Influence of the gas-liquid flow configuration in the absorption column on photosynthetic biogas upgrading in algal-bacterial photobioreactors

Elsevier Editorial System(tm) for Bioresource Technology Manuscript Draft Manuscript Number: Title: Influence of the gas-liquid flow configuration in the absorption column on photosynthetic biogas upgrading in algal-bacterial photobioreactors Article Type: Original research paper Keywords: Algal-bacterial photobioreactor; biogas upgrading; bio-methane; nutrients recovery; digestate Corresponding Author: Mr. Raul Munoz, PhD Corresponding Author's Institution: Valladolid University First Author: Alma Toledo-Cervantes, PhD Order of Authors: Alma Toledo-Cervantes, PhD; Cindy Madrid-Chirinos, BSc; Sara Cantera, MSc; Raquel Lebrero, PhD; Raul Munoz, PhD Abstract: The potential of an algal-bacterial system consisting of a high rate algal pond (HRAP) interconnected to an absorption column (AC) via recirculation of the cultivation broth for the upgrading of biogas and digestate was investigated. The influence of the gas-liquid flow configuration in the AC on the photosynthetic biogas upgrading process was assessed. AC operation in a co-current configuration enabled to maintain a biomass productivity of 15 g m-2 d-1, while during countercurrent operation biomass productivity decreased to 8.7 ± 0.5 g m-2 d-1 as a result of trace metal limitation. A bio-methane composition complying with most international regulatory limits for injection into natural gas grids was obtained regardless of the gas-liquid flow configuration. Furthermore, the influence of the recycling liquid to biogas flowrate (L/G) ratio on bio-methane quality was assessed under both operational configurations obtaining the best composition at an L/G ratio of 0.5 and co-current flow operation. Suggested Reviewers: Leslie Meier PhD Universidad de La Frontera, Chile [email protected] Expert in wastewater treatment and Biogas upgrading using microalgae cultures Cheng Yan Chen PhD School of Environmental Studies, China [email protected] Expert in digestate treatment and Biogas upgrading using microalgae cultures Simon Murray PhD Queen's University Belfast, UK [email protected] Expert in Biogas upgrading Enrica Uggetti PhD Universitat Politècnica de Catalunya, Spain [email protected] Expert in wastewater and sludge treatment, low cost technologies, microalgae, anaerobic digestion and biogas. Ashok Pandey Centre of Innovative and applied Bioprocessing (CIAB) Mohali, Punjab, India Dear Editor-in-Chief Please find enclosed our original unpublished paper “Influence of the gas-liquid flow configuration in the absorption column on photosynthetic biogas upgrading in algalbacterial photobioreactors” co-authored by Alma Toledo-Cervantes, Cindy Madrid-Chirinos, Sara Cantera, Raquel Lebrero and Raúl Muñoz. All authors are aware of the ethics policy of Bioresource Technology Journal, declare no conflict of interest and accept responsibility for the present manuscript. The manuscript is submitted for publication in Bioresource Technology for the first time, considering that it is the best-suited journal for the research area of the present work, more specifically Biological waste treatment: Environmental bioengineering (20.100). Photosynthetic biogas upgrading coupled with nutrient removal from digestate represents a competitive and environmentally friendly technology to conventional physical-chemical technologies for biogas upgrading. This innovative technology, here evaluated at pilot scale, consisted of a high rate algal pond (HRAP) treating digestate interconnected to a CO2-H2S absorption column (AC) via recirculation of the HRAP cultivation broth for biogas scrubbing. Preliminary studies in our lab have consistently showed that despite the high potential of photosynthetic biogas upgrading, N2 and O2 stripping from the recycling cultivation broth to the upgraded biogas often results in CH4 concentrations < 95 % (the minimum concentration for biomethane injection into natural gas grids in most EU countries). Thus, an optimization of biogas scrubbing in the AC of this photosynthetic biogas upgrading system is needed in order to obtain a bio-methane complying with the quality standards for injection into natural gas grids. This research assessed the influence of the gas/liquid