Facilitating multiple nitrite for anammox process directly treating municipal wastewater: Optimized organics utilization and microbial composition.

While direct anammox implementation is attractive when treating wastewater, nitrite (NO₂^--N) availability and excess organic matter significantly limit its practical application. This study proposed partial nitrification and endogenous partial denitrification/anammox (PN/A-EPD/A) for the treatment of real municipal wastewater (COD/N ratio: 2.8) within a single-stage reactor under anaerobic/aerobic/anoxic mode. Interestingly, with reducing dissolved oxygen concentration (5.0 ± 1.0 → 1.0 ± 0.5 mg-O₂/L) during aerobic phase, Comammox Nitrospira clade A became dominated and introduced vast nitrate (NO₃^--N) into the subsequent anoxic stage. Both in-situ and ex-situ tests confirmed that sufficient NO₃^--N as electron acceptors were in favor of the EPD/A occurrence with endogenous organics utilization, which was obtained by anaerobic endogenous transformation. Metagenomic results confirmed the role of Thauera in facilitating NO₃^--N→NO₂^--N process, and further supporting AnAOB. As a result, Ca. Brocadia gradually enriched on granules (from 0.08% to 3.51%) and contributed up to 51.5 % to total inorganic nitrogen removal through the PN/A-EPD/A process. Optimized carbon utilization pathway promoted the re-cooperative balance of microorganisms and this process achieved efficient nitrogen removal (93.5%) and desirable quality of effluent (3.2 mg-N/L) when treating real municipal wastewater.