Synergy between Comammox and Anammox Bacteria in Wastewater Ammonia Removal

Complete ammonia oxidizing bacteria (CAOB) coexist with canonical nitrifiers, anammox bacteria (AnAOB), and heterotrophs in mainstream wastewater nitrogen removal systems. The cooperation and competition among these populations are dependent on various environmental factors, such as ammonia and dissolved oxygen (DO). In the study, a moving bed biofilm reactor (MBBR) was operated at low DO (0.1–0.3 mg/L) and varied influent NH4+ (phases I–III: 7, 14, and 33 mg N/L) to obtain an autotrophic nitrogen removal community. CAOB was the dominant ammonia oxidizer throughout under the dual effects of low DO and low residual ammonium. In phase I, the residual NH4+ was <0.2 mg N/L, and CAOB tended toward complete ammonia oxidization for gaining more energy rather than secreting nitrite for AnAOB utilization. With the increase in influent NH4+, AnAOB started to grow and shifted from Ca. Kuenenia (phase II) to Ca. Brocadia dominance (phase III), where residual NH4+ was 0.2–7.6 mg N/L. 15N tracing indicated that nitrite reduction in the MBBR could be mainly attributable to anammox rather than endogenous denitrification. AnAOB also possessed more abundant and diverse nitrite transporter genes (e.g., focA, nirC) than did NOB-Nitrospira for potential efficient nitrite utilization. The CAOB-AnAOB synergy enabled up to ∼35% total nitrogen removal in the MBBR. Microrespirometry tests further revealed that the synergy occurred at DO in the range of 0.01–0.9 mg/L (optimum: 0.3 mg/L). NH4+ and DO concentrations that allow effective CAOB-AnAOB synergy can be applied in improving anammox-based mainstream wastewater nitrogen removal toward low aeration and a carbon footprint.