Influence of the seasonal variation of environmental conditions on biogas upgrading in an outdoors pilot scale high rate algal pond

1 Influence of the seasonal variation of environmental conditions on 1 biogas upgrading in an outdoors pilot scale high rate algal pond 2 David Marín1, 2, Esther Posadas1, Patricia Cano1, Víctor Pérez1, Raquel Lebrero1, Raúl 3 Muñoz*1 4 1 Department of Chemical Engineering and Environmental Technology, School of Industrial 5 Engineerings, Valladolid University, Dr. Mergelina, s/n, 47011, Valladolid, Spain. 6 2 Universidad Pedagógica Nacional Francisco Morazán, Boulevard Centroamérica, Tegucigalpa, 7 Honduras. 8 9 * Corresponding author: [email protected] 10 11 ABSTRACT 12 The influence of the daily and seasonal variations of environmental conditions on the 13 quality of the upgraded biogas was evaluated in an outdoors pilot scale high rate algal 14 pond (HRAP) interconnected to an external absorption column (AC) via a conical 15 settler. The high alkalinity in the cultivation broth resulted in a constant biomethane 16 composition during the day regardless of the monitored month, while the high algal-17 bacterial activity during spring and summer boosted a superior biomethane quality. CO2 18 concentrations in the upgraded biogas ranged from 0.1% in May to 11.6% in December, 19 while a complete H2S removal was always achieved regardless of the month. A limited 20 N2 and O2 stripping from the scrubbing cultivation broth was recorded in the upgraded 21 biogas at a recycling liquid/biogas ratio in the AC of 1. Finally, CH4 concentration in 22 the upgraded biogas ranged from 85.6% in December to 99.6% in August. 23 Keywords: Algal-bacterial photobioreactor; Biogas upgrading; Biomethane; Outdoors 24 operation; Yearly evaluation. 25 26 *Manuscript Click here to view linked References 2 1. Introduction 27 Biogas from the anaerobic digestion of wastewaters and organic waste constitutes a 28 renewable source of energy to generate electricity or heat (Muñoz et al., 2015). 29 However, the use of biogas as a substitute of natural gas or fuel in transportation 30 requires an effective purification to levels set by national regulations. For instance, 31 biogas injection into natural gas grids typically requires concentrations of CH4 ≥ 95%, 32 CO2 ≤ 2%, O2 ≤ 0.3% and trace levels of H2S (Muñoz et al., 2015; Toledo-Cervantes et 33 al., 2017). 34 35 Algal-bacterial processes have emerged as a platform technology capable of 36 simultaneously removing CO2 and H2S in a single stage, and constitute a cost-effective 37 and environmentally friendly alternative to conventional biogas upgrading technologies 38 (Bahr et al., 2014; Muñoz et al., 2015). Biogas upgrading in algal-bacterial 39 photobioreactors is based on the oxidation of H2S to SO42- by sulfur oxidizing bacteria 40 promoted by the high dissolved oxygen (DO) concentrations in the scrubbing 41 cultivation broth, and on the photosynthetic fixation of the absorbed CO2 by microalgae. 42 The economic and environmental sustainability of this biotechnology can be boosted via 43 digestate supplementation as a nutrient and water source, which will support an 44 effective recovery of nutrients in the form of algal-bacterial biomass (Posadas et al., 45 2017; Toledo-Cervantes et al., 2016). 46 47 Biogas upgrading coupled to digestate treatment has been typically evaluated indoors in 48 high rate algal ponds (HRAPs) interconnected to biogas absorption columns (AC) under 49 artificial illumination (Alcántara et al., 2015; Bahr et al., 2014; Meier et al., 2015; 50 3 Posadas et al., 2016, 2015; Serejo et al., 2015; Toledo-cervantes et al., 2017; Toledo-51 Cervantes et al., 2017, 2016). The optimization of this process has reached promising 52 results in terms of biomethane quality (CH4 concentrations of 96.2±0.7 %), nutrient 53 removal (total nitrogen (TN)-removal efficiencies (REs) of 98.0±1.0 % and P-PO4-3- 54 REs of 100±0.5 %) and biomass productivities (15.0 g m-2 d-1) (Toledo-Cervantes et al., 55 2017). Comparable results were also obtained by Posadas et al. (2017) in a similar 56 biogas upgrading photobioreactor configuration operated outdoors during summer in 57 Spain, when solar irradiation, temperature and the number of sun hours were most 58 favorable to support algal-bacterial activity. In this context, a systematic year-round 59 evaluation of the influence of the daily and seasonal variations of environmental 60 conditions on biogas upgrading and nutrient recovery from digestate is needed to 61 validate this technology under outdoor conditions. 