Food Waste Discharge to Sewer Significantly Regulates Denitrification for N₂O Mitigation in Downstream Biological Nitrogen Removal Process

Integrating food waste (FW) into a wastewater treatment system through FW disposers facilitates waste reduction and energy recovery; however, it may pose challenges to the downstream biological nitrogen removal (BNR) processes. Particularly, the response of nitrous oxide (N₂O, a potent greenhouse gas) emissions to FW discharge is underexplored. In this study, it was revealed that FW contributed to N₂O mitigation by regulating denitrification. Chronic FW addition enhanced N₂O reduction capacity while weakening the rates of other denitrification processes, thereby providing an N₂O sink for possible mitigation. Meanwhile, nitrite accumulation during the predenitrification phase was completely avoided in the presence of FW, reducing N₂O production in the subsequent nitrification. As a result, the overall N₂O emission factor of the mainstream BNR system decreased from 6.49% to 0.84% with FW addition. In addition, N₂O production during nitrification increased by 19.8–118.2% because of the elevated ammonia-oxidizing bacteria (AOB) denitrification pathway by chronic FW addition, while the nitrification activities were reduced. The underlying mechanisms for the altered N₂O emission by FW addition were explored from the point of view of sludge properties and microbial communities. This study provided insights into the strategic management of FW while focusing on the concern of N₂O emissions in the context of FW integration into wastewater systems.