Clarifying the mechanism of peroxydisulfate and sulfite activated by Fe²⁺ for waste activated sludge pretreatment: Enhancement of dewaterabiliy, removal of antibiotic resistance genes and pathogenic microorganisms

The remarkable performance of SO₄^-• based advanced oxidation processes in improving waste activated sludge (WAS) dewatering has recently received considerable attention. However, its potential effect on the removal of emerging pollutants is often overlooked. In this study, two types of pretreatment methods (S₂O₈^2-/Fe²⁺ and SO₃^2-/Fe²⁺) were employed to comprehensively evaluate their performance in improving the dewatering of WAS and the removal of antibiotic resistance genes (ARGs) and pathogenic microorganisms. At the optimal dosage of 1.2/1.5 mmol-S₂O₈^2-/Fe²⁺/g-VS, the capillary suction time reduction rate of WAS after S₂O₈^2-/Fe²⁺ was 1.8 times than that of SO₃^2-/Fe²⁺. Notably, both S₂O₈^2-/Fe²⁺ and SO₃^2-/Fe²⁺ could remove the 12 types of ARGs identified in WAS. Nonetheless, S₂O₈^2-/Fe²⁺ was more effective in removing ARGs, with a range of removal rate (0.96–2.17 log-units), and reduced the propagation and proliferation potential of residual ARGs. Furthermore, the coliforms content in WAS after S₂O₈^2-/Fe²⁺ decreased significantly with a minimum of 1.84 ± 0.24 log (MPN/g-TS) compared to SO₃^2-/Fe²⁺. Further analysis indicated that the redistribution of bound extracellular polymeric substances (EPS) in WAS, especially the variations in protein, was the key factor affecting the dewaterability. In contrast to SO₃^2-/Fe²⁺, the S₂O₈^2-/Fe²⁺ could quickly destroy the EPS structure to release bound water, accelerate the dissociation of WAS flocs, and cause cell lysis and DNA dissolution because of the continuous production and accumulation of SO₄^-• in the hostile conditions inside. Therefore, S₂O₈^2-/Fe²⁺ is an efficient, economical, and green method to improve WAS dewatering and the removal of pollutants.