Magnetite-augmented sulfur-siderite autotrophic denitrification: Deep nitrogen removal at ultra-low HRT from lab to pilot scale.

Persistent eutrophication and increasingly stringent discharge regulations have intensified the demand for advanced nitrogen removal technologies in wastewater treatment. Sulfur-siderite autotrophic denitrification (SSAD) presents a chemical-free, low-carbon alternative to conventional heterotrophic processes. However, its widespread application is hindered by long hydraulic retention times (HRTs) and frequent nitrite (NO-N) accumulation. To address these limitations, this study developed a sulfur-siderite-magnetite autotrophic denitrification (SSMAD) system by integrating magnetite into SSAD fillers. Both lab- and pilot-scale experiments confirmed that SSMAD significantly outperformed SSAD in terms of denitrification capacity and stability. The SSMAD system maintained robust performance at HRTs under 3 h, whereas the SSAD reactor exhibited negligible nitrate removal. In pilot-scale SSMAD reactors treating secondary effluent, total nitrogen in the effluent remained below 11.5 and 12.3 mg/L at ultra-low HRTs of 20 and 15 min, respectively. At a 30-minute hydraulic retention times (HRT), the SSMAD system achieved a denitrification load of 0.95 kgN/(m·d), exceeding those of SSAD and sulfur autotrophic denitrification (SAD) systems by factors of 1.6 and 4.4, respectively. Sulfur served as the primary electron donor, while Fe released from siderite provided an additional source of electrons. The microbial community in both SSAD and SSMAD systems was enriched with Thiobacillus and Sulfurimonas, which couple sulfur and iron oxidation with nitrate reduction. Magnetite additions enhanced both sulfur- and iron- driven denitrification and increased the abundance of these key genera. Metatranscriptomic analysis indicated that magnetite facilitated interspecies electron transfer (IET) via sulfur intermediates produced by Sulfurimonas and utilized by Thiobacillus. Additionally, extracellular electron transfer (EET) by Thiobacillus was promoted, evidenced by up-regulated expression of genes coding for extracellular c-type cytochromes. Overall, this study presents a viable strategy for achieving energy-efficient, rapid nitrogen removal at ultra-short HRTs, demonstrating the practical potential of SSMAD for advanced wastewater treatment applications.