A three-stage oxidation method improves sludge dewaterability by achieving sustained oxidation and phased enhanced coagulation: Ingeniously using H₂O₂ as a “converter”

Sludge reduction is essential for mitigating the environmental hazards associated with sludge. The use of ferric chloride (Fe(III)) catalyzed ferrate (Fe(VI)) can significantly enhance sludge dewaterability by increasing the Fe (VI) oxidation rate and maintaining in-situ enhanced coagulation. However, its weak oxidation ability at the sludge's original pH limits its practical application. To address this, hydrogen peroxide (H2O2) strengthened Fe (VI)/Fe(III) was employed to efficiently improve sludge dewaterability without pH adjustment, thereby enhancing the environmental utilization capacity of sludge. Compared to raw sludge, the treatment of Fe(VI)/Fe (III)-H2O2 resulted in approximately the 95% reduction in specific resistance to filtration (SRF) and increased the bioavailable phosphorus (P) content in dewatered sludge. During the oxidation process, highly reactive species such as tetravalent iron (Fe(IV)) and hydroxyl radicals (center dot OH) were generated, facilitating the release of tightly bound water in extracellular polymeric substances (EPS). Importantly, the oxidation process was divided into three stages (persistent moderate oxidation - fast strong oxidation - sustained weak oxidation) based on the predominant reactive species (Fe(IV) or center dot OH). This division promoted more robust and more durable oxidation processes. Phased enhanced coagulation occurred during the oxidation process. The gradual increase in Fe(III) content led to the re-flocculation of damaged EPS, enlarging the size of sludge drainage pores/channels and improving water discharge efficiency. Additionally, the costs and carbon emissions of chemicals for the Fe(VI)/Fe (III)-H2O2 process were 18% and 52% lower than those of the traditional Fenton method. This study not only achieves the goals of sludge reduction and resource collection but also provides insights into selecting sludge conditioning methods that meet pollution reduction and carbon emission reduction requirements.