Influence of biogas flow rate on biomass composition during the optimization of biogas upgrading in microalgal-bacterial processes

Abstract

The influence of biogas flow rate (0, 0.3, 0.6, and 1.2 m3 m–2 h–1) on the elemental and macromolecular composition of the algal-bacterial biomass produced from biogas upgrading in a 180 L photobioreactor interconnected to a 2.5 L external bubbled absorption column was investigated using diluted anaerobically digested vinasse as cultivation medium. The influence of the external liquid recirculation/biogas ratio (0.5 < L/G < 67) on the removal of CO2 and H2S, and on the concentrations of O2 and N2 in the upgraded biogas, was also evaluated. A L/G ratio of 10 was considered optimum to support CO2 and H2S removals of 80% and 100%, respectively, at all biogas flow rates tested. Biomass productivity increased at increasing biogas flow rate, with a maximum of 12 ± 1 g m–2 d–1 at 1.2 m3 m–2 h–1, while the C, N, and P biomass content remained constant at 49 ± 2%, 9 ± 0%, and 1 ± 0%, respectively, over the 175 days of experimentation. The high carbohydrate contents (60–76%), inversely correlated to biogas flow rates, would allow the production of ≈100 L of ethanol per 1000 m3 of biogas upgraded under a biorefinery process approach.