Influence of alkalinity and temperature on photosynthetic biogas upgrading efficiency in high rate algal ponds

Algal-bacterial photobioreactors have emerged as a cost-effective platform for biogas upgrading. The influence on biomethane quality of the inorganic carbon concentration (1500, 500 and 100 mg L-1) and temperature (12 and 35 degrees C) of the cultivation broth was evaluated in a 180 L high rate algal pond (HRAP) interconnected to a 2.5 L absorption column via settled broth recirculation. The highest CO2 and H2S removal efficiencies (REs) from biogas were recorded at the highest alkalinity (CO2-REs of 99.3 +/- 0.1 and 97.8 +/- 0.8% and H2S-REs of 96.4 +/- 2.9 and 100 +/- 0% at 12 and 35 degrees C, respectively), which resulted in CH4 concentrations of 98.9 +/- 0.2 and 98.2 +/- 1.0% at 12 and 35 degrees C, respectively, in the upgraded biogas. At the lowest alkalinity, the best upgrading performance was observed at 12 degrees C (CO2 and H2S-REs of 41.5 +/- 2.0 and 80.3 +/- 3.9%, respectively). The low recycling liquid to biogas ratio applied (0.5) resulted in a negligible O-2 stripping regardless of the alkalinity and temperature, which entailed a biomethane O-2 content ranging from 0 to 0.2 +/- 0.3%.