Iron-based mixotrophic denitrification for enhancing nitrate removal from municipal secondary effluent: Performance, microbial community dynamics, and economic feasibility

High nitrate content limits the recycling of the secondary effluent of wastewater treatment plants. In the research, one biomass-iron mixture (BIM) filter material based on mixotrophic denitrification mode (heterotrophic and iron-driven autotrophic denitrification) was developed and used to construct a novel denitrification biological filter (BIM-DNBF) for the nitrogen removal of secondary effluent. BIM-DNBF had a short start-up time (approximately 9 days), and high total nitrogen removal (81 %-89 %) without external addition of organic carbon sources during the whole operation. The coexistence of dominant heterotrophic-denitrification-like Pseudomonas and Erysipelothrix as well as iron-driven autotrophic-denitrification-like Citrobacter, Acidovorax, etc. were found in the BIM-DNBF. Moreover, biomass was recognized as one key player in promoting the reduction of Fe³⁺ to Fe²⁺, thereby facilitating the occurrence of iron-driven autotrophic denitrification. In addition, BIM-DNBF was assessed to be affordable. These findings provide evidence that BIM-DNBF can be an efficient technology for nitrogen removal of secondary effluent.