Turning Vegetable Residues into Coagulants: Sustainable Solutions for Agri-Food Wastewater

The increasing global population poses major challenges for food production. With the increasingly fast pace of people’s lives and the growing demand for healthy and ready-to-eat foods, the frozen vegetable and ready-to-eat salad sector in supermarkets is expanding, driving growth in the agri-food industry. These industries generate large amounts of vegetable residues and wastewater. Coagulation/flocculation is a common treatment method, but its reliance on metallic salts poses environmental and economic drawbacks. This study explored sustainable alternatives using tannin-based coagulants from vegetable residues. Kale residues were first investigated as a potential tannin source. A Box–Behnken design evaluated extraction time, temperature, and liquid-to-solid ratio, with distilled water as solvent. Optimum conditions (50 min, 40 mL·g^-1, 75 ºC) yielded 49.3±0.1% extraction and 23.4±0.8 mg GAE/g of extract. However, kale proved unsuitable due to its low tannin content. Given Portugal’s role as a leading chestnut producer, along with the high tannin content of chestnut shells (CSs) and their proven capability as coagulant sources [1, 2], this residue was selected as an alternative. CS tannins were chemically modified to enhance their cationic properties and for application as coagulants for agri-food industry wastewater treatment. The efficiency of the CS-based coagulant was compared with that of Tanfloc, a commercial tannin-based coagulant. The CS coagulant was able to reduce the initial color by 74% at pH 6, with a dose of 125 mg·L^-1 and 30 minutes of sedimentation. In comparison, Tanfloc achieved a similar color removal (68%) under the same pH but required a higher coagulant dose of 450 mg·L^-1 and longer sedimentation time (135 min). Regarding organic matter, Tanfloc reduced around 34% of the initial dissolved organic carbon, whereas the CS-based coagulant achieved only 3%. It is possible to conclude that the CS-based coagulant is a good and sustainable treatment solution for color removal from agri-food industry wastewater.