Unveiling driving forces and N₂O reduction capacity of endogenous denitrification in domestic wastewater treatment

Achieving efficient nitrogen removal while mitigating nitrous oxide (N₂O) emissions remains a critical challenge for sustainable wastewater treatment under low carbon-to-nitrogen ratio conditions. Although the anaerobic/oxic/anoxic (AOA) process exhibits excellent nitrogen removal performance, the electron donor sources sustaining endogenous denitrification and its inherent N₂O sink capacity remain unclear. Here, endogenous denitrification was investigated in a continuous-flow AOA system treating domestic wastewater (sludge retention time = 30 days, mixed liquor suspended solids = 3000-5000 mg/L, hydraulic retention time = 14.4 h, and dissolved oxygen = 1-2 mg/L). Results showed that endogenous denitrification was sustained by multiple endogenous electron donors, including intracellular polymers, amino acids and extracellular polymeric substances, which collectively supported stable nitrogen removal and limited N₂O accumulation. Integrated kinetic and mass-transfer analyses revealed that the anoxic zone acted as an effective N₂O sink, characterized by high apparent affinity for N₂O, a pronounced excess of reduction over production potential, and the dominance of biological consumption over physical stripping. Isotope tracing further revealed the rapid turnover and consumption of N₂O during endogenous denitrification. Molecular analyses indicated this high sink activity was associated with enzymatic regulation. This study fills knowledge gaps regarding electron donor sources and N₂O production-consumption mechanisms during endogenous denitrification.