Sustainable Coagulation–Flocculation Pathways for Water and Wastewater Treatment: Advances, Mechanisms, and Environmental Implications of Natural and Tannin-Based Coagulants
Keywords:
Natural coagulants, Tannin-based coagulants, Wastewater treatment, Coagulation–flocculationAbstract
The treatment of water and wastewater remains one of the most critical environmental engineering challenges of the twenty-first century, driven by population growth, industrialization, agricultural intensification, and the increasing scarcity of clean water resources. Among the various physicochemical treatment processes, coagulation–flocculation has historically played a central role due to its effectiveness in removing turbidity, suspended solids, organic matter, nutrients, and a wide range of contaminants. However, the extensive reliance on conventional inorganic coagulants such as alum and iron salts has raised concerns related to sludge generation, residual metal toxicity, ecological impacts, and long-term sustainability. In response, there has been a growing global shift toward natural, plant-based, and tannin-derived coagulants as environmentally benign alternatives. This article provides an in-depth, theory-driven and critically elaborated analysis of natural and tannin-based coagulants for water and wastewater treatment, strictly grounded in existing scholarly literature. The discussion integrates insights from studies on plant extracts, fruit peels, seeds, chitosan, and tannin-rich biomaterials, as well as comparative evaluations against conventional chemical coagulants. Particular emphasis is placed on wastewater streams such as textile effluents, dairy wastewater, aquaculture discharges, biodiesel wastewater, vegetable oil refinery effluents, and drinking water turbidity control. Beyond performance metrics, the article deeply explores coagulation mechanisms, extraction and valorization pathways, environmental impacts, cost considerations, and emerging research trajectories. By synthesizing these dimensions, the study contributes a comprehensive academic perspective on how natural coagulants can support sustainable water management while addressing technical, environmental, and socio-economic constraints.
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