Photosynthetic biogas upgrading in algal-bacterial photobioreactors represents a cost-effective and environmentally friendly process for simultaneously removing CO2 and H2S from biogas. Recent research has focused on process optimization for high performance of photosynthetic biogas upgrading. Previous studies revealed that maintaining a high pH in the medium in a biogas absorption column allowed for consistent and significant CO2 removals. Therefore, innovative operating strategies to achieve such conditions are needed to improve biomethane quality. In this study, six operational strategies were evaluated to enhance CO2 mass transfer in a biogas absorption column: ⅰ) internal gas recirculation, ⅱ) direct centrate feeding to the column, an increase of pH in the centrate/digestate ⅲ) with and ⅳ) without ammonium stripping and ⅴ) an addition of liquid nanoparticles (NPs). The simultaneous implementation of strategies ⅲ) and ⅳ) in combination with ⅱ) managed to reduce the CO2 concentration from 29.5% to 5.7% and 2.5% and increase the CH4 concentration in the biomethane from 70% to 91.3% and 94.3%, respectively, which fulfilled with the current legislation on the use of biogas as vehicle fuel. The addition of NPs in the culture broth boosted photosynthetic activity, resulting in an increase in biomass concentration from 1.32 to 3.48 g VSS L-1. However, the higher biomass concentration reduced light penetration in the cultivation broth, which induced a limitation in the photosynthetic activity.
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