Comparative study of biofilter and continuous stirred tank reactor for anammox fast start-up and stable operation

The performance of anaerobic ammonium oxidation (anammox) in mainstream wastewater treatment is often limited by slow start-up and biomass retention challenges. In this study, biofilter (BF) and continuous stirred-tank reactor (CSTR) configurations were evaluated for anammox cultivation, focusing on start-up performance, nitrogen removal, and microbial dynamics. The results showed that the BF achieved higher total nitrogen removal rate (TNRE) of 79.35 % ± 0.63 % and nitrogen removal rate (NRR) of 1.35 ± 0.04 kg N/(m³·d) than 74.99 ± 3.53 % of TNRE and 0.59 ± 0.04 kg N/(m³·d) of NRR in CSTR. The nitrogen removal of BF driven by the combined contribution of anammox, DN and PN/A. CSTR achieved a shorter start-up time within 34 days and a higher anammox nitrogen removal contribution of 92.22 ± 4.50 % than 59 days for start-up and 84.65 ± 4.80 % anammox nitrogen removal contribution of BF, reflecting the predominance of the anammox pathway. BF showed vertical stratification of nitrogen removal, whereas CSTR formed stable granular sludge with high activity. Moreover, microbial analysis revealed distinct enrichment patterns, with Candidatus Brocadia dominating in BF and Candidatus Kuenenia in CSTR, reflecting niche differentiation driven by reactor configuration. These findings highlight the influence of reactor design on anammox activity, microbial community formation, and nitrogen removal efficiency, providing insights for optimizing mainstream anammox applications in wastewater treatment.