Prospective environmental and economic assessment of solar-assisted thermal energy recovery from wastewater through a sequencing batch biofilter granular reactor

Prospective environmental and economic assessment of solar-assisted thermal energy recovery from wastewater through a sequencing batch biofilter granular reactor Ivan Muñoz a, *, Francisco Portillo b, c, Sabina Rosiek b, c, Francisco J. Batlles b, c, Javier Martínez-Del-Río d, Iñaki Acasuso e, Valentina Piergrossi f, Marco De Sanctis f, Silvia Chimienti f, Claudio Di Iaconi f a 2.-0 LCA Consultants, Rendsburggade 14, Room 2.345, 9000, Aalborg, Denmark b Department of Chemistry and Physics, University of Almería, 04120, Almería, Spain c CIESOL, Joint Centre of the University of Almería -CIEMAT, 04120, Almería, Spain d Department of Economics and Business, University of Almería, 04120, Almería, Spain e Hedera Helix I&B, Avda. Putxeta 1, Abanto y Zierbena, 48540, Bizkaia, Spain f CNR e Water Research Institute, Via F. De Blasio 5, 70132, Bari, Italy * Corresponding author. E-mail address: [email protected] (I. Muñoz). Abstract The integration of an off-grid solar-assisted heat pump (SHP) and a sequencing batch biofilter granular reactor (SBBGR) for thermal energy recovery from wastewater was assessed by means of a prospective life cycle assessment (LCA) and life cycle costing (LCC), by theoretically scaling up a pilot installation in Bari, Italy, to a full-scale unit designed for 5000 person-equivalents. The LCA and LCC included all activities in the life cycle of the SHP and wastewater treatment plant (WWTP), namely construction, operation and end-of-life. The thermal energy produced by the SHP was assessed as supplying heating and cooling for an air-conditioning system, displacing a conventional air-source heat pump powered by electricity from the grid. This integrated system was compared to a reference situation where wastewater is treated in a conventional WWTP applying activated sludge with no thermal energy recovery system, showing clear environmental benefits in all impact indicators, such as a 42% reduction in greenhouse-gas emissions and a cost reduction of 53%. Several sensitivity analyses confirmed these findings, with the exception of the price rebound effect, which showed that the lower cost of the integrated system could lead to overturning the environmental benefits. As a limitation of the study, the distribution of the supplied air-conditioning to meet a demand off-site the WWTP premises, such as in residential buildings or hotels, was not included. Therefore, our results constitute only a preliminary positive outcome that should be validated in a real-life application. Keywords: Life cycle assessment (LCA) Life cycle costing (LCC) Wastewater-source heat pump Solarassisted heat pump Thermal energy recovery 1. Introduction Wastewater treatment constitutes a substantial energy consumer and source of greenhouse gas (GHG) emissions. According to Cao (2011), the provision of wastewater treatment services accounts for around 1% of a country's electricity consumption. Also, wastewater is responsible for 4% and 2% of methane and nitrous oxide emissions, respectively (USEPA, 2013). At the same time, urban wastewater constitutes a promising low-grade source of thermal energy, as it is produced steadily, in high volumes and with small temperature variations; it is typically warmer than the environment during winter, but colder in summer, making it suitable for heating and cooling purposes through heat pump systems (CWWA, 2009; Frijns et al., 2013; Meggers and Leibundgut, 2011). Tapping this resource for heating, ventilation, and air conditioning (HVAC) in buildings is of particular interest, given that buildings account for around 40% of energy consumption (EIA, 2018; Settino et al., 2018) and that energy consumed by HVAC systems is responsible of the largest share of a building's environmental impact (Ochoa et al., 2005; Aliane et at. 2016; Ge et al., 2018). Centralized systems for thermal energy recovery from wastewater have been in place since the 1980's in Germany, Switzerland and Scandinavia (DWA, 2009), mainly focused on recovery from treated effluents of WWTPs, whereas recovery within sewers or wastewater treatment basins remains elusive, mainly due to biofouling of heat exchangers in contact with untreated wastewater (Chao et al., 2012; Liu et al., 2014). In this context, the novel sequencing batch biofilter granular reactor (SBBGR) constitutes a potential solution to