Mechanisms of N2O production in salinity-adapted partial nitritation systems for high-ammonia wastewater treatment

Partial Nitritation/Anammox (PN/A) can achieve green, economical, and efficient biological nitrogen removal; however, the PN process contributes significantly to nitrous oxide (N₂O, the third most important greenhouse gas) emissions. Balancing the stability of PN systems while reducing N₂O emissions, particularly under varying salinity conditions, is a key challenge in applying PN/A for high-salinity and high-ammonia wastewater treatment. This study explored the long-term effects of salinity on PN performance and N₂O emissions in PN systems treating high-ammonia wastewater. The results showed that the specific ammonia oxidation rates of the control and two salinity-acclimated PN reactors were 78.84, 75.03, and 42.60 mg N/(g VSS·h), indicating that low salinity (2.5 g NaCl/L) had minimal effect, while high salinity (10 g NaCl/L) significantly inhibited ammonia-oxidating bacteria and associated nitritation processes. Moreover, N₂O emission factors increased from 0.08 ± 0.04% to 0.24 ± 0.03% as salinity rose from 0 to 10 g NaCl/L. Further analysis revealed that salinity stimulated N₂O production in both aerobic and anoxic stages. Particularly, the N₂O production increased by 2.84–11.14 times in the aerated stage and by 0.61–2.04 times in the nonaerated stage (i.e. anoxic and settling stages). Isotopic pathway analysis indicated that salinity enhanced N₂O production primarily by stimulating the nitrite reduction pathway. Additionally, the mechanism investigation examined the combined effects of salinity-induced changes in sludge properties and microbial community on N₂O emissions. These findings provide valuable insights for applying PN systems to treat high-strength wastewater and understanding the mechanisms of N₂O emissions.