flow configurations (co-current and counter-current) in the AC on bio-methane quality and nutrient recovery from a real digestate in the form of algal-bacterial biomass. The influence of the liquid recycling to biogas flowrate (L/G) ratio on bio-methane quality was also tested under both gas-liquid flow configurations in order to minimize both O2 and N2 content in the bio-methane. Additionally, an innovative process design was evaluated by interconnecting an external coagulation-flocculation tank to the HRAP, which allowed obtaining a biomass productivity of 15 g m-2 d-1 (thus maximizing the recovery of C, N, P and S in the form of algal-bacterial biomass) while minimizing the effluent to be discharged. Process operation in a co-current configuration enabled to maintain this biomass productivity, while counter-current operation decreased biomass productivity likely due to a sulphur-mediated heavy metal deprivation. A bio-methane composition complying with most international regulatory limits for injection into natural gas grids was obtained regardless of the gas/liquid flow configuration. Furthermore, an optimal L/G ratio of 0.5 under co-current flow operation in the AC allowed obtaining a bio-methane composition of 0.8 ± 0.0 % CO2, 0.01 ± 0.0 % O2, 0.7 ± 0.2 % N2 and 98.5 ± 0.2 % CH4. We look forward to your evaluation. Best regards, Alma Toledo-Cervantes Raúl Muñoz Cover Letter Graphical Abstract (for review) Highlights  EU standard bio-methane was obtained regardless of the gas-liquid flow configuration  Optimum bio-methane composition was achieved at a L/G=0.5 under co-current operation  Counter-current operation decreased biomass productivity and the cultivation broth pH  High C, N, P and S recoveries were achieved by decoupling the HRT from the SRT *Highlights (for review) 1 Influence of the gas-liquid flow configuration in the absorption column on 1 photosynthetic biogas upgrading in algal-bacterial photobioreactors 2 Alma Toledo-Cervantes1, Cindy Madrid-Chirinos1, Sara Cantera1, Raquel Lebrero1, 3 Raúl Muñoz1* 4 5 1.-Department of Chemical Engineering and Environmental Technology, University of 6 Valladolid, Dr. Mergelina s/n., Valladolid 47011, Spain. 7 8 *Corresponding author: [email protected] 9 10 *Manuscript Click here to view linked References 2 Abstract 11 The potential of an algal-bacterial system consisting of a high rate algal pond (HRAP) 12 interconnected to an absorption column (AC) via recirculation of the cultivation broth 13 for the upgrading of biogas and digestate was investigated. The influence of the gas-14 liquid flow configuration in the AC on the photosynthetic biogas upgrading process was 15 assessed. AC operation in a co-current configuration enabled to maintain a biomass 16 productivity of 15 g m-2 d-1, while during counter-current operation biomass 17 productivity decreased to 8.7 ± 0.5 g m-2 d-1 as a result of trace metal limitation. A bio-18 methane composition complying with most international regulatory limits for injection 19 into natural gas grids was obtained regardless of the gas-liquid flow configuration. 20 Furthermore, the influence of the recycling liquid to biogas flowrate (L/G) ratio on bio-21 methane quality was assessed under both operational configurations obtaining the best 22 composition at an L/G ratio of 0.5 and co-current flow operation. 23 24 Keywords: Algal-bacterial photobioreactor; biogas upgrading; bio-methane; nutrients 25 recovery; digestate. 26 27 3 1. Introduction 28 Anaerobic digestion is a sustainable platform technology to reduce the environmental 29 impact of biodegradable organic wastes. During anaerobic digestion, ~20-95 % of this 30 residual organic matter is biologically converted into biogas (consisting of 50-70 % of 31 CH4, 30-50 % of CO2 and trace gases such as H2S, H2 and N2 (Appels et al., 2011)) and 32 digestate (a nutrient rich liquid effluent) (Möller and Müller, 2012). Biogas is a 33 renewable energy source typically used in industry for heat and power generation or as 34 natural gas substitute after upgrading. Nowadays, the high energy and chemicals 35 consumption associated to conventional physical-chemical technologies for biogas 36 upgrading (to a CH4 content of at least 95% as required by most international bio-37 methane standards) limits their environmental and economic sustainability (Muñoz et 38 al., 2015). On the other hand, digestate is applied in agriculture as biofertilizer, although 39 environmental problems such as ammonia emission, nitrate leaching or phosphorus soil 40 saturation might derive from inappropriate digestate handling, storage and application 41 (Holm-Nielsen et al., 2009). 