Optimization of photosynthetic biogas upgrading in a demo-scale algal-bacterial photobioreactor

A demo-scale system consisting of an 82 m3 high-rate algal pond (HRAP) coupled with a 0.5 m3 absorption column was operated for 710 days to photosynthetically upgrade biogas generated from the anaerobic digestion of municipal solid waste. Operation began with nutrient supplementation and was progressively adapted to enable a microalgal–bacterial consortium to utilize raw digestate as the sole nutrient source. The liquid-to-gas (L/G) ratio in the absorption column was dynamically adjusted in response to variations in biogas composition. Removal of hydrogen sulphide (H2S) was consistently >99 %, even under substantial fluctuations in inlet concentrations. Average carbon dioxide (CO2) removal over the entire operational period was 90 ± 15 %, increasing to values of up to 98 % after day 660, coinciding with a more stable biogas flowrate and an enhanced biogas composition. Biomass harvesting strategies were implemented to maintain algal concentrations between 1.0 and 2.5 g L−1, supporting process stability. Overall, this study demonstrates the long-term feasibility and robustness of a demo-scale HRAP–absorption column system for biogas upgrading, with additional benefits of nutrient recycling from digestate, reduced dependency on external inputs, and improved economic and environmental sustainability. The results provide valuable operational data to support the industrial scale-up of this integrated technology.