched separately in pure culture (bio-augmentation). In this relation, an interesting approach was published by Kovács et al. (2013). As hydrogenotrophic archaea are important for methane formation, the hydrogen forming bacteria Caldicellulosiruptor saccharolyticus and Enterobacter cloacae were added to lab-scale digester experiments. Kovács et al. observed a higher biogas productivity, which is surprising as higher levels of hydrogen might also inhibit syntrophic relations. Although in case of the hydrogen forming bacterium Enterobacter cloacae a long-lasting effect with a stable cell number was observed (Ács et al., 2015), bioaugmentation is a process with an uncertain outcome. A good example of this is a work by Strang et al. (2017), which enriched a cellulose consortium from a biogas process that increased methane formation by 22-24% in augmented biogas reactors. After the 4 most common species (Thermoanaerobacterium thermosaccharolyticum, Caldanaerobacter subterraneus, Thermoanaerobacter pseudethanolicus and Clostridium cellulolyticum) were isolated and enriched, the positive effects in augmented reactors were reduced by half compared to the effects by the original consortium. This shows that the metabolic complexity is still beyond our understanding. The work of Strang et al. also shows that even microorganisms that occur in very small quantities can make an important contribution. Another important influencing factor is the competitiveness of the microorganisms involved. As explained earlier, it is a probable scenario that added microorganisms are repressed by the native microorganisms of the underlying microbiome. In order to overcome this hurdle, it is of particular importance that selection pressures are used which, in particular, favor the added type of microorganisms. The following examples can be given here: Strang et al. (2017) added lignocellulolytic bacteria using lignocellulolytic biomass as selection pressure, as such biomass degrades only very slowly. In addition to lignocellulolytic biomass, fats are also poorly degradable. This can therefore also be seen as a selection pressure, which [Document text truncated for crawler view.]