The Suppression of Nitrite-Oxidizing Bacteria Using Free Nitrous Acid and Limited Available Dissolved Oxygen to Maintain the Stability of Toilet Wastewater Biofilm Nitritation

Researchers have found that maintaining the long-term stability of nitritation becomes challenging when relying on a single inhibitor. Currently, a feasible solution to this problem is to apply two or more inhibitors to achieve the synergistic suppression of NOB. However, studies on this solution have mainly focused on mainstream wastewater, while few have focused on non-mainstream wastewater. Moreover, most of the studies relating to non-mainstream wastewater have only focused on the spontaneous achievement of nitritation within a short operation time or have described nitritation collapse. Since toilet wastewater (TW), as non-mainstream wastewater, can endogenously produce free nitrous acid (FNA) through spontaneous nitritation, an attempt was made in this study through a series of field experiments to combine another inhibitor—a low concentration of dissolved oxygen (DO) available to NOB in the inner layer of biofilm—for biofilm nitritation. Under different levels of DO in the nitritation unit, the working effect and mechanism of high FNA–low available DO dual-factor suppression in maintaining nitritation stability were investigated. The results showed that the dual-factor suppression maintained the long-term stability of TW biofilm nitritation and triggered negative feedback regulation when the nitritation was unstable. A feasible method for establishing a low level of available DO based on a normalized FNA inhibitor when the COD/TN in the nitritation unit exceeds 0.50 is possible when the influent COD/TN of the unit is over 1.57. This study aimed to construct an endogenous and unregulated synergistic suppression strategy for stabilizing nitritation in non-mainstream wastewater to support the application of efficient and sustainable N-removal technology.