Evaluating the Application of Cationic Acacia Tannins in Wastewater Treatment as Bio-Based Alternative to Synthetic Flocculants

Polyacrylamide (PAM), the most widely used flocculant in wastewater treatment, has raised sustainability concerns because it is derived from fossil-based raw materials. This study evaluated cationic tannins derived from Acacia as a bio-based alternative to PAM for the chemical treatment of dairy wastewater. Jar tests combined with design of experiments (DoE) and response surface optimization were used to investigate the effects of FeCl3, PAM, and tannin dosages on the removal of suspended solids (Susp), total phosphorus (TP), and chemical oxygen demand (COD). Samples were subjected to rapid mixing (150 rpm, 2 min), followed by slow mixing (80 rpm, 5 min) and a sedimentation time of 30 min. The combination of 360 µL L−1 FeCl3 and 100 mg L−1 tannins achieved treatment efficiencies comparable to those obtained with 360 µL L−1 FeCl3 and 25 mg L−1 PAM, indicating that approximately four times more tannin than PAM was required. At optimized FeCl3–tannin dosages, removal efficiencies approaching 100% for Susp, TP, and COD were obtained. Tannins used without FeCl3 achieved approximately 98%, 70%, and 76% removal of Susp, TP, and COD, respectively. Treatment costs were estimated at 2.78 SEK m3 for the FeCl3–tannin system compared with 1.83 SEK m3 for the FeCl3–PAM system, corresponding to an additional cost of approximately 0.09 € /m3. However, costs could be reduced by optimizing the FeCl3-to-tannin ratio. Furthermore, sludge containing 100–200 mg L−1 tannins showed enhanced biogas production. These results demonstrate that cationic Acacia tannins are a promising bio-based alternative to synthetic flocculants for dairy wastewater treatment, although further optimization and more full-scale validation are required.