Abstract
High levels of extracellular polymeric substances (EPS) and bound water in sludge negatively impact sludge dewaterability. This study applied a novel sludge conditioning process involving calcium hypochlorite (Ca(ClO)2) oxidation, ferric coagulant re-flocculation, and walnut shell (WS) skeleton construction to remove EPS and bound water and improve sludge dewaterability. The study also described the underlying mechanisms for this activity, by investigating the changes in EPS distribution, morphological structure, flocs morphology, rheological behavior, and bound water. Using single factor experiments, the optimal dosages of the composite conditioner included 10 mg/g dry solid (DS) of Ca(ClO)2, 60 mg/g DS of ferric chloride (FeCl3) or 100 mg/g DS of polymeric ferric sulfate (PFS), and 600 mg/g DS of WS. Using a novel pressure filtration system, the moisture content of the dewatered sludge cake conditioned with Ca(ClO)2/ferric coagulant/WS was reduced to below 50 wt%. The study described a three-step mechanism for the composite conditioning of sludge. First, protein-like substances and polysaccharide-like materials in the EPS and bound water were degraded by Ca(ClO)2 oxidation. Second, the porous structure and compact flocs were re-flocculated using high charge neutralization and the bridging ability of ferric coagulant. The flowability increased and the viscosity decreased compared with raw sludge. Third, a rigid networked structure was formed by adding the WS skeleton. The combined Ca(ClO)2/ferric coagulant/WS treatment had a significant synergistic effect supporting dewatering. The filter liquor of the combined treatment has a lower impact on the environment. The combined treatment is an appropriate was to treat sludge prior to landfilling and incineration. Furthermore, the economic analysis showed that using the Ca(ClO)2/FeCl3/WS treatment had economic advantages over other treatments.
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