62 63 This study investigated for the first time the year-round performance of biogas 64 upgrading in an outdoors pilot HRAP interconnected to an external AC by monthly 65 monitoring the daily variations of biogas quality and cultivation broth parameters under 66 continental climate conditions. 67 68 2. Materials and methods 69 2.1. Biogas and centrate 70 A synthetic biogas mixture composed of CO2 (29.5%), H2S (0.5%) and CH4 (70%) was 71 used as a raw biogas in the present study (Abello Linde; Spain). Centrate was monthly 72 obtained from the centrifuges dehydrating the anaerobically digested mixed sludge of 73 Valladolid wastewater treatment plant (WWTP) and stored at 4 ºC. The composition of 74 4 centrate varied along the experimental period as a result of the seasonal operational 75 variations of the WWTP: total organic carbon (TOC) = 16-523 mg L-1, inorganic carbon 76 (IC) = 450-600 mg L-1, TN = 374-718 mg L-1, P-PO43- = 26-135 mg L-1 and SO42- = 0-77 38 mg L-1. The IC concentration in the centrate was adjusted to 1999 ± 26 mg L-1 via 78 addition of NaHCO3 and Na2CO3 in order to maintain the required high alkalinity and 79 pHs (≥9) in the cultivation broth to support an effective CO2 and H2S absorption in the 80 AC (Posadas et al., 2017). 81 82 2.2. Experimental set-up 83 The experimental set-up, constructed according to Posadas et al. (2017), was located 84 outdoors at the Department of Chemical Engineering and Environmental Technology of 85 Valladolid University (41.39º N, 4.44º W). The pilot plant consisted of a 180 L HRAP 86 with an illuminated area of 1.20 m2 (width = 82 cm; length = 170 cm; depth = 15 cm) 87 and two water channels divided by a central wall and baffles in each side of the 88 curvature. The internal recirculation velocity of the cultivation broth in the HRAP was ≈ 89 20 cm s-1, which was supported by the continuous rotation of a 6-blade paddlewheel. 90 The HRAP was interconnected to an external 2.5 L bubble AC (height = 165 cm; 91 internal diameter = 4.4 cm) provided with a metallic biogas diffuser of 2 µm pore size 92 located at the bottom of the column. The HRAP and the AC were interconnected via an 93 external liquid recirculation of the algal-bacterial cultivation broth from an 8 L conical 94 settler (Fig. 1). The efficiency of the settler in terms of biomass removal was almost 95 complete. 96 <Figure 1> 97 2.3. Operational conditions and sampling procedures 98 5 Process operation was carried out from November the 1st 2016 to October the 30st 2017. 99 The HRAP was inoculated to an initial concentration of 210 mg TSS L-1 with a 100 microalgae inoculum composed of Leptolyngbya lagerheimii (54%), Chlorella vulgaris 101 (28%), Parachlorella kessleri (9%), Tetrademus obliquus (5%) and Chlorella 102 minutissima (2%) from an indoor HRAP treating biogas and centrate at the Department 103 of Chemical Engineering and Environmental Technology of Valladolid University 104 (Spain). Five different operational stages (namely I, II, III, IV and V) were defined as a 105 function of the temperature, photosynthetic active radiation (PAR), number of sun hours 106 and biomass productivity imposed (Table 1). The synthetic biogas was sparged into the 107 AC under co-current flow operation at 74.9 L d-1 under a recycling liquid to biogas ratio 108 (L/G) of 1.0 according to Posadas et al. (2017), which resulted in gas and liquid 109 retention time of 48 min and. The liquid velocity accounted for 2 m h-1. The HRAP was 110 fed with IC-supplemented centrate as a nutrient source at a flow rate of 3.5 L d-1, which 111 entailed a hydraulic retention time of 50 d. Tap water was supplied in order to 112 compensate water evaporation losses and allow process operation without effluent 113 (Table 1). 114 <Table 1> 115 The pH, temperature and DO concentration in the cultivation broth of the HRAP, AC 116 and settler, along with PAR, were monitored every thirty minutes during the daytime of 117 one day every month where the environmental conditions were representative of the 118 conditions in the entire month. Gas samples of 100 µL from the upgraded biogas were 119 drawn every hour to monitor the gas concentrations of CH4, CO2, H2S, O2 and N2. 