this problem. The SBBGR is an attachedbiomass system operating with a complete separation of the biomass from the liquid phase, thus allowing for a reactor zone free of suspended solids where a heat exchanger can be placed (Di Iaconi et al., 2010, 2017; De Sanctis et al., 2017). Due to some additional features of SBBGR biomass (i.e. long sludge age and concentration) it not only prevents biofouling of heat exchangers in the bioreactor, but also favours thermal energy recovery by maximising the conversion of chemical energy stored in water pollution into biochemical heat, as well as by withstanding wastewater temperature changes (Piergrossi et al., 2018). Coupling of a SBBGR with a solar-assisted heat pump (SHP) has been recently demonstrated at pilotscale in Bari, Italy (Piergrossi et al., 2018). While the environmental performance of these two technologies have been separately assessed in the past by means of life cycle assessment (LCA) (Di Iaconi et al., 2017; Batlles et al., 2010), the resulting benefits or otherwise from their integration remain to be quantified. Furthermore, besides environmental sustainability, economic feasibility is also required for a new technology to find its way into the market and for this reason an evaluation of life cycle costs (LCC) constitutes an equally necessary exercise. In this article we present the results of a prospective LCA and LCC applied to the integration of a SBBGR and SHP for HVAC purposes, as described in Piergrossi et al. (2018), considering its implementation at a larger scale, namely in a hypothetical WWTP designed for 5000 person-equivalents (PE) in a Mediterranean context. 2. Material and methods 2.1. Pilot plant description The basis for the prospective assessment is a pilot plant installed in Bari, where an existing SBBGR unit was upgraded with a SHP in September 2016 (Fig. 1). Below we provide a brief description of the installation. For detailed descriptions on the SBBGR the reader is referred to Piergrossi et al. (2018). Fig. 1. The THERBIOR pilot plant. The SBBGR prototype mainly consists of two columns with a total volume of 300 L. One of such column constitutes the biofilter (3200 mm in height and 220 mm in diameter), which is packed with plastic media where the biomass develops and the biological degradation takes place. The second compartment is known as the aerator (273 mm in diameter and 3200 mm in height). The reactor is operated as a sequencing-fed batch process, consisting of three stages: anaerobic filling, aerobic recirculation between the two columns and eventually a withdrawal phase. The treatment cycle takes 6 h in total, after which the effluent can be discharged without the need for a settling tank. During the recirculation phase, air is provided by means of discontinuous 5-min blowing periods starting every 25 min. In addition, pure oxygen is continuously provided in order to maintain a dissolved oxygen concentration in the aerator within the 15e20 mg/L range. The influent fed to the reactor is real urban wastewater from the local sewer in Bari. The SHP is a solar-assisted fully off-grid energy system capable of producing, recovering and storing thermal energy. It consists of the main following components: • A heat pump coupled to a titanium tubular heat exchanger submersed in the SBBGR unit, particularly in the aerator column. • A reciprocating compressor with 4810 W refrigeration capacity. • Two short-term thermal latent energy hot and cold storage units filled with phase-change materials (PCM), operating at 53 ◦C and 0 ◦C, respectively and with a capacity of 0.3 m3 and 0.5 m3, respectively. • Energy dissipation devices (condenser and evaporator). • A rooftop photovoltaic plant with 5.1 kWp capacity and 32 m2 module area, powering the entire SHP system. The SHP is fully off-grid, with the exception of control and HVAC distribution devices, which are supplied from the grid in order to ensure 24-h operation. From a hydraulic point of view, the system is composed of a hot water line and a cold water line. When sufficient solar energy is available, the SHP produces and stores heat in the form of PCM. Conversely, the cold water line produces and stores chilled water in the cold PCM tank. The SHP is used in a lowtemperature air-conditioning system supplying a so-called experimental test laboratory (ETL), a room that simulates demand onsite. 