Enhancing simultaneous nitrate and phosphate removal in sulfur‑iron (II) autotrophic denitrification biofilters by endogenous magnetic fields: Performance and mechanism

The sulfur‑iron (II) autotrophic denitrification (SIAD) technology can achieve simultaneous nitrogen and phosphorus removal from wastewater with a low C/N ratio without an organic carbon source. However, the low nutrient removal load limits its large-scale application. To enhance N removal load and decrease the effluent SO 4 2− concentration, we propose a novel bioaugmentation technology by adding magnetite to a sulfur‑iron (II) biofilter to generate an endogenous magnetic field (EMF). The treatment characteristics of two biofilters with sulfur‑iron (II) (S1) and sulfur‑iron (II)-magnetite (S2) were comparatively studied in long-term experiments. The maximum nitrate removal load of the S2 system was 1.02 ± 0.04 kg N/(m 3 ·d), 1.27 times higher than that of S1. The S2 system also showed a 40 % higher PO 4 3− removal efficiency compared to S1. The production of SO 4 2− was reduced by >12.34 % in S2 with 5.40 g SO 4 2− /g N. The EMF significantly increased nitrate reductase activity by 3.50 times and enhanced the size of the attached microbial bacterial population by 30 %–50 %. Functional gene prediction indicated that amino acid metabolism, cell activity, N/S metabolism were improved under the EMF, and the genera Ferritrophicum and Thiobacillus were the predominant autotrophic bacteria, forming a symbiotic relationship. Our study demonstrates a novel solution to resolve the limits of high sulfate byproduct and low nutrient removal load in SIAD biofilters, which could promote large-scale application of SIAD process as a highly efficient and low-cost postdenitrification process.