Combined sludge conditioning with NaCl-cationic polyacrylamide-rice husk powders to improve sludge dewaterability

Strong water binding ability of extracellular polymeric substances (EPS) and high compressibility of sludge during compression dewatering have greatly hindered the improvement of sludge dewaterability. In this paper, a novel combined conditioning method was proposed to enhance the sludge dewaterability. The sludge was firstly disintegrated by NaCl, then the sludge particles were re-flocculated by cationic polyacrylamide (CPAM), meanwhile the sludge filter cake was modified with rice husk (RH) as filter aid. The results showed that combined NaCl-CPAM-RH conditioning greatly improved the sludge dewaterability compared with that of raw sludge. The optimal NaCl pretreatment lasted 120 min with a dosage of 0.25 mol/L, the optimal CPAM and RH dosage was 30 mg/L and 50 wt%, respectively, which led to a moisture content of sludge cake of 65.44%. The changes of sludge liquid and solid phase conditioned by NaCl-CPAM-RH were analyzed. The NaCl pretreatment disintegrated the sludge floc structure with the release of protein and polysaccharide from EPS and Ca2+ and Mg2+ from sludge flocs to liquid, accompanying with the release of water trapped in sludge matrix. The addition of CPAM and RH reconstructed the sludge flocs with a big mean diameter d(0.5) of 126.84 μm, and the sludge filter cake showed a porous and rigid structure with a porosity increment of 138.62% and a compressibility reduction of 39.62% compared to that of raw sludge. [Display omitted] •NaCl disintegrated sludge flocs and released trapped water.•CPAM reflocculated sludge particles by charge neutralization and bridging.•Rice husk (RH) powders modified structure of sludge filter cake as filter aid.•NaCl-CPAM-RH combined conditioning significantly enhanced sludge dewaterability.