Minimization of biomethane oxygen concentration during biogas upgrading in algal–bacterial photobioreactors

Abstract

Novel operational strategies to reduce the O2 concentration in the upgraded biogas were evaluated in a 180L algal–bacterial photobioreactor interconnected to a 2.5L external absorption column during the simultaneous treatment of diluted anaerobically digested or raw vinasse and biogas upgrading. The lowest biomethane O2 levels (0.7±0.2%) were recorded when raw vinasse was fed directly into the absorption column, which resulted in CO2 and H2S removals from biogas of 72±1% and 100±0%, respectively. Process operation at a Hydraulic Retention Time (HRT) of 7d under the above configuration also supported the maximum total carbon, nitrogen and phosphorus removals of 72±4%, 74±3% and 78±5%, respectively. Biomass productivity ranged from 11.4±1.8 to 13.5±2.2gm−2d−1 during microalgae cultivation in diluted anaerobically digested vinasse, while this productivity increased to 16.9±0.7gm−2d−1 when feeding diluted raw vinasse. The good settling characteristics of the algal–bacterial flocs resulted in an average harvesting efficiency of 98.6±0.5% at a HRT in the settler of 23min, regardless of the treated vinasse. The morphological and molecular characterization of the microbial communities showed a high microalgae diversity and bacterial species richness, regardless of the operational conditions (Shannon–Wiener indices ranging from 2.8 to 3.3).