2.2. LCA and LCC methods LCA was carried out with the ISO 14040 and 14044 standards as main methodological guidelines (ISO, 2006a; 2006b), and consequential modelling was used in the inventory analysis, as defined in Weidema (2003, 2009). The software used to model the life cycle was SimaPro version 8.5 (Pre´, 2016). Environmental LCC was carried out as described in Hunkeler et al. (2008), that is, accounting for internal costs associated with the life cycle of the product that are covered by any one or more of the actors in the product life cycle. Both LCA and LCC were aligned in terms of functional unit, system boundaries, etc. As highlighted in the article title, the overall assessment is prospective in the sense that 1) consequential modelling is used in the LCA and 2) the assessment takes a step forward from its actual pilot scale to a commercial scale model. These two methodological aspects allow for a more realistic depiction of a potential deployment of this technology in the market. 2.3. Goal and scenarios assessed The goal was to assess the expected life-cycle environmental and economic performance of coupling a SHP for thermal energy recovery from urban wastewater with a SBBGR unit, with the aim of providing cooling/heating for residential buildings in a touristic Mediterranean region. We call this the THERBIOR scenario. This is compared to a reference situation for wastewater treatment, in which urban wastewater is treated in a conventional WWTP applying activated sludge (AS) and with no thermal energy recovery system. Fig. 2 shows a conceptual flow diagram for these two scenarios and the respective activities included in the study. Fig. 2. Flow diagram for the two assessed scenarios: reference (left) and THERBIOR (right). AS: activated sludge; SHP: solarassisted heat pump 2.4. Geographical scope The case study considers the implementation of the proposed concept in a hypothetical WWTP located in Bari, where the pilot plant has been installed. This geographical setting is assumed to represent a typical Mediterranean location. The average urban wastewater production in this region is approximately 0.15 m3 per person-equivalent (PE) and day. 2.5. Technological scope: wastewater treatment and sludge disposal We consider the deployment of this technology in a WWTP designed for treating 5000 PE, or 750 m3/day. The decision to consider a higher scale than the actual experimental one is based on the fact that research scales such as laband pilot-scales are not suitable for meaningful environmental (and economic) assessments (Munñoz et al., 2015; Gavankar et al., 2015; Piccinno et al.,2016). An important aspect linked to the size of the WWTP is the fact that touristic areas are commonly subject to substantial population fluctuations, with peaks in the summer season leading to similar fluctuations in terms of wastewater production. In order to cope with these peaks, WWTPs need to be designed for larger capacities, resulting in suboptimal use of the infrastructure for the most part of the year. In our study we assume that the WWTP is oversized by a factor of 50%. This results in an actual capacity of 3333 PE, which equals 500 m3/day or 182,500 m3/year. In both scenarios the WWTPs need to discharge a treated effluent complying with the European Directive 91/271/EEC concerning urban wastewater treatment. The AS plant considered as reference includes the following unit operations: mechanical pretreatment, primary settling, biological treatment with nitrogen removal, secondary settling, sand filtration and disinfection. Excess sludge is subject to aerobic estabilization and dewatering. The SBBGR WWTP involves substantially less unit operations, namely only mechanical pre-treatment and biological treatment in the SBBGR unit (aerator-biofilter). Excess sludge does not require a stabilization process in this case, but only dewatering. Concerning nutrient (nitrogen and phosphorus) removal, this is in principle not required by current legislation given the size of the assessed WWTPs, however for nitrogen we see in the European Environment Agency's WATERBASE database (EEA, 2016) that 37% of plants in Italy designed for 5000 PE do feature nitrogen removal and for this reason we decided to include it for the reference AS plant. On the other hand, chemical phosphorus removal is less common in these plants, with only 13% of them featuring this operation; for this reason, it is not included in our model. In the case of the SBBGR, both nitrogen and (biological) phosphorus removal are considered, based on previous experimental results with this pilot plant (De Sanctis et al., 2017). Sludge disposal in the two scenarios is assumed to be through incineration. This is based on an analysis of sludge disposal trends in Europe, using Eurostat data (see SM). This analysis shows that from four main disposal options (use in agriculture, landfilling, incineration and composting), only incineration and composting show a growth trend over time, with this trend being more than three times higher for incineration compared to composting. 