Carbon-ferrous-sulfur mixotrophic denitrification driven by polyferric sulfate (PFS)-conditioned recovered sludge fermentation liquor enhanced nitrogen removal in woolen wastewater

Reducing pollution and carbon emissions synergistically in wastewater treatment is an important part of carbon neutralization. Recycling carbon sources (CSs) in sludge to refeed denitrification has been considered a promising sustainable pathway, but it is limited by CSs purification and recovery. This study evaluated the effects of polyferric sulfate (PFS), polyaluminum chloride (PAC), and polyacrylamide (PAM) conditioning on fermentation CSs recovery and denitrification efficiency. Results indicated PFS-conditioned recovered fermentation liquor showed higher denitrification efficiency for woolen wastewater. CSs recovery test showed that PFS conditioning improved the purity of high-bioavailability CSs by depleting fewer short-chain fatty acids (SCFAs) and removing more total nitrogen (TN), total phosphorus (TP), and humic acids. Under the 0.05 g/gTSS PFS conditioning and recovery, the preservation of soluble chemical oxygen demand (SCOD) and SCFAs were 71.00 % and 84.36 %, respectively, and the nitrate degradation rate achieved 3.19 mgNO3−·h−1·gVSS−1. Importantly, Fe(III) dissociated from PFS oxidized the S2− from woolen wastewater, and generated Fe(II) and S0, which composed a synergistic electron donor system together with SCFAs. They constituted a mixotrophic denitrification dominated by heterotrophic denitrification and reinforced by ferrous and sulfur-autotrophic denitrifications, which contributed 69.56, 17.72, and 12.72 % to nitrogen removal, respectively. In addition, the refeeding of PFS-conditioned recovered fermentation liquor promoted the proliferation of microorganisms involved in iron/sulfur autotrophic denitrification and the abundance of denitrification functional genes, especially the napA, nirS, cnorB, qnorB, and nosZ, forming an efficient Carbon-Ferrous-Sulfur hybrid denitrifiers community to enhanced nitrogen removal from woolen wastewater.