Improving in-situ biomethanation of sewage sludge under mesophilic conditions: Performance and microbial community analysis

Abstract

This research investigated the application of in-situ biological hydrogen methanation within a continuous stirred tank reactor (CSTR) system under mesophilic conditions, with sewage sludge used as the substrate. Two CSTRs with an effective capacity of 5 L were installed and loaded with inoculum sludge with a volatile solid (VS) concentration of 1.2–1.5 %. They were fed mixed waste sludge with an organic loading rate (OLR) of 1.5 g VS/L and an average sludge retention time (SRT) of 19 days under mesophilic conditions at 37 °C. One of the reactors operated as a control, while the other was injected with H₂ through a microceramic membrane diffuser with a H₂:CO₂ ratio of 4:1. The results of this study revealed that the addition of H₂ and the recirculation of residual hydrogen in biogas led to a substantial increase in the production of methane from 157 L/kg VS to 275 L/kg VS. Increasing the methane content in biogas from 52 % to 78 % yielded an impressive 42.8 % higher methane production rate. Metataxonomic analysis of the microbial community via high-throughput sequencing techniques revealed that the dominant acetoclastic and hydrogenotrophic methanogens were Methanosaeta and Methanoregula, respectively, with greater abundances of both groups in the experimental bioreactor. The dynamics of their activity in both bioreactors were analyzed via qPCR, and the functional genes encoding methyl-coenzyme M reductase (mcrA gene) and hydrogenase Ni-Fe presented comparable changes between RI and RII. By optimizing key operational parameters and closely examining the dynamics of the microbial community, this approach can contribute significantly to sustainable bioenergy solutions while minimizing environmental impact.