2.6. Technological scope: solar-assisted heat pump Similar size considerations apply to the SHP as those discussed for the WWTPs. The SHP installation at the pilot plant, coupled to a SBBGR treating 269 L wastewater/day, does not properly reflect the expected size and number of components for coupling with a WWTP treating 500 m3/day. In order to overcome this limitation, we carried out a theoretical scale-up of the pilot SHP, by redimensioning the individual equipment and/or increasing the number of units. This process was done in detail for the most important components such as photovoltaic plant, tanks, compressor, etc. Also, equipment considered to be part of the pilot plant only for research purposes was discarded in the full-scale design. The resulting SHP model had a capacity of 244 kW heating and 160 kW cooling, powered by 1225 m2 of photovoltaic modules. 2.7. Technological scope: substituted grid power-driven heat pump The heating and cooling service fulfilled by the implementation of the proposed concept is assumed to substitute heating and cooling supplied by an air-source heat pump driven by electricity from the grid, having the same heating and cooling capacity as the SHP. This is in accordance with a similar study (Batlles et al., 2010) evaluating a solar HVAC system in Almería, Spain, with very similar climatic conditions to Bari. The choice of a single heat pump over e.g. a combined heat pump (for cooling) and a boiler (for heating) can be justified based on the fact that given the relatively mild winters in the Mediterranean region, the installation of two different systems is not justified, as the heat pump can deliver both functions. A survey in Spain (IDAE, 2016) shows that in the Mediterranean regions of the country heat pumps are more commonly used for residential and commercial uses compared to the Northern Atlantic regions, with colder climate, where lower cooling and higher heating requirements are found. As for the type of heat pump, this same survey in Spain shows that air-source heat pumps dominate both in terms of installed units (78% of total) and installed capacity (71% of total) over water-source and geothermal heat pumps. 2.8. Functional unit The function of the system under study is established as providing treatment for urban wastewater. The THERBIOR scenario, however, provides an additional function, namely the recovery of thermal energy to be used for cooling/heating. This additional function is dealt with in the LCA and LCC by means of substitution. The functional unit and reference flow used in the study is the treatment of 1 m3 of urban wastewater with the following composition, obtained from the WWTP at Putignano (Southern Italy): 937 mg chemical oxygen demand (COD)/L, 93 mg total nitrogen (TN)/L, 625 mg suspended solids (SS)/L and 10 mg total phosphorus (TP)/L. 2.9. Limitations We can highlight two main limitations in our assessment, one related to energy use and the other related to wastewater treatment. From the energy point of view, the purpose of our research project was to assess the potential for thermal energy recovery from wastewater through the innovative SHP. In the pilot plant the recovered energy is used to provide heating and cooling to the simulated ETL attached to the plant. In a full-scale deployment of this technology, however, the heating and cooling demand could be located outside of the WWTP premises, in hotels, residences or office buildings. In our study we do not include the transport of the produced energy to a hypothetical building off-site. We instead quantify the service as if it was supplied on-site (no distribution network, no losses). From the wastewater treatment side, a relevant aspect that could be addressed is the fate and potential toxicity impacts of heavy metals and organic micropollutants present in the raw wastewater and how these differ when a SBBGR is used instead of activated sludge. Unfortunately, in this study we lacked information on the corresponding performance of SBBGR vs. activated sludge with regard to these pollutants. The concentration of some heavy metals and organic micropollutants in SBBGR influent and effluent was measured during a previous study on wastewater reuse in agriculture (De Sanctis et al., 2017). Most of the selected micropollutants were not detected in the wastewater used in that study. In the inventory analysis we do not quantify emissions to air, water or soil from these pollutants in wastewater. 