Seasonal variation of biogas upgrading coupled with digestate treatment in an outdoors pilot scale algal-bacterial photobioreactor

Abstract The yearly variations of the quality of the upgraded biogas and the efficiency of digestate treatment were evaluated in an outdoors pilot scale high rate algal pond (HRAP) interconnected to an external absorption column (AC) via a conical settler. CO 2 concentrations in the upgraded biogas ranged from 0.7% in August to 11.9% in December, while a complete H 2 S removal was achieved regardless of the operational month. CH 4 concentrations ranged from 85.2% in December to 97.9% in June, with a limited O 2 and N 2 stripping in the upgraded biogas mediated by the low recycling liquid/biogas ratio in the AC. Biomass productivity ranged from 0.0 g m −2  d −1 in winter to 22.5 g m −2  d −1 in summer. Finally, microalgae diversity was severely reduced throughout the year likely due to the increasing salinity in the cultivation broth of the HRAP induced by process operation in the absence of effluent.