Effects of influent immigration and environmental factors on bacterial assembly of activated sludge microbial...

The functional mechanism of microbial assembly of activated sludge (AS) in urban wastewater treatment plants (UWTPs) remains unclear. A comprehensive quantitative evaluation of the contribution of influent immigration and environmental factors to AS community composition requires investigation. In this study, the microbial characteristics of six full-scale UWTPs with different influent compositions and environmental factors (altitude, temperature, dissolved oxygen (DO), pH, chemical oxygen demand (COD), total nitrogen (TN), ammonia nitrogen (NH₄⁺-N), and total phosphorus (TP)) were analyzed to determine the main forces affecting the bacterial assembly of AS microbial communities. Abundant and core taxa were screened out based on the abundance and frequency of operational taxonomic units (OTUs) occurrence in all samples. Abundant OTUs (18.7% occurrence) accounted for 87.7% of the total 16S rRNA sequences, while rare OTUs (71.7% occurrence) accounted for only 7.8% of the total 16S rRNA sequences. A total of 135 OTUs were identified as core taxa, accounting for 14.6-26.2% of the total reads, of which 83 OTUs belonged to abundant taxa. The richness and uniformity of the influent community were significantly lower than those of the AS system. The community composition in influent varied from that in AS. Moreover, about 89.7% (86.5% of 16S rRNA sequences) OTUs in AS samples showed positive growth rates, indicating that immigration of influent communities had a limited effect on the microbial composition of AS. Redundancy analysis (RDA) combined with co-occurrence network showed that the bacterial assembly of microbial communities was significantly correlated with altitude, pH, and TN (P < 0.05), and these three parameters could explain 23.3%, 21.1%, and 17.7% of the bacterial assembly of AS microbial communities in UWTPs, respectively.