Conversion of sewage sludge into environmental catalyst and microbial fuel cell electrode material: A review

At present, environmentally friendly and cost-effective disposal of sewage sludge (SS) is the major challenge of wastewater treatment that prompted the concept of sludge valorization. A recent technology, SS conversion into biochar as an efficient catalyst for environmental application, shows great promise to sludge valorization. This review presents the literature and advances of sludge biochar-based catalysts (SBCs), including their synthesis route, physiochemical characteristics, catalytic applications, reaction mechanisms, chemical stability, feasibility, and future aspects. Two major applications of SBCs such as organic pollutants degradation and employing as an electrode material in a microbial fuel cell (MFC) were summarized. The literature has indicated that carbonization of raw or organic/ inorganic-laden sludge produces various metal phase structure and surface functional groups which perform various catalytic reaction such as Fenton-like reaction, ozonation, H₂O₂/ persulfate activation, and photoreaction in the organic pollutants degradation tests. The degradation efficiency and chemical stability of SBCs have found very satisfying. Moreover, catalysts are highly recyclable, separable, and ensure negligible metal leaching. Secondly, high-temperature carbonized sludge exhibits excellent electrical conductivity which is suitable to use as MFC electrodes. The low-cost sludge biochar-based electrodes (SBEs) performance is comparable to many commercial electrodes. This new technology is concurrently advantageous for environmental pollution remediation, energy production, and harmful metals immobilization, which offer a new route towards SS valorization.