Rapid in-situ enrichment of anammox bacteria in a thiosulfate-driven partial denitrification and anammox biofilter packed with iron sponge.

Despite its high efficiency and low-carbon advantages, the engineering application of anaerobic ammonium oxidation (anammox) technology has long been constrained by two major bottlenecks: slow start-up and limited sources of microbial inoculum. This study established a thiosulfate-driven partial denitrification/anammox biofilter (TDDA-BF) system, achieving rapid start-up using conventional activated sludge and sulfur‑autotrophic denitrifying sludge, without the need for external anammox inoculation. The TDDA-BF system successfully achieved in-situ enrichment of anammox bacteria within 78 days, with a total nitrogen removal efficiency of 85%. Microbial community analysis revealed a substantial increase in the abundance of Candidatus_Brocadia from 0.38% to 21.01%. Hydrazine dehydrogenase activity assays and predictions suggested that Fe (II) from iron sponge boosted anammox key enzyme activity by enhancing heme c synthesis. This study successfully demonstrates the feasibility of rapid in-situ enrichment of anammox bacteria in a TDDA-BF system. The implementation of this strategy not only substantially shortens the conventional start-up period but also promotes a more carbon-efficient pathway for nitrogen removal, highlighting its potential for sustainable wastewater treatment applications.