Dynamics of nitrogen oxides emissions in the pilot wastewater treatment plant: Characteristics and mechanisms

Nitrogen oxides (NO_x), consisting of nitric oxide (NO) and nitrogen dioxide (NO₂), pose significant risks to human health and the environment, potentially causing respiratory and cardiovascular diseases. In contrast to extensive research on industrial NO_x emissions, there has been a paucity of attention on the dynamics of NO_x in wastewater treatment plants (WWTPs). This study investigated NO_x emission patterns in a pilot-scale WWTP, revealing that NO dominated NO_x emissions (0.013 % of the nitrogen load), peaking nocturnally (4 a.m.- 6 a.m.) and primarily originating from the oxic zone due to ammonia-oxidizing bacteria activity and aeration stripping synergistically increasing NO_x emission. NO₂ emissions were minimal (0.003 % of the nitrogen load), with limited conversion from NO. Pearson correlation analysis identified ammonium removal efficiency and nitrate accumulation efficiency as key drivers of NO_x production. The microbial community composition was closely linked to NO_x emissions, with nitrifying bacteria such as Nitrosomonas contributing to NO_x generation in the oxic zone, while denitrifying bacteria like Methylotenera contributed to NO_x emissions under oxygen-limited conditions in the anoxic zone. This study enhanced the comprehension of the intricate processes of the nitrogen cycle during wastewater treatment and provided theoretical guidance for further optimization of WWTPs.