Overlooked pathways of endogenous simultaneous nitrification and denitrification in anaerobic/aerobic/anoxic sequencing batch reactors with organic supplementation

•Organic supplementation promoted nitrification and denitrification in the A/O/A-SBR.•Endogenous heterotrophic nitrification dominated in the enhanced nitrification.•Batch tests verified endogenous heterotrophic nitrification driven by PHAs.•Novel SND pathway based on endogenous nitrification/denitrification was unraveled.•CN12 was optimal for endogenous simultaneous nitrification and denitrification.