Nitrogen removal and N₂O emissions in anammox reactors: Influence of reactor design on process performance and...

The assessment and mitigation of N₂O emissions from anammox-related processes is challenging for environmentally friendly wastewater treatment. This study evaluated the nitrogen removal efficiency (NRE), N₂O emissions, and microbial diversity in three laboratory-scale anammox reactors: a sequencing batch reactor (SBR) with a recirculation line, a continuous stirred tank reactor (CSTR) without a recirculation line (CSTR1), and a CSTR with a recirculation line (CSTR2). Across two operational phases with anammox biomass (dry weight) of 1.63 g L^-1 (phase I) and 5.44 g L^-1 (phase II), the SBR had a higher NRE and lower N₂O emissions than the CSTRs. The NREs in phase II were 70.7 ± 14.1 % for the SBR, 68.9 ± 15.7 % for CSTR2, and 41.9 ± 15.8 % for CSTR1. N₂O emissions from the SBR were reduced by 56 % in phase II relative to phase I. Microbial diversity declined, and community composition shifted during reactor operation. In phase II, the Shannon entropy indices were 4.77 (SBR), 4.61 (CSTR2), and 5.04 (CSTR1); higher diversity in CSTR1 correlated with lower anammox abundance and thus lower performance. Candidatus Jettenia caeni became the predominant anammox species. Gene analysis revealed a positive correlation between the abundance of anammox-specific 16S rRNA genes (targeted by Amx809f/Amx1066r) and NRE, while nirS and nirK gene copy numbers were inversely related to reactor performance (NRE and N₂O emissions). The copy numbers of nosZ genes (clade I and clade II) varied in phase II across different reactors, which potentially contributed to the differences in N₂O emission reductions observed during this phase.