Effective gel-like floc matrix destruction and water seepage for enhancing waste activated sludge dewaterability under hybrid microwave-initiated Fe(II)-persulfate oxidation process

Chemical conditioning before mechanical dewatering is an indispensable step to enhance the waste activated sludge (WAS) dewaterability and solid-liquid separation. Feasibility of utilizing Fe(II)/S₂O₈²⁻ oxidation integrated with microwave irradiation (MW) in improving gel-like floc destruction, water seepage and WAS dewaterability was investigated. Comprehensive characterization of the treated WAS was conducted to explore the effects of MW on the catalyzing kinetics of Fe(II)/S₂O₈²⁻ oxidation and reveal the underlying dewatering principle. The results demonstrated that MW-Fe(II)/S₂O₈^2–process was more cost-efficient, reagent-saving than single Fe(II)/S₂O₈²⁻ oxidation or MW irradiation in stimulating WAS dewaterability and the optimal conditions were 0.4/0.5 mmol-Fe(II)/S₂O₈²⁻ g⁻¹-TS (total solids) and 500 W with 94.6% capillary suction time (CST) reduction within 120 s of conditioning. Thermal effect of MW reduced the activation energy of S₂O₈²⁻ decomposition and stimulated the generation of more SO₄⁻· while athermal effect could create additional gel-network destruction and cell lysis, which reduced the water-binding energy and induced the seepage of more extracellular polymeric substances (EPS)-bound and cell water. Further analysis via fluorescence excitation-emission matrix combined with parallel factor analysis demonstrated that protein-like, humic- and fulvic-like substances in slime EPS (S-EPS) and loosely bound EPS (LB-EPS) together affected sludge dewaterability. Additionally, the hybrid process could further remove the released COD and ammonia, facilitating the subsequent advanced treatment.