Strategies for decreasing the O₂ content in the upgraded biogas purified via microalgae-based technology

Microalgae-bacteria consortium based technology using a High Rate Algal Pond (HRAP) interconnected to an Absorption Bubble Column (ABC) has emerged as an environmentally friendly promising option to upgrade biogas. However, the oxygenic photosynthesis of microalgae induces oxygen contamination in upgraded biogas, which could limit its further applications. Several strategies were proposed to favor the oxygen desorption and oxygen uptake in parts and accessories of the upgrading system. The effect of the volumetric ratio liquid recirculation rate/biogas rate (L/G = 5.0, 1.0 y 0.5) was evaluated in conjunction with the application of a novel accessory called Open Trickling Column (OTC). The O₂ content in upgraded biogas was around 2.1%v, attaining CO₂ removal efficiencies around 90%, at L/G ratio of 1.0 during diurnal and nocturnal periods. The inclusion of an OTC at the previous L/G, enhanced 54% the removal of O₂ by stripping and uptake compared with the basal condition. Mass balances of H₂S and methane showed that L/G > 1.0 favored the complete oxidation of H₂S but promoted the loss of methane in dissolved form. Additionally the effect of increasing linear velocity of liquid broth in the lab-scale HRAP (from 15 cm s^-1 to 20 cm s^-1) showed to improve the O₂ stripping with a consequential increase of biomass concentration under steady-state (from 0.7 to 1.4 g L^-1) besides achieving O₂ content in the upgraded biogas around 1.5%v.