Synergy between Comammox and Anammox Bacteria in Wastewater Ammonia Removal

Complete ammonia oxidizing bacteria (CAOB) coexist withcanonicalnitrifiers, anammox bacteria (AnAOB), and heterotrophs in mainstreamwastewater nitrogen removal systems. The cooperation and competitionamong these populations are dependent on various environmental factors,such as ammonia and dissolved oxygen (DO). In the study, a movingbed biofilm reactor (MBBR) was operated at low DO (0.1-0.3mg/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 effectsof low DO and low residual ammonium. In phase I, the residual NH4 (+) was <0.2 mg N/L, and CAOB tended toward completeammonia oxidization for gaining more energy rather than secretingnitrite 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. N-15 tracing indicated thatnitrite reduction in the MBBR could be mainly attributable to anammoxrather than endogenous denitrification. AnAOB also possessed moreabundant and diverse nitrite transporter genes (e.g., focA, nirC) than did NOB-Nitrospira for potential efficient nitrite utilization. The CAOB-AnAOB synergyenabled up to & SIM;35% total nitrogen removal in the MBBR. Microrespirometrytests further revealed that the synergy occurred at DO in the rangeof 0.01-0.9 mg/L (optimum: 0.3 mg/L). NH4 (+) and DO concentrations that allow effective CAOB-AnAOB synergy canbe applied in improving anammox-based mainstream wastewater nitrogenremoval toward low aeration and a carbon footprint.