120 Liquid samples of 100 mL from the cultivation broth of the HRAP, AC and settler were 121 drawn every two hours to monitor the concentrations of dissolved TOC, IC, TN. 122 123 6 2.4. Analytical procedures 124 PAR was measured using a LI-250A light meter (LI-COR Biosciences, Germany), 125 while pH was determined with an Eutech Cyberscan pH 510 (Eutech instruments, The 126 Netherlands). Temperature and DO were measured using an OXI 330i oximeter (WTW, 127 Germany). Gas concentrations of CH4, CO2, H2S, O2 and N2 were determined using a 128 Varian CP-3800 GC-TCD according to Posadas et al. (2015) (Palo Alto, USA). 129 Dissolved TOC, IC and TN concentrations were measured using a Shimadzu TOC-130 VCSH analyzer (Japan) coupled with a TNM-1 chemiluminescence module. 131 132 3. Results and discussion 133 3.1. Biogas Upgrading 134 <Figure 2> 135 3.1.1 CO2 biomethane concentration 136 Negligible variations in CO2 concentration in the biomethane were recorded throughout 137 the daytime regardless of the operational month likely due to the high alkalinity of the 138 cultivation broth (Fig. 2; Fig. S6). These results were in agreement with Posadas et al. 139 (2017), who observed a constant CO2 concentration in the upgraded biogas during the 140 daytime in a similar set-up operated with a high ionic strength cultivation broth (IC 141 concentration ≈2660±48 mg L-1). This study also suggested that the influence of the 142 cultivation broth temperature on CO2 absorption (Henry’s law constant ranged from 143 HCO2≈1.27 at 8.3 ºC in November to HCO2≈0.59 at 40.3 ºC in July) was lower than that 144 of the IC concentration (Sander, 2015). Hence, the biomethane CO2 concentration in 145 stage I ranged from 1.4% in January to 11.6% in December. This concentration varied 146 from 0.1% in March to 3.9% in May during stage II, and from 0.6% in June to 2.2% in 147 July in stage III. CO2 concentrations in stage IV and V ranged from 0.4% to 1.8% and 148 7 from 0.8% to 1.2%, respectively (Fig. 2). Thus, the concentration of CO2 in the 149 biomethane produced in the algal-bacterial photobioreactor complied during most of the 150 year with European regulations, which require CO2 concentrations ≤2% prior injection 151 into natural gas grids or use as a vehicle fuel (Muñoz et al., 2015). The high CO2 REs 152 here obtained (estimated from ≈60.7% in December to 99.7% in May) were promoted 153 by the optimum L/G ratio reported by Posadas et al. (2017) and the high pHs/alkalinity 154 of the cultivation broth in the AC, which enhanced CO2 absorption (Lebrero et al., 155 2016; Posadas et al., 2015; Toledo-Cervantes et al., 2016). These results were in 156 accordance with Rodero et al. (2017), who reported an increase in the CO2-RE from 157 30.8% to 99.3% when alkalinity increased from 102±7 mg IC L-1 to 1581±135 mg IC L- 158 1 at 35.0ºC in a similar photobioreactor configuration under indoor conditions. 159 160 This year-round evaluation of the performance of the algal-bacterial photobioreactor 161 confirmed the key role of biotic mechanisms on this biogas upgrading technology (Fig. 162 2). Hence, despite the low temperatures of the cultivation broth during winter increased 163 CO2 aqueous solubility, the lower pHs of the cultivation broth supported by the low 164 photosynthetic activity (from 8.1 to 9.0) resulted in higher CO2 concentrations in the 165 upgraded biogas. The higher photosynthetic activity mediated by the favorable 166 environmental conditions prevailing during spring and summer, along with the 167 accumulation of IC in the cultivation broth from 1785 mg L-1 to 4599 mg L-1 from stage 168 II to V, increased the pH from 8.8 to 9.8, which resulted in biomethane CO2 169 concentrations complying with most international regulations. In this context, although 170 a 60% decrease in CO2 solubility is expected when the cultivation broth temperature 171 increases from 10 to 40°C, the high CO2 concentration gradient supported by the high 172 8 alkalinity/pH of the cultivation broth during stages II - V compensated this decrease in 173 CO2 solubility. 174 175 3.1.2 H2S biomethane concentration 176 H2S was completely removed in the system regardless of the environmental parameters 177 and alkalinity. This higher elimination compared to the removal of CO2 was attributed 178 to the higher H2S aqueous solubility (Henry´s law constant ranging from HH2S ≈3.58 at 179 8.3 ºC to HH2S ≈1.80 at 40.3 ºC) (Sander, 2015). The high pHs also promoted the 180 complete removal of this acidic gas in the AC (Bahr et al., 2014). These results were in 181 accordance to Posadas et al. (2017), who reported a complete removal of H2S during the 182 simultaneous treatment of centrate and biogas in a similar outdoors experimental set-up, 183 and to Toledo-Cervantes et al. (2016) who also observed a complete depletion of H2S 184 during the optimization of photosynthetic biogas upgrading under laboratory conditions. 