Contrasting EPS Protein Responses to Fe(II)/PDS and Ca(OCl)2/FeCl3 Conditioning of Food Waste Digestate

This study compared time-dependent EPS and protein oxidation indicator responses during Fe(II)-activated peroxydisulfate (Fe(II)/PDS) and calcium hypochlorite/ferric chloride (Ca(OCl)2/FeCl3) conditioning of food waste digestate (FWD). Dewatering performance was evaluated across reagent dosages, while molecular indicators were followed over 0–60 min at the selected dosages. Protein (PN) and polysaccharide (PS) concentrations were measured in soluble, loosely bound, and tightly bound EPS (S-EPS, LB-EPS, and TB-EPS), together with thiol, protein carbonyl, and advanced oxidation protein product (AOPP) responses. Under the selected conditions, Fe(II)/PDS reduced capillary suction time (CST) from 1289.9 to 61.2 s and cake water content from 94.67% to 59.33%, whereas Ca(OCl)2/FeCl3 reduced them to 30.5 s and 69.17%, respectively. Ca(OCl)2/FeCl3 yielded the lower CST, while Fe(II)/PDS yielded the lower cake water content after filter-press compression. Fe(II)/PDS showed greater depletion of detectable S-EPS PN, persistent thiol loss, and a transient AOPP increase. Ca(OCl)2/FeCl3 showed a greater normalized protein carbonyl response without a corresponding AOPP increase. These findings show that the two conditioning treatments produced contrasting dewatering outcomes and distinct EPS and protein oxidation indicator responses.