42 43 In this context, photosynthetic biogas upgrading coupled to nutrient removal from 44 digestate can enhance the sustainability and economic viability of biogas and digestate 45 management (Bahr et al., 2014; Posadas et al., 2015; Serejo et al., 2015). During 46 photosynthetic biogas upgrading, microalgae use light energy to fix the CO2 from 47 biogas via photosynthesis, while sulphur-oxidizing bacteria oxidize H2S to sulphate 48 using the O2 photosynthetically produced. Both microalgal and bacterial growth can be 49 supported by the N and P contained in the digestate, with the subsequent reduction of its 50 eutrophication potential. The algal–bacterial biomass produced during photosynthetic 51 biogas upgrading can be used as slow-release bio-fertilizer or as a feedstock for biofuel 52 4 production, thus contributing to improve the economic and environmental viability of 53 this innovative technology (Posadas et al., 2014). 54 Despite the high potential of photosynthetic biogas upgrading, N2 and O2 stripping from 55 the recycling cultivation broth to the upgraded biogas often results in CH4 56 concentrations < 95 %. (Muñoz et al., 2015). N2 and O2 are often present in the 57 recycling cultivation broth at concentrations of ~14 mg-N2 L-1 and > 8 mg-O2 L-1 as a 58 result of its direct contact with the atmosphere (in open HRAPs) and the intensive 59 microalgal photosynthetic activity in the photobioreactor, respectively (Toledo-60 Cervantes et al., 2016). In fact, the O2 stripped out from the cultivation broth is a 61 function of the biomass productivity, which is directly linked to the irradiation 62 impinging into the cultivation broth. All studies evaluating the performance of this 63 technology to date were conducted under low light intensities (75-420 μmol m-2 s-1), 64 which could have partially biased the results obtained in terms of final bio-methane 65 quality (Posadas et al., 2015; Serejo et al., 2015; Toledo-Cervantes et al., 2016). On the 66 other hand, the liquid to biogas flow (L/G) ratio in the external absorption column (AC) 67 has been recently identified as one of the key operational parameters determining the 68 final composition of bio-methane. Unfortunately, the influence of the biogas/recycling 69 liquid flow configuration in the AC (counter-current vs co-current) on bio-methane 70 composition has not been yet systematically assessed. Meier et al. (2015) operated a 71 counter-current flow bubble column interconnected to a stirred tank photobioreactor and 72 reported a bio-methane O2 content of ~1.2 % at a L/G of 6.3. Likewise, bio-methane O2 73 concentrations ranging from 0.7 to 1.2 were recorded by Posadas et al. (2015) in a 74 HRAP interconnected to a bubble column operated at co-current flow. These O2 75 concentrations were significantly higher than the limit of 0.3 % required by most 76 international regulations for bio-methane injection into natural gas networks, which 77 11 productivity, P concentration in the cultivation broth remained below the detection limit 228 of the spectrophotometric method used at both operational configurations. The low 229 phosphorous content measured in the biomass (0.005 g-P/g-biomass) and the ability of 230 microalgae to accumulate energy in the form of polyphosphate suggested a total P 231 recovery during both steady states (Alcántara et al., 2015). 232 The carbon mass balance conducted estimated that 88 ± 4 % of the carbon supplied 233 (considering both the inorganic and organic carbon in the digestate and the C-CO2 234 absorbed in the AC) was recovered as biomass during stage I. Carbon recovery 235 decreased in stage II down to 57 ± 5 % due to the above mentioned decrease in biomass 236 productivity. In contrast, IC-RE significantly increased (t-test, ≤ 0.0 ) (Figure 2a) from 237 90 ± 1.1 % to 95 ± 0.5 % mainly due to the enhanced CO2-stripping (38.6 ± 5 %) 238 mediated by the decrease in pH (Table 1). Additionally, the sulphur mass balance 239 estimated that 38 and 24 % of the sulphur contained in the biogas was assimilated into 240 biomass during stage I and II, respectively (0.007 g-S/g-biomass). 