Insights into the effects of exogenous folate on the recovery of SPNA system after starvation: Performance, microbial community and metabolism

To investigate the effects of folate addition on the recovery process of the single-stage partial nitritation-anammox (SPNA) system, two continuous stirred tank reactors (CSTRs), named R1 (adding folate) and R2 (control group), were operated for 105 days at room temperatures. The highest nitrogen removal rate (NRR) observed in R1 was 0.47 kg N/(m3·d) with the addition of 3.0 mg/L folate. Additionally, the activities of HZO and HZS in R1 were higher than those in R2. The relative abundance of Candidatus Brocadia reached 23.7% in R1 and 18.2% in R2. Moreover, the relative abundance of nitrogen metabolism genes and carbon metabolism genes increased with folate addition. However, 3.0 mg/L folate addition reduced the relative abundance of nirK/S and the activity and growth of ammonia-oxidizing bacteria, which was not conducive to continually improving the NRR. This study provided a novel perspective on improving the activity and abundance of anammox bacteria during the restoration process.