The sewer microbiome: A missing link to understand community composition and dynamics in wastewater treatment plants

The link between the sewer microbiome and microbial communities in activated sludge (AS) in wastewater treatment plants (WWTPs) is currently poorly known even though the systems are directly interconnected. Moreover, recent studies highlight microbial immigration from wastewater as a key factor in AS community assembly. The sewer system transports bacteria in the wastewater; however, it also harbors a distinct microbiome in biofilms and sediments. We wanted to investigate the sewer microbiome as a potential source of process-critical bacteria in WWTPs and elucidate their immigration patterns. We sampled biofilm, sediment, and sewer wastewater for 1 year in domestic sewers of Aalborg, Denmark with concurrent sampling of influent wastewater and AS from downstream WWTPs. Distinct microbial communities were observed in all sample types, with greater diversity in WWTP-influent and sewer wastewater compared to biofilms and sediments; a natural consequence considering the diverse origins of bacteria in wastewater. The core community of sewer biofilm and sediment included process-critical genera important for phosphorus and nitrogen removal in WWTPs. Moreover, a shift in the bacterial composition of wastewater was seen during transportation, transitioning from a dominance of gut-bacteria in sewer wastewater to a prevalence of biofilm and sediment bacteria in WWTP-influent. In AS, biofilm and sediment bacteria made up >30% of the total abundance, signifying a substantial representation of sewer-originating species. A holistic understanding of the sewer and AS is important, as bacterial immigration, also from sewers, is essential for community structure and dynamics in full-scale WWTPs and may explain the differences observed across the world.