2.10. Impact assessment method The method used for impact assessment in the LCA study is Stepwise 2006, version 1.5. The method is described and documented in Annex II in Weidema et al. (2007) and in Weidema (2009). Stepwise is capable of providing results at the level of midpoints (characterization) and endpoints (damage). At the endpoint level, each impact category is expressed in monetary units (Euro), measuring environmental damage. In total, Stepwise2006 includes a total of 16 impact categories commonly used in LCA, however, given the lack of data at the inventory level on the presence and fate of micropollutants in wastewater, we decided to exclude freshwater ecotoxicity from the set of environmental impact categories assessed. 2.11. Data collection for life cycle assessment The life cycle model was built using the consequential library available in ecoinvent v.3.2 (Ecoinvent, 2018) as background database, whereas a variety of primary data sources were used in the inventory analysis. In this section we outline the main sources and assumptions, whereas a summarised mass and energy balance is provided in Fig. 3 for the two scenarios (excluding sludge disposal, capital equipment and the background system). For complete and detailed inventory tables the reader is referred to the SM. Fig. 3. Summarised mass and energy balance for the two assessed scenarios during the operation life cycle phase: reference (left) and THERBIOR (right). * 0.14 kWh is the electricity consumption of SHP and all peripheral components such as circulation pumps and fan-coil unit. Construction of a conventional WWTP designed for treating 750 m3/day was estimated with a linear regression using existing inventories for WWTPs of different capacities in Switzerland, available in the ecoinvent database. These same data were the basis for the SBBGR WWTP construction, for which no data were found. We took the assumption that infrastructure material inputs are proportional to investment costs, meaning that the lower investment cost of a SBBGR WWTP (see 2.12) leads to a proportional infrastructure material reduction. WWTP operation included inputs of electricity, sodium hypochlorite for disinfection, polyelectrolyte for sludge dewatering, sodium hydroxide for sand filter cleaning and sand replacements for this filter. All material inputs were quantified based on literature (see SM) with the exception of electricity consumption, which was taken from the WWTP at Putignano. This plant has recently been partially upgraded to a SBBGR treatment, allowing us to get accurate data for both conventional (activated sludge) and SBBGR treatment. A detailed theoretical mass balance was established for the WWTPs, based on the influent and effluent composition, using WW LCI, a life cycle inventory model for chemicals discharged in wastewater (Muñoz et al., 2016; Kalbar et al., 2017). With this model we estimated direct emissions from the WWTP to air (CO2, N2O), to water (NO3, NH4, TP) as well as sludge production, which resulted in 0.51 kg dry mass/m3 for the conventional WWTP after aerobic digestion and 0.14 kg dry mass/m3 for the SBBGR WWTP. WW LCI was also used to model two additional processes. Firstly, emissions of CO2 and N2O resulting from the ultimate degradation in the aquatic environment of organics and nutrients in the treated effluent. Secondly, to model sludge disposal by incineration, which involves transport of dewatered sludge, drying to a content of 90% dry mass and combustion with energy recovery (see SM for details). The inventory for construction of the SHP, based on a theoretical scale-up of the pilot, was accomplished by first compiling a detailed bill of materials for this pilot, which resulted in 1.7 tonnes of equipment. In the scale-up we enlarged, changed and adapted all this equipment, resulting in a SHP embedding 56 tonnes of equipment, including a 190 kWp photovoltaic plant with 1225 m2 module area. In order to annualize the consumption of equipment, service lives in years were defined for each type of component, ranging from 7 years for