Technology validation of photosynthetic biogas upgrading in a semi-industrial scale algal-bacterial photobioreactor

1 Technology validation of photosynthetic biogas upgrading in a semiindustrial scale algal-bacterial photobioreactor María del Rosario Roderoa,c, Raquel Lebreroa,c, Esteban Serranob, Enrique Larab, Zouhayr Arbibb, Pedro A. García-Encinaa,c, Raúl Muñoz*a,c a Department of Chemical Engineering and Environmental Technology, University of Valladolid, Dr. Mergelina s/n., Valladolid 47011, Spain. b FCC Servicios Ciudadanos, Av. del Camino de Santiago, 40, edificio 3, 4ª planta,28050 Madrid, Spain c Institute of Sustainable Processes, University of Valladolid, 47011, Valladolid, Spain *corresponding author: mut[email protected] ABSTRACT In this work, the performance of photosynthetic biogas upgrading coupled to wastewater treatment was evaluated in an outdoors high rate algal pond (HRAP) interconnected to an absorption column at semi-industrial scale. The influence of biogas flowrate (274, 370 and 459 L h-1), liquid to biogas ratio (L/G = 1.2, 2.1 and 3.5), type of wastewater (domestic versus centrate) and hydraulic retention time in the HRAP (HRT) on the quality of the biomethane produced was assessed. The highest CO2 and H2S removal efficiencies (REs) were recorded at the largest L/G due to the higher biogas-liquid mass transfer at increasing liquid flowrates. No significant influence of the biogas flowrate on process performance was observed, while the type of wastewater was identified as a key operational parameter. CO2 and H2S-REs of 99% and 100% at a L/Gmax=3.5 were recorded using centrate. The maximum CH4 content in the biomethane (90%) was limited by N2 and O2 desorption. Keywords: algal-bacterial photobioreactor; biogas upgrading; microalgae; semiindustrial scale HRAP; wastewater treatment. © 2019 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 2 1. Introduction Biogas from the anaerobic digestion of organic waste, such as sludge from wastewater treatment plants (WWTPs), constitutes a valuable bioenergy vector able to reduce our current dependence on fossil fuels. Biogas from WWTPs is typically composed of CH4 (60-75%), CO2 (30-40%) and other pollutants at trace level concentrations such as H2S (0.02-2%), O2 (0-1%), N2 (0-2%), NH3 (<1%) and siloxanes (0-0.2%) (Ryckebosch et al., 2011). The high concentration of CO2 increases hydrocarbon and carbon monoxide emissions during biogas combustion, reduces its specific calorific value and increases its transportation cost. On the other hand, H2S is a malodorous and toxic gas contaminant that generates corrosion and mechanical wear in pipelines and internal combustion engines (Lebrero et al., 2016). Several technologies are nowadays commercially available to remove these contaminants from biogas in order to generate a high quality biomethane similar to natural gas. Physical-chemical technologies for CO2 separation such as pressure swing adsorption, membrane separation and water/organic/chemical scrubbing often need a previous H2S cleaning step (i.e. adsorption on activated carbon or metal ions-based in situ precipitation) and a high energy input (0.2-0.7 kWh/m3biogas), with the associated increase in operational costs. Thus, the high energy and chemical requirements of conventional biogas upgrading processes, among other factors such as the cost of acquisition of the organic matter and the type of process, limit the cost-effective use of biomethane as a renewable substitute of natural gas (Rodero et al., 2018a). On the other hand, biological technologies such as biofiltration or in situ microaerobic anaerobic digestion for H2S removal followed by hydrogenotrophic biogas upgrading (power to gas) for CO2 bioconversion into CH4 entail 3 the need of a two-stage process and can be only applied in locations with a sustained surplus of renewable electricity (Angelidaki et al., 2018; Muñoz et al., 2015a). In