The experimental optimization and comprehensive environmental risk assessment of heavy metals during the enhancement of sewage sludge dewaterability with ethanol and Fe(III)-rice husk

The optimal concentrations of ethanol, Fe3+ and rice husk (RH) to enhance sludge dewaterability were determined by response surface methodology (RSM). Results showed the optimal concentrations of ethanol, Fe3+ and RH were 22.2 g/g DS, 239.9 mg/g DS and 348.9 mg/g DS, respectively, and the CST reduction efficiency reached 72.3%. The transformation behavior and mechanism of the heavy metals (HMs) during conditioning process were determined in terms of total HMs content, leaching tests, and fraction distribution. The environmental risk of HMs was quantitatively evaluated after conditioning in terms of bioavailability and ecotoxicity, potential ecological risks, and pollution levels. Results showed that the high ecological risk of HMs in raw sludge cake is primarily dominated by Cd and the use of Fe3+ alone negatively affected the immobilization of HMs and reduction of leaching toxicity. However, after repeated conditioning with Fe3+ and ethanol, the total HMs content reduction values in sludge cake were 75%, 93%, 100%, 91%, and 74% for Pb, Cr, Cd, Zn, and Cu, respectively. The potential ecological risk index (PERI) and geoaccumulation indicated low or no overall environmental risk after repeated conditioning. Particularly, the risk of Cd was reduced from high risk to low risk after repeated conditioning according to the PERI. Ethanol/Fe3+-RH can effectively reduce HMs risk from the sludge cake in the dewatering tests.