High-rate iron sulfide and sulfur-coupled autotrophic denitrification system: Nutrients removal performance and microbial characterization

Iron sulfides-based autotrophic denitrification (IAD) is a promising technology for nitrate and phosphate removal from low C:N ratio wastewater due to its cost-effectiveness and low sludge production. However, the slow ki-netics of IAD, compared to other sulfur-based autotrophic denitrification (SAD) processes, limits its engineering application. This study constructed a co-electron-donor (FeS and S0 with a volume ratio of 2:1) iron sulfur autotrophic denitrification (ISAD) biofilter and operated at as short as 1 hr hydraulic retention time (HRT). Long-term operation results showed that the superior total nitrogen and phosphate removals of the ISAD biofilter were 90-100% at 1-12 h HRT, with the highest denitrification rate up to 960 mg/L/d. Considering low sulfate pro-duction, HRT of 3 h could be the optimal condition. Such superior performance in the ISAD biofilter was ach-ieved due to the interactions between FeS and S0 , which accelerated the denitrification process and maintained the acidity-alkalinity balance. Metagenomic analysis found that the enriched nitrate-dependent iron-oxidizing (NDFO) bacteria (Acinetobacter and Acidovorax), sulfur-oxidizing bacteria (SOB), and dissimilatory nitrate reduction to ammonia (DNRA) bacteria likely supported stable nitrate reduction. The metabolic pathway analysis showed that completely denitrification and DNRA, coupled with sulfur oxidation, disproportionation, iron oxidation and phosphate precipitation with FeS and S0 as co-electron donors, were responsible for the high-rate nitrate and phosphate removal. This study provides the potential of ISAD as a highly efficient post-denitrification technology and sheds light on the balanced microbial S-N-Fe transformation.

Automated summary

Iron sulfides-based autotrophic denitrification (IAD) is a cost-effective and low sludge production technology for nitrate and phosphate removal from low C:N ratio wastewater. However, its slow kinetics compared to other sulfur-based autotrophic denitrification (SAD) processes limits its application. This study constructed an iron sulfur autotrophic denitrification (ISAD) biofilter with co-electron-donors (FeS and S0) and achieved superior total nitrogen and phosphate removals at short hydraulic retention times (HRTs) through the interaction between FeS and S0. Metagenomic analysis revealed the presence of nitrate-dependent iron-oxidizing (NDFO) bacteria, sulfur-oxidizing bacteria (SOB), and dissimilatory nitrate reduction to ammonia (DNRA) bacteria, supporting stable nitrate reduction.