The green footprint of anammox processes under simulated actual operating conditions: Focusing on the nitrous oxide and methane production

The anammox process has attracted increasing attention due to its advantages of low-carbon and energy-saving, nevertheless, greenhouse gas was still generated during its engineering applications process. Hence, it is vital to comprehensively understand the production characteristics and mechanisms of N₂O and CH₄ in anammox processes by responding to practical conditions including dissolved oxygen, temperature, and salinity. Results showed that N₂O production increased by 192 %-358 %, while nitrogen removal efficiency (NRE) increased by 64.2 %-86.8 % with increasing temperature. The increased salinity inhibits 40.60 %-65.33 % N₂O production with a decrease NRE of 7.85 %-18.2 %. CH₄ production was the highest at 18-27 °C, reaching 3.07 ± 0.11-4.06 ± 0.16 mg·L^-1, which were 1.59-2 and 1.29-1.38 times higher than that at 8-17 °C and 28-37 °C, respectively. Denitratisoma, Thauera, and Nitrosomonas were the main functional microbes for greenhouse gas production in anammox consortia. Notably, H₂O₂-induced intracellular Fenton reaction may be critical for the CH₄ production in anammox consortia. This work provides valuable insights into achieving efficient nitrogen removal and minimizing carbon footprint in anammox systems and provides a theoretical basis for implementing the net-zero emission idea in wastewater treatment plants.