electronics to 30 years for structural elements. One of the difficulties faced to build this inventory lies in the current lack of data sets in ecoinvent to represent some of the machinery and equipment installed in the SHP. In the SM we report in detail how we adapted and linked our data to the ecoinvent database. The operation of the SHP includes some grid electricity needed to keep the heating/cooling distribution running during the night, but more importantly, the amount of heating/cooling produced and recovered by this system, expressed in MJ. This consists of two contributions: the solar-powered heat pump itself and the thermal energy recovery from wastewater. The first contribution is quantified considering the SHP heating and cooling capacity, of 244 kW and 160 kW, respectively and an enduser heating and cooling demand of 632 h/year and 864 h/year, respectively for a Mediterranean location (Batlles et al., 2010). Concerning thermal energy recovery from wastewater, as described in Piergrossi et al. (2018), it was found that a substantial share of the recovered thermal energy was sensible heat from solar radiation, i.e. heating of the SBBGR columns by direct exposure to sunlight, as they are placed above ground (see Fig. 1). However, in a full-scale SBBGR unit the tanks would be instead placed on the ground (similarly to conventional activated sludge tanks), and therefore the heating contribution from sunlight would not occur. A detailed energy balance of the SBBGR reactor revealed that in average, approximately 50% of the recovered energy is due to solar exposure of the reactor (Portillo, 2017). In our scale-up calculations we excluded this contribution and quantified the recoverable thermal energy as 13.8 MJ per m3 wastewater, and the actually recovered energy as 4.6 MJ/m3, given that the SHP can only operate during daylight (8 h/day). Considering the two contributions (SHP and wastewater), the total thermal energy recovered is 10.4 MJ/m3. The inventory for the substituted air-source heat pump was quantified based on a mirror design of the up-scaled SHP where the photovoltaic system was removed and power from the grid was instead considered. Finally, electricity production in Italy, which supplies all the above activities, was modelled considering only flexible suppliers in the period 2012e2020, resulting in a grid mix containing 88% renewably-sourced electricity (see SM for details). 2.12. Data collection for life cycle costing Costs assessed included investment and operation, whereas decommissioning costs were neglected (Muñoz, 2006). Investment costs were annualized using the so-called capital recovery factor (CRF), as a function of service life in years and interest rate. The latter was taken as 1%, which corresponds to the GDP-weighted Euro area ten-year sovereign bond yield, according to the European Central Bank at the beginning of 2018 (ECB, 2018). As in the previous section, below we only provide a summary of the main data sources and assumptions, whereas detailed cost calculations are available in the SM. Investment costs for a conventional and SBBGR WWTP were established as 350V and 224V per PE, respectively, based on information supplied by the integrated water services operator in the Apulia region, which operates the Putignano WWTP upgraded to SBBGR. Investment cost for the up-scaled SHP was estimated close to 906,000 V, based on the total equipment cost plus installation, engineering costs, etc. The CRF for these investments was calculated assuming a service life of 30 years for the WWTP and 20 years for the SHP. Operation costs for the WWTP included electricity costs, other operation costs and sludge disposal. Electricity costs for Italian industrial consumers was obtained from EUROSTAT (2018) as 0.185 V/kWh. Other operation costs were estimated as 3% of the investment cost, annually (COWI, 2010), and sludge disposal costs included transport and incineration costs, estimated for Italy as 203 V per tonne in wet weight, based on Diaz et al. (2015) and UNHabitat (2008). Operation costs for the SHP included grid electricity consumption for equipment running 24 h and maintenance costs. The latter were estimated as 3% of the investment costs, annually. This is the average value obtained from a study on life cycle costs of HVAC systems (Wu and Clements-Croome, 2007). The substituted air-source heat pump costs were quantified following a similar approach as in the SHP. Investment costs were estimated as 167,600V with a service life of 15 years. Operation costs included electricity and maintenance, taking the same approach as in the SHP. In this case, the grid electricity consumption is 0.144 kWh per MJ heating/cooling. Overall, the unitary cost of this pump results in 0.043V per MJ heating/cooling. 