Mechanisms of persistence and impact of ordinary heterotrophic organisms in aerobic granular sludge

[Display omitted] •The persistence and impact of ordinary heterotrophs (OHO) in AGS were assessed.•OHOs were dominant in the sludge but resulting in long-term stable granules.•C, NH4 and P were removed in accord to OHOs, PAO, GAO and nitrifiers abundances.•Predation, fermentation and other mechanisms contribute to OHO survival in AGS.•The results improve the understanding of microbial competition and stability of AGS. The stability of granules, contaminant removal and microbial structure of an aerobic granular sludge (AGS) process were investigated with a focus on ordinary heterotrophic organisms (OHOs). Long-term stable granules and high removals of COD (97 %), NH4+ (98 %), P (85 %) and total N (77 %) were achieved. Sequencing analyses identified 6.6 % of phosphorus-accumulating organisms in the sludge, concordant with the observed bio-P removal capacity. However, OHOs were the most abundant bacteria in the sludge (70–93 %) without resulting in unstable aggregates. Under current dogmas of microbial competition in activated sludge, it seemed contradictory that OHOs could persist in the long term in the AGS where COD was depleted beginning in the anaerobic phase. Microbial analyses showed that OHOs could survive in granules by micropredation, proteolysis, fermentation and EPS consumption. Heterotrophic-nitrification/ aerobic-denitrification was an active pathway in the AGS. These findings contribute to a better understanding of microbial competition in AGS and its stability.