this context, biogas upgrading using algal-bacterial processes has emerged as a costcompetitive and environmentally friendly platform capable of removing CO2 and H2S in a single step process (Bahr et al., 2014). Photosynthetic biogas upgrading is based on the concomitant CO2 fixation by microalgae using solar energy and oxidation of H2S to S0/SO42by sulfur-oxidizing bacteria using the oxygen photosynthetically produced (Sun et al., 2016). Moreover, this biotechnology simultaneously supports wastewater treatment since residual nutrients can sustain algal-bacterial growth, which contributes to improve its environmental and economic sustainability (Posadas et al., 2015a; Zhang et al., 2017). Biogas upgrading combined with wastewater treatment in algal-bacterial photobioreactors has been successfully validated indoors at lab-pilot scale (Bahr et al., 2014; Meier et al., 2017; Ouyang et al., 2015; Posadas et al., 2016; Rodero et al., 2018b; Serejo et al., 2015; Toledo-Cervantes et al., 2017a, 2016; Yan et al., 2016). Likewise, promising results in terms of biogas upgrading (CH4 contents of 85.2-97.9%) and centrate treatment (total nitrogen removal efficiencies (REs) of 80-87% and P-PO43REs of 8592%) were obtained in an outdoors 180 L high rate algal pond (HRAP) interconnected to an absorption column (Marín et al., 2018; Posadas et al., 2017a). However, this innovative biogas upgrading technology has not been yet validated at semi-industrial scale, which is a must in order to foster its acceptance by the industrial sector. This work investigated for the first time the influence of biogas flow rate and the liquid to biogas ratio (L/G) on biomethane quality in an outdoors algal-bacterial photobioreactor treating real biogas at semi-industrial scale. Moreover, the influence of the type of 4 wastewater (domestic versus centrate) and the hydraulic retention time (HRT) in the HRAP on biogas upgrading and nutrient recovery efficiency was also assessed. 2. Materials and methods 2.1. Biogas and wastewaters Biogas was produced in a semi-industrial 20 m3 anaerobic digester treating sewage sludge at Chiclana de la Frontera WWTP (Spain). Biogas composition averaged 69.2±5.7% CH4, 32.7±2.8% CO2 and 1183±1006 ppm H2S. Fresh domestic wastewater was pumped into the HRAP directly after screening and degreasing of the influent raw wastewater. The average composition of the domestic wastewater was (mg L-1): chemical oxygen demand (COD) = 496±145, inorganic carbon (IC) = 46±11, total nitrogen (TN) = 41±11, ammonium (N-NH4+) = 44±9, phosphate (P-PO43-) = 6±2 and total suspended solids (TSS) = 140±40. Urea, H3PO4, NaHCO3 and Na2CO3 were added to the raw domestic wastewater to achieve a final IC, TN and P-PO43concentration of 500, 500 and 75 mg L1, respectively, in order to simulate a medium-strength centrate composition. 2.2. Experimental set-up The experimental set-up was located outdoors at Chiclana de la Frontera WWTP (36.42 N; 6.15 W) (Spain). The set-up consisted of a 9.6 m3 HRAP made of concrete blocks with an illuminated surface of 32 m2, 0.3 m of depth, two water channels divided by a central wall and two flow rectifiers in each side of the curvature. The cultivation broth in the HRAP was continuously agitated by a 6-blade paddlewheel operated at 7 rpm, resulting in an internal liquid velocity of 0.30 m s-1. The HRAP was interconnected to a 150 L absorption column provided with a polypropylene fine bubble biogas diffuser (Ecotec AFD 270) via an external liquid recirculation of the supernatant from a 7 m3 conical settler 5 (Figure 1). The algal-bacterial biomass accumulated at the bottom of the settler was continuously recirculated to the HRAP to avoid an excessive biomass accumulation in the settler. The algal-bacterial biomass was wasted from an overflow located in the HRAP in order to maintain the depth of the photobioreactor at 0.3 m. <Figure 1> 2.3. Operational conditions