3. Results and discussion 3.1. Life cycle impact assessment Table 1 shows the life cycle impact assessment results for the two scenarios, at both midpoint and endpoint level. While at midpoint each indicator has its own specific units, at endpoint level they are all expressed in monetary units. At midpoint level we can see that the THERBIOR scenario shows lower environmental impact in all 15 indicators, with a reduction in impact ranging from 19% in mineral extraction to 93% in ozone layer depletion. In nature occupation, the THERBIOR scenario presents a negative score, meaning a net beneficial effect on this indicator. The inclusion of results at endpoint level is useful to identify those impact indicators where the system has the highest contribution to environmental damages. It can be seen in Table 1 that in our case study this corresponds to global warming, respiratory inorganics and freshwater eutrophication, with the first one of these three having the highest magnitude. Table 1. Life cycle impact assessment results at midpoint and endpoint level per m3 wastewater. Impact category Midpoint Endpoint Unit Reference THERBIOR Unit Reference THERBIOR Human toxicity, carcinogens kg C 2 H 3 Cl-eq into air 0.023 0.011 V 0.006 0.003 Human toxicity, non-carcinogens kg C 2 H 3 Cl-eq into air 0.042 0.015 V 0.011 0.004 Respiratory inorganics kg PM 2.5 -eq 0.00086 0.00035 V 0.058 0.024 Ionizing radiation Bq Carbon-14 into air 0.60 — 0.19 V 1.2E-05 —3.8E-06 Ozone layer depletion kg CFC11-eq 7.5E-08 5.0E-09 V 7.7E-06 5.2E-07 Ecotoxicity, terrestrial kg triethylene glycol-eq into soil 5.5 3.1 V 1.2E-04 1.7E-04 Nature occupation m 2 agr. land 0.037 — 0.001 V 0.0045 — 0.0001 Global warming kg CO 2 -eq 0.82 0.47 V 0.068 0.039 Acidification m 2 unprotected ecosystem 0.044 0.015 V 0.0003 0.0001 Eutrophication, aquatic kg NO 3 -eq 0.33 0.21 V 0.033 0.021 Eutrophication, terrestrial m 2 unprotected ecosystem 0.11 0.03 V 0.0014 0.0004 Respiratory organics Person$ppm$h 0.00059 0.00023 V 1.5E-04 6.0E-05 Photochemical ozone, vegetation m 2 $ppm$hour 6.8 2.4 V 0.0025 0.0009 Non-renewable energy Megajoule 8.9 2.5 V 0 0 Mineral extraction Megajoule 0.12 0.10 V 5.0E-04 4.0E-04 Fig. 4 shows a contribution analysis of the global warming indicator, expressing GHG emissions in CO2-eq. It must be stressed that biogenic CO2 emissions from the degradation of organic matter in wastewater are considered to be climate-neutral in our analysis. The graph shows that GHG emissions are especially lower for the THERBIOR scenario in two aspects: sludge disposal and heat pump substitution. With regard to sludge disposal, there is a clear advantage for the SBBGR due to its substantially lower sludge production. This means less transport, less fuel requirements to dry sludge prior to incineration, and less incineration emissions. Overall, GHG emissions related to sludge disposal are reduced by a factor four. Concerning heat pump substitution, this is shown with a negative sign in the graph, meaning that 0.27 kg CO2-eq are avoided per m3 wastewater. These emissions are related to the (avoided) production, operation and end of life of a conventional heat pump powered by the grid. There is no such saving in the reference scenario, as there is no thermal energy production in that case. Overall, in the THERBIOR scenario the SHP leads to a total emission over the life cycle (construction, operation, end of life) of 0.16 kg CO2-eq/m3, but this is more than compensated by the aforementioned saving of 0.27 kg CO2-eq/m3. It is also worth mentioning that the THERBIOR scenario also involves a GHG reduction during WWTP construction, however this result is subject to uncertainty, as we have assumed that infrastructure impacts are directly linked to investment costs. It seems reasonable though that the much simpler layout of a SBBGR WWTP should involve, to some extent, a lower need for construction materials such as concrete and reinforced steel, among others. Framework. International Standard Organization (ISO), Geneve. ISO, 2006b. ISO 14044 Environmental Management - Life Cycle Assessment e Requirements and Guidelines. International Standard Organization (ISO), Geneve. 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