Coagulant Recovery from Water Treatment Residuals: A Review

Coagulation–flocculation is widely used to treat surface water, resulting in the generation of huge quantity of water treatment residuals (WTRs). Disposal of WTRs in landfills or natural water bodies can lead to environmental problems and bioaccumulation of toxic substances. As a result, sustainable and environmentally friendly management options for WTRs are being explored, with a focus on beneficial reuse strategies. Various beneficial reuses of WTRs have been studied, including direct use in wastewater treatment and recovery of coagulants from WTRs for reuse in wastewater treatment. Among the various approaches being studied, the recovery of coagulants from WTRs for reuse as a coagulant in wastewater treatment is extensively reported. This approach has several advantages, including reducing the volume of WTRs, minimizing the demand for fresh coagulants, lowering the cost of sludge disposal, reducing the toxic elements in water bodies, and reusing aluminum or iron present in WTRs as a recovered coagulant for removing contaminants. This paper presents various methods, including acidification, basification, membrane separation processes, and thermal treatment, used for coagulant recovery from WTRs. Factors affecting coagulant recovery and studies on optimizing the coagulant recovery processes are discussed. A brief overview of the use of recovered coagulant in wastewater treatment is also presented.