241 242 On the other hand, the alkaline conditions prevailing during HRAP-operation (pH >9.5), 243 together with the high average IC (1550 ± 471 mg L-1) and sulphate (539 ± 113 mg L-1) 244 concentrations, promoted the dominance of the unialgal culture of Mychonastes 245 homosphaera (Skuja) Kalina & Puncochárová. The morphological identification of this 246 microalga was confirmed by the DGGE analysis with observation of bands 2 and 3 247 (Figure S2, Supplementary information), which belonged to the genus Chlorophyta and 248 were related to Chlorella species. The DGGE analysis also revealed 12 bands belonging 249 to four different phyla: Cyanobacteria/Chloroplast (4 bands), Proteobacteria (5 bands), 250 Chloroflexi (2 bands) and Bacteroidetes (1 band) (Table S1, Supplementary material). 251 Aerobic bacteria from the genus Sphingomonas (band 11) and Sphingobacteriales order 252 12 (band 12) likely supported the biodegradation of the organic matter contained in the 253 digestate (Shokrollahzadeh et al., 2008; Ye and Zhang, 2013) 254 255 Finally, the low effluent flow rate (0.5 L d-1), together with the low N and P effluent 256 concentrations recorded, entailed a low environmental impact in terms of wastewater 257 discharge to the environment. At this point it should be also stressed that the 258 coagulation-flocculation process implemented in the interconnected tank was efficient 259 at removing biomass from the cultivation broth to an average effluent TSS 260 concentration of 28 ± 4 mg L-1, which complies with the limit established by the 261 European Union legislation (European Directive 91/271/CEE). 262 263 3.2 Influence of gas-liquid flow configuration on biogas upgrading performance 264 Conventional water scrubbing for biogas upgrading relies on the contact between the 265 biogas flowing upwards through a packed absorption column and a pressurized water 266 stream trickling down in a counter-current mode. The column is typically filled with 267 random packing materials in order to increase the specific gas-liquid contact area and 268 thus maximize the gas-liquid mass transfer. State of the art water scrubbers can provide 269 a bio-methane with a CH4 content of 96-98 % (Ryckebosch et al., 2011). In contrast, the 270 absorption columns coupled to photobioreactors have been mostly operated at co-271 current flow with no packing materials to avoid biomass clogging (Toledo-Cervantes et 272 al., 2016), with only one experimental study conducted using a counter-current flow 273 configuration (Meier et al., 2015). The study here reported constitutes, to the best of our 274 knowledge, the first systematic comparison addressing the influence of the biogas-275 recycling liquid flow configuration on bio-methane composition. Statistically different 276 (t-test, p≤ 0.05) CO2-REs of 98.8 ± 0.8 % (co-current) and 96.9 ± 1.6 % (counter-277 13 current) were recorded during stages I and II, respectively, while statistically similar 278 REs ~100 % were obtained for H2S. The CO2 and H2S REs observed under a co-current 279 configuration were in agreement with those reported by Toledo-Cervantes et al. (2016). 280 The lower CO2-REs recorded under counter-current flow operation were attributed to 281 the decrease in the pH of the cultivation broth from 10.2 to 9.5 (Table 1), mediated by 282 the decrease in microalgal photosynthetic activity (See section 3.1). These results 283 confirmed that CO2 removal highly depends on the photosynthetic activity of 284 microalgae in spite of the high buffer capacity of the digestate. Furthermore, the nearly 285 complete H2S removal observed at both configurations highlighted the robustness of 286 this biological technology for the abatement of H2S from biogas. 