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. Please find enclosed our original unpublished paper "Influence of the gas-liquid flow configuration in the absorption column on photosynthetic biogas upgrading in algalbacterial photobioreactors"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 CO 2 -H 2 S 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, N 2 and O 2 stripping from the recycling cultivation broth to the upgraded biogas often results in CH 4 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 b...

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