Co-existence of flocs and granules in aerobic granular sludge system: Performance, microbial community and proteomics

[Display omitted] •A coexistence system with 9–11% flocs was obtained.•The flocs/granules coexistence system exhibited excellent nutrient removal.•Azoarcus and Thauera were enriched in flocs, while GAO was abundant in granules.•Granules and flocs in coexistence system behaved differently on protein level.•GAO and Thauera were the main contributors to the DEPs between granules and flocs. In aerobic granular sludge (AGS) system, flocs typically coexist with granules. The inherent differences between flocs and granules can result in their different behaviors in the same reactor. In this study, a flocs/granules co-existence system was investigated and differences between the flocs and granules in co-existence system were compared on proteomic level, providing an in-depth insight into co-existence of flocs and granules. Results showed that the co-existence system obtained under 4.5 kg COD m−3 d-1 exhibited excellent nutrient removal (COD: 96–97 %, TN: 98–99 %) with approximately 9–11 % flocs. A comparison of flocs and granules in the co-existence system showed different functional bacteria in flocs and granules. The dominant bacteria in granules was Candidatus_Competibacter, while Azoarcus and Thauera were enriched in flocs. Furthermore, proteomic analysis suggested that the metabolic pathways associated with glycogen synthesis and denitrification were more active in granules than that in flocs, while the differentially expressed proteins (DEPs) related to aromatic compounds and xenobiotics metabolism in flocs were more abundant than that in granules. Species annotation revealed that the DEPs between granules and flocs were mainly contributed by Candidatus_Competibacter and Thauera. The in-depth insight into co-existence of flocs and granules provides important reference for the design, optimization and operation of AGS technology in further applications.