A review on sustainable reuse applications of Fenton sludge during wastewater treatment

• The sustainable approaches related to Fenton sludge reuse systems are summarized. • Degradation mechanism of Fenton sludge heterogeneous catalyst is deeply discussed. • The efficient utilization directions of Fenton sludge are proposed. The classical Fenton oxidation process (CFOP) is a versatile and effective application that is generally applied for recalcitrant pollutant removal. However, excess iron sludge production largely restricts its widespread application. Fenton sludge is a hazardous solid waste, which is a complex heterogeneous mixture with Fe(OH)₃, organic matter, heavy metals, microorganisms, sediment impurities, and moisture. Although studies have aimed to utilize specific Fenton sludge resources based on their iron-rich characteristics, few reports have fully reviewed the utilization of Fenton sludge. As such, this review details current sustainable Fenton sludge reuse systems that are applied during wastewater treatment. Specifically, coagulant preparation, the reuse of Fenton sludge as an iron source in the Fenton process and as a synthetic heterogeneous catalyst/adsorbent, as well as the application of the Fenton sludge reuse system as a heterogeneous catalyst for resource utilization. This is the first review article to comprehensively summarize the utilization of Fenton sludge. In addition, this review suggests future research ideas to enhance the cost-effectiveness, environmental sustainability, and large-scale feasibility of Fenton sludge applications.