287 288 Table 2 shows the bio-methane composition under co-current and counter-current flow 289 configurations. The CO2 and CH4 contents of the bio-methane were statistically 290 different, with a higher CH4 content under a co-current flow configuration (96.2 ± 0.7 291 %) (Figure 3a). The bio-methane obtained in both operational stages presented a low 292 oxygen content due to the active oxygen demand resulting from the oxidation of H2S to 293 sulphate (Figure 3b). No significant differences in O2 and N2 content were observed at 294 both operational configurations. Toledo-Cervantes et al. (2016) reported a similar N2 295 concentration (2.4 ± 0.2 %) but a lower O2 content (0.03 ± 0.04 %) in the biogas 296 upgraded in a similar experimental set-up operated under co-current flow configuration 297 at a L/G=1. The lower O2 content observed by these authors was likely due to the lower 298 irradiance (420 ± 0 μmolm-2 s-1) and biomass productivity (7.5 g m -2 L-1) used in 299 their experimentation, which entailed a lower DO concentration in the cultivation broth 300 (9.6 ± 0.4 mg O2 L-1). Moreover, since the same L/G ratio was applied at both 301 operational configurations, the nitrogen content in the bio-methane (stripped out from 302 14 the cultivation broth) was statistically similar (Figure 3b). 303 304 The bio-methane obtained under both gas-liquid flow configurations complied with the 305 regulatory limits of most international standards for bio-methane injection into natural 306 gas grids regardless of the operational configuration. Nonetheless, several operational 307 problems were observed during counter-current flow operation. First, elemental sulphur 308 accumulation at the bottom of the AC resulted in diffuser clogging, while biomass 309 accumulation at the top of the AC caused the obstruction of pipelines. The elemental 310 sulphur accumulation observed under counter-current configuration was attributed to 311 the stripping and gradual DO consumption along the AC, which resulted in a low DO 312 concentration at the bottom of the AC where biogas was sparged. Therefore, the 313 dissolved H2S at the bottom of the column was not completely oxidized to sulphate but 314 to elemental sulphur, which accumulated at the surface of the diffuser and the 315 o o ol m ’ walls. The limited H2S oxidation at the bottom of the AC was also 316 responsible of the trace metal precipitation hypothesized in section 3.1. 317 318 3.3 Influence of the L/G ratio on bio-methane composition under co-current and 319 counter-current operation 320 The recycling liquid to biogas ratio constitutes as a key operational parameter 321 determining the final quality of bio-methane in algal-bacterial photobioreactors 322 (Toledo-Cervantes et al., 2016). Theoretically, an increased overall concentration 323 gradient and volumetric mass transfer coefficient were expected under counter-current 324 flow operation. Nonetheless, the decrease in pH and biomass productivity during stage 325 II counterbalanced the beneficial mass transfer effects of counter-current flow 326 operation. In this context, a systematic evaluation of the influence of the L/G ratio on 327 15 bio-methane composition was carried under both gas-liquid flow configuration. This 328 experimentation was carried out from days 95 to 98 and therefore it was not biased by 329 the above-mentioned secondary effects of the counter-current flow operation on the 330 cultivation broth (i.e. lower biomass productivity and pH) and allowed to minimize the 331 O2 and N2 content in the bio-methane without compromising the CO2 removal. 332 333 Table 3 shows the REs recorded under both gas-liquid flow configurations. The CO2-334 REs at the L/G ratios tested were significantly different under co-current and counter-335 current flow operation (t-test, p≤ 0.05), except for the REs obtained under counter-336 current flow operation at a L/G ratio of 1 and 0.8. The CO2-REs observed at a L/G ratio 337 of 1 were in agreement with those previously reported by Toledo-Cervantes et al. 338 (2016) (98.8 ± 0.2 %) using a similar experimental fed with synthetic digestate. The 339 results obtained also indicated that the CO2-REs increased at increasing the L/G ratio up 340 to 1, likely due to the higher carry over capacity when increasing the recycling liquid 341 rate. As expected, higher CO2-REs were observed under counter-current flow operation 342 (Table 3) due to the enhanced overall concentration gradient and mass transfer 343 coefficient (kLa-CO2), the latter mediated by an extended gas-liquid contact time. In 344 contrast, the CO2-REs recorded at a L/G ratio of 0.3 were lower at both operational 345 configurations (70.3 ± 1.0 % and 60.4 ± 1.9 % under co-current and counter-current 