Differences in responses of activated sludge to nutrients-poor wastewater: Function, stability, and microbial community

Whether activated sludge (AS) systems exhibit different patterns in response to different levels and types of nutrient-poor wastewater is inconclusive, and the microbiological mechanisms are unclear. This study evaluated comparatively the impacts of nitrogen (N) and phosphorus (P)-poor wastewater on AS. The results showed that AS could adapt to N (C/N of 100/2) and P-limited (C/P of 100/0.3) wastewater, and N deficiency (C/N of 100/0) reduced the abundance of denitrifying phosphorus-accumulating bacteria and P removal, while P deficiency (C/P of 100/0) reduced the abundance of nitrifying bacteria and N removal. In addition, both N and P deficiency led to a decrease in AS aggregation, and N deficiency reduced sludge settling performance. Although the reduction in the AS aggregation was related to the massive secretion of exopolysaccharide (PS) in N and P deficiency, the elevated PS content due to N deficiency was associated with Candidatus_Competibacter and unclassified_f_Rhodo-bacteraceae, and the elevated PS content due to P deficiency was associated with Defluviicoccus, Flavobacterium, Phreatobacter, Taonella and Candidatus_Contendobacter. N and P deficiency also resulted in different bacterial metabolism. The pathways related to energy metabolism were significantly up-and down-regulated in N-and P -poor system, leading to the enrichment of different types of glycogen-accumulating organisms (Candida-tus_Competibacter and Defluviicoccus). Finally, the Variance Partitioning Analysis (VPA) revealed that N-poor system depended on the compound impact of multiple environmental factors, while the influence of single factor is more obvious in P-poor system. The study provides insights of the responses of AS to N and P poor wastewater from a microbial perspective, which can be useful for further applications of activated sludge.