and sampling procedures The HRAP was inoculated with a consortium of cyanobacteria/microalgae and bacteria from an outdoors HRAP treating domestic wastewater at Chiclana de la Frontera WWTP prior to the experiment start-up. Three different operational conditions were tested to assess the influence of the HRT and the type of wastewater used as a nutrient source (domestic wastewater vs centrate) in the HRAP on biogas upgrading efficiency. During stages I and II, the HRAP was fed with domestic wastewater at a HRT of 3.5 and 8 days, respectively, which correspond to typical values used during wastewater treatment in HRAPs (Arbib et al., 2013; Posadas et al., 2015b). In stage III, simulated centrate was used as a nutrient source at a high HRT (≈73 days) in order to avoid inhibition of microalgae growth by its high NH4+ concentration. The high nutrient content of centrate entailed lower wastewater flowrates to satisfy nutrient requirements. L/G ratios of 1.2 and 2.1 were tested under counter-current flow operation at different biogas flowrates (274±12, 370±7 and 459±36 L h-1) under steady state in the three operational stages. Moreover, a L/G ratio of 3.5 was tested only at the lowest biogas flow rate of 274 L h-1 since the maximum flow rate of the recycling liquid pump was 1000 L h-1. 6 The temperature, dissolved oxygen concentration (DO) and pH in the cultivation broth of the HRAP were monitored every five minutes. Liquid samples of 1 L from the influent wastewater (obtained along 24 hours) and 500 mL from the clarified effluent were withdrawn twice a week to monitor the concentration of COD, N-NH4+, P-PO43-, N-NO2- , N-NO3-, IC and TN. Liquid samples were also drawn from the cultivation broth of the HRAP to monitor algal-bacterial TSS and volatile suspended solids (VSS) concentration. The algal-bacterial biomass was dried for 24 h at 105 °C to determine its elemental composition (C, N and S) under steady state in each operational stage. 2.4. Analytical procedures The pH, DO concentration and temperature were monitored and recorded using Crison pH 4603 and DO 6050 probes coupled to a Crison Multimeter 44 display (Spain). CH4, CO2, H2S and O2 were measured using a COMBIMASS® Portable Gas-analyzer GA-m5. The concentrations of dissolved TN and IC were determined by means of a Shimadzu TOC-VCSH analyzer (Japan) equipped with a TNM-1 chemiluminescence module. NH4+ was analyzed using a selective electrode (Thermo Scientific Orion, USA). COD, P-PO43- , N-NO2-, N-NO3-, TSS and VSS were measured using Standard Methods (Eaton et al., 2005). The elemental composition of the algal-bacterial biomass (C, N and S content) was determined using a LECO CHNS-932 analyzer (LECO, Italy). 2.5. Statistical analysis The results here presented were provided as the average values along with their standard deviation from replicate measurements. An analysis of variance (ANOVA) was performed to determine the influence of the biogas flowrate, HRT and L/G ratio on the quality of biomethane. 7 3. Results and discussion 3.1. Environmental parameters The ambient temperature and the diurnal solar radiation cycle seasonally varied along the three experimental stages, with the subsequent variations in the cultivation broth temperatures (23.5±2.5, 12.4±2.3 and 18.8±3.0 ºC during stages I, II and III, respectively) (Table 1). These variations in environmental conditions are inherent to any outdoors experimentation. In this context, Rodero et al. (2018b) found a negligible impact of the temperature on biogas upgrading performance when using a moderate alkalinity cultivation broth (i.e. centrate), while at low alkalinity (i.e. domestic wastewater) the CH4 content of the biomethane increased by 3.3% when the temperature decreased from 35 ºC to 12 ºC. The average pH of the cultivation broth under steady state during stages I, II and III was 7.3±0.2, 7.1±0.5 and 8.9±0.3, respectively. The higher pH recorded