Influence of biogas supply regime on photosynthetic biogas upgrading performance in an enclosed algal-bacterial photobioreactor

Abstract

Photosynthetic biogas upgrading has recently emerged as an alternative to physical/chemical technologies in order to decrease the energy demand and environmental impact of biomethane production. Despite enclosed photobioreactors (PBRs) provide higher photosynthetic efficiency and CO2 capture capacity than open ponds, the number of studies of PBRs devoted to biogas upgrading is scarce. The effect of biogas supply regime (12 h feeding during the light period vs 24 h feeding during the light and dark periods) on biomethane quality and biochemical biomass composition was evaluated in a 132 L Tubular-PBR interconnected to a 2.5 L biogas scrubbing column operated at high alkalinity. Process operation at a biogas flow rate of 14.4 L d−1 during the light period supported a biomethane composition of 95.9 ± 1.3% v/v of CH4, 2.5 ± 0.8% v/v of CO2, 1.4 ± 0.6% v/v of N2, 0.1 ± 0.1% v/v of O2, and negligible H2S concentrations. On the contrary, CH4 and CO2 concentrations of 88.8 ± 1.6% v/v and 9.3 ± 1.1% v/v, respectively, were recorded when biogas was continuously supplied (28.8 L d−1). The oxygenation capacity of the PBR during the light period was sufficient to oxidize NH4+ to NO3−, which was further used during the dark period to oxidize H2S and prevent microalgae inhibition. Biomass productivity and composition remained constant regardless of the biogas supply regime, with a slight increase in carbohydrate content from 23.6 ± 5.7% (12 h supply) to 30.9 ± 3.5% (continuous supply).