Effect of external carbon source type on effluent dissolved organic nitrogen characteristics in postdenitrifying moving bed biofilm reactors: Chemical molecular and microbial insights

•Carbon source type affected postdenitrifying MBBRs effluent DON characteristics.•Produced DON contributed to the differences in the bioavailability of effluent DON.•Phyla and metabolic pathways responsible for produced DON were identified.•Bioavailable DON dominated phytoplankton growth rather than DIN.•Acetate was recommended as the preferred carbon source in postdenitrifying MBBRs.