Optimization of Photosynthetic Biogas Upgrading in a Demo-Scale Algal-Bacterial Photobioreactor

Abstract

A demo-scale 82 m3 high-rate algal pond (HRAP) coupled to a 0.5 m3 absorption column was operated to photosynthetically upgrade biogas from the anaerobic digestion of municipal solid waste. The system was operated for 710 days, progressively adjusting the nutrient composition to facilitate the acclimation of a microalgal-bacterial consortium to raw digestate as the sole nutrient source. The liquid-to-gas (L/G) ratio in the absorption column was adjusted based on the biogas composition. Despite considerable fluctuations in H2S and CO2 concentrations in the biogas, near-complete H2S removal was consistently achieved. Average CO2 removal was maintained at 90 ± 15%, increasing to 94.7 ± 3% from day 660 onwards, coinciding with the stabilization of digester performance. Biomass harvesting strategies were optimized to maintain microalgal concentrations between 1 and 2.5 g L-1. This study demonstrates the long-term feasibility and robustness of a demo-scale HRAP - absorption column system for biogas upgrading, highlighting its potential for nutrient recycling from digestate, reduced dependency on external inputs, and enhanced economic and environmental sustainability. These results provide valuable data and operational insights to support the scale-up of this integrated technology.

Keywords: Absorption column, Biogas upgrading, Biomethane, Digestate, High-rate algae pond, Microalgae harvesting