346 flow operation, respectively). These low CO2-REs were attributed to the decrease in pH 347 in the recycling cultivation broth from 10 to 8.5 ± 0.1 induced by the increase in the 348 liquid HRT in the AC. No significant differences were observed in the H2S-REs under 349 both operational configurations regardless of the L/G ratio, which confirmed the 350 robustness of this technology in terms of H2S. 351 352 16 Counter-current flow operation involved higher mass transfer rates, which resulted in 353 higher O2 and N2 desorption rates from the cultivation broth concomitant with enhanced 354 CO2 removals in the AC, but slightly lower CH4 concentrations than under co-current 355 operation (Figure 4). However, the two gas-liquid flow configurations tested allowed 356 obtaining a bio-methane complying with most international regulations. Under co-357 current flow operation at a L/G of 0.5, a bio-methane composition of 0.8 ± 0.0 % of 358 CO2, 0.01 ± 0.0 % of O2, 0.7 ± 0.2 % of N2 and 98.5 ± 0.2 % of CH4 was obtained, 359 which to the best of our knowledge constitutes the best composition ever reported for 360 any stand-alone biological biogas upgrading technology. 361 362 4. Conclusions 363 Microalgae photosynthetic activity was identified as a key process parameter 364 determining both the quality of bio-methane and the extent of the nutrients removal 365 mechanisms. Process design here evaluated, allowed decoupling biomass productivity 366 from the HRT, which overcame the light limitation problem associated with the use of 367 high strength digestates. Despite counter-current flow operation supported a more 368 efficient gas-liquid mass transfer, both the enhanced N2/O2 stripping and the lower 369 microalgal activity observed, resulted in a lower bio-methane quality. However, the bio-370 methane composition achieved under both operational configurations complied with the 371 regulatory limits required for its injection into natural gas grids. 372 373 Acknowledgments 374 This research was supported by MINECO and the European Union through the FEDER 375 program (CTM2015-70442-R and Red Novedar), the Regional Government of Castilla 376 y León (Project VA024U14 and UIC 71) and INIA (RTA2013-00056-C03-02). 377 17 CONACyT-México is also gratefully acknowledged for the Postdoctoral grant of Alma 378 Toledo (No. Reg: 237873). Authors acknowledge Saúl Blanco Lanza for the taxonomic 379 identification of microalgae. 380 381 References 382 [1] Alcántara, C., Fernández, P.A., García-Encina, R. Muñoz. Mixotrophic 383 metabolism of Chlorella sorokiniana and algal-bacterial consortia under extended dark-384 light periods and nutrient starvation. Appl. Microbiol. Biotechnol. 2015, 99 (5), 2393-385 2404 386 [2] Appels, L., Lauwers, J., Degrève, J., Helsen, L., Lievens, B., Willems, 387 K., Impe, J.V., Dewil, R. 2011. Anaerobic digestion in global bio-energy production: 388 Potential and research challenges. Renew. Sust. Energ. Rev. 2011, 15 (9), 4295-4301 389 [3] Bahr, M., Díaz, I., Domínguez, A., González Sánchez, A., Muñoz, R. 390 Microalgal-biotechnology as a platform for an integral biogas upgrading and nutrient 391 removal from anaerobic effluents. Environ. Sci. Technol. 2014, 48, 573-581 392 [4] Coder, D.M., Goff, L.J. The host range of the Chlorellavorous bacterium 393 (“Vampirovibrio chlorellvorus”). J. Phycol. 1986, 22 (4), 543-546 394 [5] de Godos, I., Guzman, H.O., Soto, R., García-Encina, P.A., Becares, E., 395 Muñoz, R., Vargas, V.A. Coagulation/flocculation-based removal of algal–bacterial 396 biomass from piggery wastewater treatment, Bioresour. Technol. 2011, 102 (2), 923-397 927 398 [6] Eaton A.D., Clesceri L.S., Greenberg A.E. Standard methods for the 399 examination of water and wastewater. 21 st edition. 2005. American Public Health 400 Association/American Water Works Association/ Water Environment Federation 401 [7] European Directive 91/271/CEE on discharge of domestic wastewaters, 402 18 1991 403 [8] Holm-Nielsen, J.B., Seadi, T.A., Oleskowicz-Popiel, P. The future of 404 anaerobic digestion and biogas utilization. Bioresour. Technol. 2009, 100 (22), 5478-405 5484 406 [9] Möller, K., Müller, T. Effects of anaerobic digestion on digestate nutrient 407 availability and crop growth: A review. Eng. Life Sci. 2012, 12 (3), 242–257 408 [10] Muñoz, R., Meier, L., Díaz, I., Jeison, D. A review on the state-of-the-art 409 of physical/chemical and biological technologies for biogas upgrading. Rev Environ Sci 410 Biotechnol. 