Improving dewaterability and filterability of waste activated sludge by electrochemical Fenton pretreatment

The electrochemical Fenton (electro-Fenton) pretreatment was explored as an attractive dewatering method for excess sludge produced in municipal wastewater treatment plants (MWTPs). The effects of initial pH, applied current density, zero valent iron (ZVI) dosage, and H₂O₂ concentration on sludge dewaterability and filterability were demonstrated by measuring water content (WC) and capillary suction time (CST) of conditioned sludge, respectively. Using radical scavenging tests, the significantly reduced WC and CST values in the electro-Fenton pretreatment compared to unconditioned sludge were attributed to both surface-bound ^[rad]OH and aqueous ^[rad]OH responsible for attacking and degrading extracellular polymeric substances (EPS). The component of tightly bound EPS (TB–EPS) was degraded to a greater extent as compared to soluble EPS (S–EPS) and loosely bound EPS (LB–EPS), resulting in the release of bound water to the aqueous phase. Protein and polysaccharide measurements in EPS fractions, together with FT–IR spectra, confirmed the preference of protein oxidation by ^[rad]OH. On the other hand, in the electro-Fenton process, ZVI was first electro-oxidized to Fe²⁺ for use in the Fenton reaction, then to Fe³⁺ that was subsequently reduced to Fe²⁺ under the applied electrical field, thereby leading to the cycle of iron (Fe²⁺ → Fe³⁺ → Fe²⁺). Moreover, ZVI/Fe³⁺ acted as scaffolds for agglomeration of flocs into large and compact particles for improved filterability. Results of this study could provide a basis to optimize the sludge dewatering process using the electro-Fenton pretreatment in MWTPs.