A potential closed-loop reutilization of iron sludge for wastewater treatment through the solid state reaction with spent coffee grounds

We proposed herein a closed-loop concept for wastewater treatment by reutilizing iron sludge through a solid state reaction with spent coffee grounds. In addition to characterizing the physical and chemical features of the resulting iron-based Fenton catalysts, the environmental implication of adopting this strategy was also evaluated. Our results showed that iron sludge calcinated at 600 ℃ with 30–50 % of spent coffee grounds achieved the highest dye degradation. The high efficiency was closely related to the enhanced electrical conductivity of the biochar support and the reduced resistance at the hematite/electrolyte interface. Notably, the iron-based Fenton catalysts demonstrated high stability, maintaining efficient dye degradation through five consecutive treatment cycles, and their reactivity could be reactivated through a simple heat treatment at 400 ℃. This closed-loop iron sludge reutilization strategy could replace the activated carbon, reducing 0.0025 kgCO₂/L emissions. This value implied an approximation of 35 tonnes of CO₂ reduction for a factory treating 14,000 tonnes of wastewater monthly. Unfortunately, the proposed closed-loop reutilization of iron sludge has a very limited contribution to iron sludge reduction. A 30–40 % iron sludge reduction could only be achieved through optimizing the dosage of iron sulfate coagulant. Given its cost-effectiveness and high organic removal efficiency, chemical coagulation remains an indispensable water treatment method. Therefore, future efforts should prioritize identifying large-scale and practical applications for iron sludge, ensuring its reutilization is both economically and environmentally viable.