In¯uence of methane enrichment by aeration of recirculated supernatant on microbial activities during anaerobic digestion

A methane enrichment process (MEP) was evaluated that involved air purging of recycled digester contents to strip CO2 and increase biogas methane content. The objective of this work was to determine if the aeration resulted in oxygen inhibition of microbial activities involved in anaerobic digestion of municipal solid waste. To assess the degree of biological perturbation associated with the MEP, the reactor e‚uent was sampled twice while the MEP was operating and twice while it was not operating. Analyses were run on composite samples (representing several di€erent sample ports of the non-mixed reactor) and samples of digester e‚uent entering and leaving the MEP process. The analyses included volatile organic acids (VOA), e‚uent solids content, dehydrogenase activity, speci®c methanogenic activities (SMA), and enzyme-linked immunosorbent assay (ELISA) for methanogenic, sulfate-reducing, and cellulolytic bacterial species. Methane enrichment in these experiments occurred with the methane content of the biogas exceeding 90%. There were no e€ects of the MEP on e‚uent VOA concentration. The MEP had no e€ect on volatile solids (VS) levels of composite samples representing the non-mixed digester contents, however, VS was reduced in e‚uent passing through the MEP. Although MEP had an inhibitory e€ect on anaerobic populations leaving the MEP process, no inhibitory e€ects were observed in measurements of microbial activities in digester samples, including speci®c methane production rate, dehydrogenase, and numbers of speci®c organisms estimated using ELISA techniques.