Microbial Communities in Septic Tank Anaerobic Digesters and Their Interactions with Digester Design and Chemical Environment

Abstract

Anaerobic digester design and operation influences the biomass degradation efficiency performed by complex and diverse microbial communities. Optimum anaerobic digester design and operational parameters in residential on-site wastewater treatment sites (OWTS) establishes physiochemical environments suitable for the growth and stability of the microbial communities responsible for organic waste degradation. A comparative study of the microbial communities and their functional profiles between different OWTS designs and operational parameters have not been done despite their functional importance in residential organic waste removal. Using whole-metagenome shotgun sequencing, microbial community compositions and functions were compared between two digester designs: conventional box septic tanks and septic tanks equipped with a novel closed-conduit tube called the InnerTubeTM. Wastewater was sampled along the length of each digester to explore the microbial community stratification during the anaerobic digestion treatment process. Additionally, the effect of effluent, aerobic recirculating-lines on the digester microbiome was also explored. Physiochemical characteristics in the form of oxygen demand, nitrogen and solids content was used as endpoints and correlated with microbial community and functional gene abundances to explore the microbes driving anaerobic digestion. Conventional digesters were characterized by syntrophic proprionate-oxidizing microbes and acetoclastic methanogens, while InnerTube™ digesters were characterized by syntrophic sulfate-reducing microbes and hydrogenotrophic methanogens. Recirculating digesters were enriched with denitrifying microbial consortia in syntrophy with hydrogenotrophic methanogens. Microbial communities were organized according to hydrolytic, acidogenic, acetogenic, and methanogenic groups along the digester treatment process. Insight into the core microbiome of OWTS can inform bioaugmentation and digester design and operation optimization strategies to improve the treatment of decentralized residential sewage sources.