Abstract

Potable water scarcity, resulting from the imbalance between increasing water demand and limited freshwater availability, poses a significant global challenge that necessitates the development of sustainable wastewater treatment technologies. Treated greywater can be reused for non-potable applications, such as toilet flushing and landscape irrigation, thereby reducing pressure on freshwater resources. This study critically reviews the application of plant-based coagulants in greywater treatment for non-potable reuse. Coagulation and flocculation, which are essential processes in wastewater treatment, have traditionally relied on chemical coagulants such as aluminum sulfate (alum) and ferric chloride. However, these conventional coagulants are associated with several environmental and operational challenges, including residual chemical contamination, sludge generation, and alterations in water pH. In response to these limitations, natural coagulants derived from plants, animals, and microorganisms have emerged as promising environmentally friendly alternatives. Numerous studies have demonstrated their effectiveness in removing turbidity, chemical oxygen demand (COD), biochemical oxygen demand (BOD), total suspended solids (TSS), and total dissolved solids (TDS). Compared with conventional chemical coagulants, plant-based coagulants are biodegradable, less toxic, renewable, and capable of providing sustainable wastewater treatment with reduced environmental impacts. Despite their considerable potential, several challenges continue to hinder the widespread adoption of natural coagulants. These include variability in raw material composition, the absence of standardized extraction and preparation methods, limited pilot-scale and full-scale evaluations, and uncertainties regarding economic feasibility. Consequently, future research should focus on optimizing extraction techniques, improving process standardization, evaluating hybrid treatment systems that combine natural and conventional coagulants, and assessing long-term operational performance under field conditions. Unlike most existing reviews, which primarily focus on drinking water or industrial wastewater treatment, this review specifically examines the application of plant-based coagulants in domestic greywater treatment and provides a comparative evaluation of different natural coagulants. By synthesizing current knowledge and identifying key research gaps, this review contributes to the advancement of sustainable greywater treatment technologies and supports the broader goal of improving water reuse and sustainable water resource management.

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