Comparative study of natural and chemical coagulants for continuous-flow treatment of textile wastewater

• Chestnut shell coagulants are a sustainable solution for treating textile effluent. • In continuous-mode operation natural coagulants outperformed Al in color removal. • Chestnut shell coagulants achieved over 80% removal of initial phosphorus content. • Natural coagulants effectively reduced organic matter in continuous treatment flow. Coagulation/flocculation are widely used techniques for treating textile effluents, with increasing interest in natural coagulants, particularly tannin-based ones. The textile industry consumes approximately 93 billion m 3 of water annually, with limited reuse. Given Portugal’s significant role as both a chestnut producer and a textile exporter, this study explores the application of a chestnut shell (CS) based coagulant for textile wastewater treatment. Tannins were extracted from frozen chestnut production residues chemically modified via the Mannich reaction using NH 4 Cl and formaldehyde, following established methods. The performance of the CS-based coagulant in color and dissolved organic carbon (DOC) removal was compared to Tanfloc and aluminum sulfate in batch mode (Jar test). All coagulants exhibited optimal color removal at pH 5, with concentrations of 125 mg·L −1 for Tanfloc and 150 mg·L −1 for both the CS-based and Al-based coagulants, achieving removal rates of 75.8 ± 0.3 %, 84.6 ± 0.3 %, and 81.6 ± 0.3 %, respectively. After determining optimal conditions, continuous-mode coagulation was tested. In this mode, coagulants showed similar color removal performance, with efficiencies of 70.4 ± 0.1 % (CS), 84.8 ± 0.03 % (Tanfloc), and 61.8 ± 0.2 % (Al 2 (SO 4 ) 3 ). Unlike in batch mode, all coagulants contributed to DOC reduction in continuous mode—a noteworthy result, given that tannin-based coagulants often increase DOC in treated water. DOC removal efficiencies were 12.2 ± 0.5 % (CS), 63 ± 2 % (Tanfloc), and 64 ± 2 % (Al 2 (SO 4 ) 3 ). All coagulants effectively removed phosphorus, with natural coagulants performing better. Only chemical coagulants achieved significant nitrogen removal. A SWOT analysis confirmed CS-based coagulants as a viable and sustainable option for textile effluent treatment, especially in continuous-mode operations.