[Potential Source Environments for Microbial Communities in Wastewater Treatment Plants (WWTPs) in China]

To better understanding the microbial composition of activated sludge (AS), unique OTUs and their indicators were examined and SourceTracker was used to identify possible source environments for AS communities to thrive in 15 cities across China, based on 16S rRNA gene sequencing. This may help reveal the underlying mechanisms of AS community composition formations in various areas. The results indicated that the dominant phyla in AS in each city were similar; Proteobacteria, Bacteroidetes, and Chloroflexi made up 94.53%-97.04% of the whole microbial communities. Geographic location and environmental conditions were identified as having an effect on microbial compositions. Cities located toward to the edge of China or along the coast tended to harbor more unique OTUs. Moreover, the composition of indicators in wastewater treatment plants (WWTPs) near the sea were more similar, while no indicators were detected in WWTPs in Chengdu and Chongqing. Source environments for AS microbial communities also depended on location and environmental conditions. Overall, microbes from 'plant' and 'soil' sources accounted for a large proportion of the AS microbial communities in each city, while bacteria from 'marine' sources were only detected in coastal AS communities. Cities with more rivers and lakes tended to have more bacteria originating from 'fresh water' sources. Since most of the WWTPs in China play a role in treating industrial wastewater, industrial wastewater was also an important source environment. However, few bacteria originating from the human digestive system were identified in the AS communities. Moreover, a large proportion of bacteria (36%-70%) originated from unknown sources not included in the source database, such as sewage infrastructure, indicating that the source database will need to be expanded in the future.