in the latter stage was attributed to the higher pH and alkalinity of the centrate fed to the HRAP in comparison with the domestic wastewater used during stages I and II. The maximum DO concentrations in the cultivation broth (8.3±2.8, 6.6±1.3 and 9.4±1.4 mg L-1 in stages I, II and III, respectively) (Table 1) were recorded during the daytime, and never exceeded inhibitory levels for microalgae activity (<25 mg O2 L-1) (Jiménez et al., 2003). On the other hand, minimum daily DO concentrations of 0.3±0.2, 2.8±1.4 and 4.3±0.7 were recorded in stages I, II and III, respectively, during the nighttime due to absence of photosynthetic activity and the occurrence of an active organic matter oxidation and NH4+ nitrification (Posadas et al., 2013). It is worth noticing that the lowest DO concentration was observed during the treatment of domestic wastewater at a HRT of 3.5 days due to the higher biological oxygen consumption resulting from the higher organic loading rates mediated by the shorter HRT (Arbib et al., 2017). 8 Finally, the average water losses by evaporation during stages I, II and III accounted for 14.7±18.7, 4.3±3.2 and -0.1±0.6 L m-2 d-1 (Table 1). The highest evaporation rate herein recorded was ~ 2.2 times higher than the maximum values reported by Marín et al. (2018) in a 180 L outdoors HRAP located at Valladolid (Spain) during one year operation. This high value was attributed to the higher temperatures of the cultivation broth and the high turbulence at the HRAP surface caused by the wind in Chiclana de la Frontera. On the other hand, the negative value obtained during stage III was caused by the higher average rain recorded (4.4 L m-2 d-1) during steady state in this period compared to 1.0 L m-2 d-1 recorded during state II and the absence of rain during stage I. This value agreed with the observations of Posadas et al. (2014), who reported negative evaporation rates in an outdoors HRAP. 3.2. Biogas upgrading performance 3.2.1. CO2 removal CO2 removal efficiency was a function of the gas-liquid mass transfer in the absorption column, which itself was influenced by CO2 consumption by microalgae in the HRAP. During stage I, CO2-REs of 59.2±3.2, 76.6±1.8 and 88.9±1.5%, which corresponded to CO2 concentrations of 17.3±2.2, 11.8±1.4 and 5.8±1.0% in the upgraded biogas, were recorded at L/G ratios of 1.2, 2.1 and 3.5, respectively, at a biogas flowrate of 274 L h-1. CO2-REs increased with the L/G ratio due to the increase in the overall gas-liquid mass transfer coefficient and the lower CO2 transferred per volume of recirculating medium, which prevented the acidification of the recycling cultivation broth along the absorption column as a result of the acidic nature of biogas (Anbalagan et al., 2017; Posadas et al., 2017a). Indeed, a lower decrease in pH between the top and the bottom of the absorption 9 column was observed with the increase in the L/G ratio (ΔpH of 1.7, 1.5 and 1.2 at a L/G ratio of 1.2, 2.1 and 3.5, respectively) during stage I. Similarly, CO2-REs varied from 59.6±2.5 to 74.2±0.5% and from 64.4±2.2 to 81.0±0.3% when the L/G increased from 1.2 to 2.1 at a biogas flowrate of 370 and 459 L/h, respectively (Figure 2a). In this context, a slight increase in CO2-RE was recorded at the highest biogas flowrate as a result of the higher turbulence in the absorption column, which enhanced the gas-liquid mass transfer coefficient in this unit. <Figure 2> During stage II, CO2-REs of 56.4±2.5, 77.2±1.5 and 90.4±0.4% were recorded at a L/G ratio of 1.2, 2.1 and 3.5, respectively, and a biogas flowrate of 274 L h-1 (Figure 2b). No significant differences (p >0.05) were observed in CO2-RE values compared to stage I, which revealed a negligible influence of the HRT on CO2 removal efficiency when domestic wastewater was used to support algal-bacterial growth. In fact, although higher pH values were expected at longer HRTs based on the lower acidification