185 In brief, the H2S concentration in the biomethane herein obtained complied with most 186 European regulations for biomethane injection into natural gas grids or use as a vehicle 187 fuel, which requires H2S levels ≤ 5 mg m-3 (Muñoz et al., 2015). 188 189 3.1.3 N2 and O2 concentrations in the biomethane 190 Despite no clear trend in the evolution of biomethane N2 concentration along the 191 daytime was recorded, the highest O2 concentrations in the upgraded biogas were 192 recorded around midday, concomitantly with the highest DO concentrations in the 193 cultivation broth (Fig. S3; Fig. S8). Biomethane N2 and O2 concentrations during stage I 194 ranged from 0.0% in November to 5.5% and 1.8%, respectively, in January. During 195 stage II, N2 and O2 concentrations varied from 1.2% (April) and 0.3% (March), 196 respectively, to 5.9% (March) and 2.4% (May), respectively. In stage III, these 197 9 concentrations ranged from 0.1% and 0.0% (July), respectively, to 3.3% (June) and 198 1.5% (July), respectively. During stage IV, N2 and O2 concentrations fluctuated from 199 0.0% (August) to 5.2% and 1.9% (September), respectively. Finally, N2 and O2 200 concentrations during stage V ranged from 1.9% and 0.4%, respectively, to 3.2% and 201 1.2%, respectively (Fig. S8). Overall, the highest N2 and O2 concentrations in the 202 upgraded biogas were recorded during stages I and II (and during September in stage 203 III) likely due to the lower ambient temperatures, which increased the solubility of these 204 gases in the HRAP and their further desorption in the AC. 205 206 The previous optimization of the L/G ratio in the AC entailed a low N2 and O2 207 desorption (Posadas et al., 2017). Thus, the O2 concentrations here recorded in the 208 biomethane were in accordance to Posadas et al. (2017) and Serejo et al. (2015), who 209 reported values ranging from 0% to 2% and from 0% to 4%, respectively, in a similar 210 experimental set-up (under outdoors and laboratory conditions, respectively) at a L/G of 211 0.5. The O2 concentration in the upgraded biogas only complied with international 212 regulations during the periods of low PAR (≤1%), which requires a further optimization. 213 214 3.1.4 CH4 biomethane concentration 215 Negligible variations in the CH4 concentration of the upgraded biogas were recorded 216 throughout the daytime regardless of the operational month (Fig. 2). Hence, CH4 217 concentration in the biomethane in stage I ranged from 85.6% in December to 94.8% in 218 January. During stage II, CH4 concentration varied from 90.4% in March to 97.2% in 219 May, and from 94.5% to 99.0% in stage III (July). Finally, the range of CH4 220 concentrations in stage IV and V were 93.0%-99.6% and 94.5%-96.0%, respectively 221 (Fig. 2). Therefore, the CH4 concentration in the biomethane here produced during 222 Figure 2. Time course of the concentration of CO2 (■) and CH4 (▲) in the upgraded biogas during one diurnal cycle under steady state as a function of the operational months. 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0 20 40 60 80 100 CO2 and CH4 (% vol) January December April March February CO2 and CH4 (% vol) July June May November August CO2 and CH4 (% vol) October September CO2 and CH4 (% vol) Time (h) Time (h) Time (h) Figure 2 Click here to download Figure: Figure 2.docx Table 1. Environmental and operational parameters during the five operational stages. Parameter Stage Month Average ambient temperature (ºC) Average photosynthetic active radiation (µmol m-2 s-1) Nº of sun hours (h) Biomass Productivity (g m-2 d-1) I November 30, 2016 4.4 ± 1.6 170 ± 33 10 ± 1 0.0 December 28, 2016 7.5 ± 4.9 349 ± 119 10 ± 1 January 31, 2017 10.2 ± 3.9 339 ± 174 10 ± 1 February 28, 2017 14.1 ± 6.6 921 ± 237 12 ± 1 II March 29, 2017 14.2 ± 6.2 1213 ± 191 13 ± 1 7.5 April 26, 2017 8.6 ± 1.5 301 ± 138 14 ± 1 May 31, 2017 23.1 ± 5.8 1399 ± 183 15 ± 1 III June 28, 2017 20.3 ± 2.7 297 ± 105 15 ± 1 15.0 July 27, 2017 28.5 ± 6.5 1411 ± 155 15 ± 1 IV August 25, 2017 26.0 ± 6.3 1070 ± 199 13 ± 1 22.5 September 27, 2017 20.7 ± 7.2 1009 ± 237 12 ± 1 V October 26, 2017 18.4 ± 7.0 113 ± 83 10 ± 1 15.0 Table 1 Electronic Annex Click here to download Electronic Annex: Supplemetary Material.docx