Advanced dewatering of digested sludge through oxidative decomposition of humic substances: A comprehensive study

Abstract

Advanced dewatering of digested sludge, aimed at reducing the moisture content of digested sludge from 80 % to <60 %, has gained significant importance due to the increasing use of anaerobic digestion. Unlike the residual sludge, the digested sludge exhibits strong bonding with water and contains a higher concentration of humic substances. Therefore, the conversion of bound water in organic biomass into free water and its subsequent removal have become critical challenges. In this study, three different oxidants, namely H2O2, Fenton's reagent, and KMnO4, were selected to pretreat digested sludge, focusing on the hydroxyl radicals generated by the Fenton's reagent in the decomposition of organic compounds in digested sludge, exploring the changes in dewaterability and the mechanism of water conversion. Additionally, a lab-scale plate frame was used to simulate the advanced dewatering characteristics. The findings revealed that among the oxidants, only Fenton's reagent demonstrated an improved dewaterability. When dosed at 4.30 mmol/g dry sludge (Ds) and subjected to a pressure of 1000 kPa for 30 min, the moisture content of the sludge cake was reduced to 50.32 %. The accumulation of humic substances during the digestion process resulted in a shift in control from polysaccharides and proteins to humic substances that governed the hydrophilic groups. The selective oxidative decomposition of humic substances using hydroxyl radicals produced by Fenton's reagent enables the conversion of bound water into free water, which can be effectively removed by applying pressure.