2015, 14 (4), 727-759 411 [11] Posadas, E., Bochon, S., Coca, M., García-González, M.C., García-412 Encina, P.A., Muñoz, R. Microalgae-based agro-industrial wastewater treatment: a 413 preliminary screening of biodegradability. J. Appl. Phycol. 2014, 26, 2335-2345 414 [12] Posadas, E., Serejo, M., Blanco, S., Pérez, R., García-Encina, P.A., 415 Muñoz, R. Minimization of bio-methane oxygen concentration during biogas upgrading 416 in algal–bacterial photobioreactors. Algal Research. 2015, 12, 221-229 417 [13] Ryckebosch, E., Drouillon, M. and Vervaeren, H., Techniques for 418 transformation of biogas to biomethane. Biomass bioenergy, 2011, 35 (5), 1633-1645 419 [14] Serejo, M., Posadas, E., Boncz, M., Blanco, S., García-Encina, PA., 420 Muñoz, R. Influence of biogas flow rate on biomass composition during the 421 optimization of biogas upgrading in microalgal-bacterial processes. Environ. Sci. 422 Technol. 2015, 49, 3228-3236 423 [15] Shokrollahzadeh, S., Azizmohseni, F., Golmohammad, F., Shokouhi, H. 424 and Khademhaghighat, F. Biodegradation potential and bacterial diversity of a 425 petrochemical wastewater treatment plant in Iran. Bioresour. Technol. 2008, 99(14), 426 6127-6133. 427 19 [16] Soo, R.M., Woodcroft, B.J., Parks, D.H., Tyson, G.W. Hugenholtz, P. 428 Back from the dead; the curious tale of the predatory cyanobacterium Vampirovibrio 429 chlorellavorus. Peer J. 2015, 3, e968 430 [17] Tippayawong, N., Thanompongchart, P. Biogas quality upgrade by 431 simultaneous removal of CO2 and H2S in a packed column reactor. Energy. 2010, 35 432 (12), 4531-4535 433 [18] Toledo-Cervantes, A., Serejo, M., Blanco, S., Pérez, R., Lebrero, R., 434 Muñoz, R. Photosynthetic biogas upgrading to bio-methane: boosting nutrient recovery 435 via biomass productivity control. Algal Research. 2016, 17, 46-52 436 [19] Uggetti, E., Sialve, B., Latrille, E., Steyer, J-P. Anaerobic digestate as 437 substrate for microalgae culture: The role of ammonium concentration on the 438 microalgae productivity, Bioresour. Technol. 2014, 152, 437-443 439 [20] Ye, L. and Zhang, T. Bacterial communities in different sections of a 440 municipal wastewater treatment plant revealed by 16S rDNA 454 pyrosequencing. 441 Appl. Microbiol. Biot. 2013, 97(6), 2681-2690. 442 20 Figure caption. 443 444 Figure 1. Time course of the total suspended solids concentration in the HRAP. The 445 vertical line indicates the change in the gas-liquid flow configuration in the AC. 446 447 Figure 2. a) Removal efficiencies of chemical oxygen demand (COD), ammonium 448 (NH4+), total nitrogen (TN), inorganic carbon (IC), sulphate (SO4-2) and phosphate (PO4- 449 3) in the HRAP and b) effluent concentrations of nitrate (N-NO3-), nitrite (N-NO2-) and 450 sulfate (SO4-2) under co-current (black bars) and counter-current (white bars) gas-liquid 451 flow operation. Vertical lines represent standard deviations from replicate 452 measurements under steady state operation. All REs were significantly different (t-453 student test, p<0.05) except those of NH4+ and PO4-3. 454 455 Figure 3. Time course of the concentration of a) CO2 (○) CH4 (), and b) O2 (□) 456 and N2 (◊) o-methane. Vertical lines represent standard deviations from 457 replicate measurements. 458 459 Figure 4. Influence of the recycling liquid to biogas ratio on the concentration of a) O2, 460 b) N2, c) CH4 and d) CO2 in the bio-methane under co-current (□) and counter-current 461 (○) gas-liquid flow operation. Vertical lines represent standard deviations from replicate 462 measurements. 463 Table 3. Influence of the L/G ratio on the carbon dioxide and hydrogen sulphide removal efficiencies under co-current and counter-current flow configurations in the absorption column. L/G ratios RE at co-current flow (%) RE at counter-current flow (%) CO2 H2S CO2 H2S 1 98.8 ± 0.0 100 a 99.2 ± 0.1 b 99.2 ± 1.4 a 0.8 98.3 ± 0.0 100 a 98.9 ± 0.2 b 96.1 ± 3.6 a 0.5 97.3 ± 0.1 100 a 98.1 ± 0.1 98.3 ± 1.4 a 0.3 70.3 ± 1.0 98.3 ± 2.4 a 60.4 ± 1.9 100 a *Same letter means no significantly different (t-test, P≤ 0.05) Electronic Annex Click here to download Electronic Annex: Supplementary information.docx