Nitrogen Removal and Nitrous Oxide Emission in a Constant-Flow Multiple Anoxic and Aerobic Process

Nitrogen removal and nitrous oxide (N2O) emission in a lab-scale constant-flow multiple anoxic (A) and aerobic (O) process, combined with the addition of suspended carriers, were investigated. Under steady state, 99.9% of ammonia nitrogen (NH4-N), 76.8% of orthophosphate, and 80.2% of total inorganic nitrogen removal efficiency was achieved. The N2O emission factor during nitrification was 0.8–1.9% of the oxidized NH4-N. The emission factor increased to 7.4–45.9% with the coexistence of heterotrophic activities. Extending the anoxic time from 30 to 90 min reduced the N2O emission factor from 1.6 to 1.0%. N2O emission was stimulated with nitrite (NO2-N) as the electron acceptor, with the N2O emission factor of 5.2–5.6%. Denitrification with internal organic carbon contributed 6.8% of reduced NO2-N to N2O. NO2-N might exert a crucial role in N2O emission independent of carbon source. For the acclimated microbial communities, Nitrospira and Nitrosospira were the dominant nitrifiers and responsible for the N2O emission during nitrification. Azospira, Dechloromonas, Flavobacterium, Pseudomonas, and unclassified genus of family Comamonadaceae might be responsible for N2O emission during denitrification. These findings may guide the design of multiple AO process for controlling N2O emission.