caused by the reduction in CO2 production due to the lower organic matter load, a similar pH of the cultivation broth was recorded in the HRAP in both stages as a result of the higher nitrifying activity during stage II (as discussed in section 3.3) (de Godos et al., 2016; Posadas et al., 2017b). The decrease in pH along the absorption column in stage II was similar to that recorded in stage I (ΔpH of 2.1, 1.7 and 1.5 at a L/G ratio of 1.2, 2.1 and 3.5, respectively), which was attributed to the similar IC concentration of the cultivation broth in both stages (25.6±5.5 and 29.5±9.4 mg L-1 during stage I and II, respectively, under steady state conditions). Similarly, CO2-REs varied from 64.3±4.7 to 84.0±1.4% and from 63.6±0.4 to 80.1±0.4% when the L/G increased from 1.2 to 2.1 at biogas flowrates of 370 and 459 L h-1, respectively (Figure 2b). These results were in accordance 16 of previously reported data (Ward et al., 2014). The main differences were recorded in S content, which varied from 0.68±0.08% during stages I and II to 0.30±0.05% during stage III. These results agreed with those reported by Posadas et al. (2017a), who observed a decrease in S content in the biomass from 0.4% to 0.2% concomitantly with the increase in the IC concentration of the cultivation broth. However, this decrease in S requires further investigation. 3.5 Biogas upgrading technology costs Despite the fact the investment cost of photosynthetic biogas upgrading is ~1.5-2.2 times higher than that of conventional-physical chemical technologies, and the needed of higher extensions of land (a total HRAP surface of ~13.4 ha to treat 300 Nm3 h-1 of biogas considering a water depth of 0.2 m) (Toledo-Cervantes et al., 2017b), the environmental sustainability (CO2 trapped in form of algal bacterial biomass and wastewater treatment), the simultaneous H2S removal and the lower energy requirements, make this technology an attractive alternative for biogas upgrading (Table 3). Moreover, algal-bacterial biomass valorization as bio-fertilizer outbalanced the high investment costs of this process. <Table 3> 4. Conclusions This work constitutes, to the best of our knowledge, the first demo-scale validation of the simultaneous photosynthetic biogas upgrading and wastewater treatment under outdoor conditions. The type of wastewater played a key role on biogas upgrading (with higher CO2 and H2S removals using centrate due to its higher pH and alkalinity), while the influence of the HRT and biogas flowrate on biogas upgrading performance was negligible. Despite higher L/G ratios supported higher CO2 and H2S removals, the 17 associated N2 and O2 stripping resulted in a lower biomethane quality. Finally, an efficient wastewater treatment was achieved regardless of the operational conditions. Acknowledgements This work was supported by the INCOVER project. The project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 689242. 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Energy 130, 392–401. https://doi.org/10.1016/j.energy.2017.04.157 21 Figure 1. Schematic diagram of the experimental set-up. 22 Figure 2. Influence of the L/G ratio on the removal efficiency of CO2 at a biogas flowrate of 274 (black), 370 (white) and 459 (grey) L h-1 during stage I (a), stage II (b) and stage III (c). 23 Figure 3. Influence of the L/G ratio on the removal efficiency of H2S at a biogas flowrate of 274 (black), 370 (white) and 459 (grey) L h-1 during stage I (a), stage II (b) and stage III (c). 24 Figure 4. Influence of the L/G ratio on the CH4 enhancement factor at a biogas flowrate of 274 (black), 370 (white) and 459 (grey) L h-1 during stage I (a), stage II (b) and stage III (c). 25 Figure 5. Steady state removal efficiencies of total nitrogen (TN), ammonium (N-NH4+), phosphate (P-PO43-) and chemical oxygen demand (COD) during stage I (white), II (black) and III (grey). 0 20 40 60 80 100 RE (%) TN N-NH4+ P-PO43COD