Carbon Source-Shaped Differences in Keystone Taxa Driving SNEDPR in Anaerobic/Oxic/Anoxic–Aerobic Granular Sludge (AOA–AGS) Systems Treating Low C/N Wastewater

Anaerobic/oxic/anoxic–aerobic granular sludge (AOA–AGS) is an emerging and promising process for wastewater treatment, performing simultaneous nitrification, endogenous denitrification, and phosphorus removal (SNEDPR). However, as carbon source type influences microbial community and metabolisms, its impacts on the process remain unclear. In this study, the impacts of acetate, propionate, butyrate, and glucose as sole carbon sources on SNEDPR of AOA–AGS systems were investigated. Acetate and propionate achieved superior nitrogen and phosphorus removal efficiency (over 85% on average) compared to butyrate and glucose. Varied carbon sources led to distinct microbial ecology and metabolisms. Acetate preferred PHA metabolisms and had the fastest responses to nitrogen and phosphorus removal, primarily via Candidatus_Accumulibacter and Candidatus_Competibacter. Propionate obtained sufficient reducing power by enhancing the glyoxylate cycle, mainly via Lysobacter. Butyrate had moderate treatment performance, and glucose required additional fermentation steps, mainly completed via Candidatus_Saccharibacteria. Keystone taxa and abundant taxa shaped by each carbon source contributed to systems’ metabolisms. This study demonstrated that carbon sources shape microbial communities via determining pathways, thereby shaping the functions, microbial ecology, and metabolisms of the systems, providing mechanistic insights and practical guidance for rational carbon source selection and management in AOA–AGS systems treating low C/N wastewater.