Impact of initial sludge pH on enhancing the dewaterability of waste activated sludge by zero-valent iron-activated peroxydisulphate

Abstract

In this study, the usefulness of the $\mathrm{ZVI}/S_2{O}_8^{2-}$ system in enhancing the dewaterability of waste activated sludge was studied at different initial sludge pH levels to reveal the associated mechanisms. Results showed that conditioning of sludge by the $\mathrm{ZVI}/S_2{O}_8^{2-}$ system at acidic initial sludge pHs enhanced sludge filterability and the dryness of dewatered sludge cake, while only the sludge filterability was improved at neutral initial sludge pH. In particular, the conditioning treatment using 0.353 g/g DS of ZVI and Na₂S₂O₈ with a mole ratio of 1.25:1, at initial sludge pH 2.52, was the optimum condition to improve sludge dewaterability, which reduced the moisture content of dewatered sludge cake to only 69.8% and meanwhile reduced sludge CST and SRF to only 70.9% and 40.7% of that of raw sludge, respectively. During the conditioning treatment, sludge microbial cell lysis resulting from decreasing the initial sludge pH led to the reduction in the moisture content of dewatered sludge cake, while the oxidation of sludge EPS and the coagulation of the disrupted sludge flocs achieved by the $\mathrm{ZVI}/S_2{O}_8^{2-}$ system improved sludge filterability. A kaolin suspension experiment revealed that with a decrease in system pH, the oxidation effect was gradually inhibited and the coagulation effect offset the disruption effect on sludge flocs to improve the filterability of sludge. Therefore, the conditioning of waste activated sludge using the ZVI/Na₂S₂O₈ system at acidic initial sludge pHs is useful to enhance the sludge filterability and the dryness of dewatered sludge cake, both of which are crucial for improving sludge dewatering performance.

Keywords: Waste activated sludge; coagulation; dewaterability; oxidation; zero-valent iron-activated peroxydisulphate.