Altering Extracellular Biopolymers and Water Distribution of Waste Activated Sludge by Fe(II) Persulfate Oxidation with Natural Zeolite and Polyelectrolyte as Skeleton Builders for Positive Feedbacks to Dewaterability

Ferrous persulfate (Fe(II)/S₂O₈ ^2-) oxidation has gained much attention due to its outstanding oxidizability and high efficiency in upgrading waste activated sludge (WAS) dewaterability. Even though the Fe(II)/S₂O₈ ^2- technique possesses a myriad of advantages, its potential could be further magnified for future upscaling and real-world application especially when coupled with other alternative chemicals. In this study, the potential benefits of Fe(II)/S₂O₈ ^2- oxidation coupled with natural zeolite and poly(dimethyl diallyl ammonium chloride) (PDADMAC) for enhancing WAS dewaterability were investigated. The vacuum filtration test was used to evaluate the dewatering effectiveness. Variations in different extracellular polymeric substance (EPS) fractions, water distribution, functional groups, and microstructures were identified to elucidate the underlying dewatering principles and kinetics. The results demonstrated that the combination of Fe(II)/S₂O₈ ^2- with zeolite and PDADMAC had a significant effect on enhancing WAS dewaterability. The optimal conditions obtained were 2.25/1.8 mmol-Fe(II)/S₂O₈ ^2-/g-VS, 0.5 g-zeolites/g-VS, and 0.3 g-PDADMAC/g-VS with up to 117 g-H₂O/g-VS removal (moisture content: 64.5%). Further analysis revealed the three-step dewatering mechanisms: (i) the SO₄·^- generated by Fe(II)/S₂O₈ ^2- oxidation attacked the WAS flocs and cells, broke the bonds of O-H, C-C, and O= C-NH-in high-molecular-weight biopolymers, and decomposed them into micromolecule organics and even inorganics, thus liberating EPS-and cell-bound water, (ii) the broken WAS flocs were then reflocculated via adsorption bridging and charge neutralization induced by PDADMAC, and (iii) during the subsequent filtration, zeolites created channels/cavities for water release by forming a multiple void skeletal lattice while alleviating the clogging of filtration cloth. In addition, the cost-benefit analysis revealed that the combined Fe(II)/S₂O₈ ^2-/zeolites/PDADMAC process represented much greater attractiveness in saving cost and real implementations than the zerovalent iron/persulfate and classical Fenton process.