Improvement of sewage sludge dewaterability and immobilization of the heavy metals by using pretreated steel slag

Pretreated steel slag was used to enhance sludge dewatering and the potential risk posed by heavy metals (HMs) was discussed in this study. Specific resistance to filtration and capillary suction time decreased by 98.8% and 78.6%, respectively, when 40 mg/g of dry sludge steel slag were added. The zeta potential showed that the steel slag addition considerably affected sludge with small particle. New fluorescence peaks indicated that the extracellular polymeric substance (EPS) structure was damaged and partially dissolved into solution EPS (S-EPS). The floc surface charge was reduced, and the small particles were enlarged owing to the flocculation of a small amount of S-EPS. Microstructure analysis showed that the inorganic components in the steel slag provided skeletal support during mechanical dewatering and ensured the outlet channel. Analysis of the HMs indicated that the Cu and Mn concentrations decreased by 60% and 85.2% in the liquid phase, respectively, and the Zn concentration increased 2.2 times. Moreover, the results of the toxicity characteristic leaching procedure showed that the Cu, Mn, and Zn concentrations in the sludge cakes decreased by 93.5%, 27.1%, and 22.1%, respectively. Therefore, the addition of steel slag reduced the risk and enhanced the immobilization of HMs.