Treatment performance, nitrous oxide production and microbial community under low-ammonium wastewater in a CANON process

To investigate the characteristics of anaerobic ammonia oxidation for treating low-ammonium wastewater, a continuous-flow completely autotrophic nitrogen removal over nitrite (CANON) biofilm reactor was studied. At a temperature of 32 ± 1 °C and a pH between 7.5 and 8.2, two operational experiments were performed: the first one fixed the hydraulic retention time (HRT) at 10 h and gradually reduced the influent ammonium concentrations from 210 to 50 mg L⁻¹; the second one fixed the influent ammonium concentration at 30 mg L⁻¹ and gradually decreased the HRT from 10 to 3 h. The results revealed that the total nitrogen removal efficiency exceeded 80%, with a corresponding total nitrogen removal rate of 0.26 ± 0.01 kg N m⁻³ d⁻¹ at the final low ammonium concentration of 30 mg L⁻¹. Small amounts of nitrous oxide (N₂O) up to 0.015 ± 0.004 kg m⁻³ d⁻¹ at the ammonium concentration of 210 mg L⁻¹ were produced in the CANON process and decreased with the decrease in the influent ammonium loads. High-throughput pyrosequencing analysis indicated that the dominant functional bacteria ‘Candidatus Kuenenia’ under high influent ammonium levels were gradually succeeded by Armatimonadetes_gp5 under low influent ammonium levels.