Enhanced paper sludge dewatering and in-depth mechanism by oxalic acid/Fe/persulfate process.

As a typical advanced oxidation process, Fe-persulfate (PDS) oxidation technology has been widely and efficiently reported for enhancing sludge dewaterability. However, higher dosage of Fe must be added, which will restrain the oxidation efficiency of Fe-PDS process. In this work, the oxalic acid (OA)/Fe-PDS process was studied to improve paper sludge dewatering. With the OA dosage of 6 μmol (g total solid (TS)), Fe dosage of 0.3 mmol (g TS), and PDS dosage of 0.6 mmol (g TS), sludge dewaterability was improved more efficiently. The specific resistance to filtration and water content of sludge cake were decreased by 70.7% and 8.0%, respectively. In comparison with Fe-PDS process, the viscosities of sludge suspension and supernatant were further reduced by 3.73% and 51.77%, respectively, and the contents of extracellular polymeric substances fractions were increased. The improved sludge dewaterability in OA/Fe-PDS process was mainly contributed by the synergistic effect of oxidative disintegration by free radicals and flocs re-flocculation, the contributions of which were the orders: metal cations > sulfate radical > hydroxyl radical. OA enhanced the efficient disintegration of sludge flocs, released more bound water, generated more Fe-oxalate complexes, and finally increased the sludge particle size significantly, forming a larger aggregation and obvious cracks. Additionally, the stabilization of several heavy metals was improved due to the chelating capacity of OA. These works will provide a novel approach for sludge dewatering in the PDS oxidation process.