Rubber wastewater treatment via sequential coagulation-flocculation and catalytic UV-Fenton process: finding synergy through kinetic analysis and cost comparison

The wastewater from natural rubber manufacturing units in rural areas is characterized by high concentrations of organic acid, chemical oxygen demand (COD), acidic pH, nitrate, turbidity, solids, and phosphate. This can result in soil degradation, groundwater contamination, surface water pollution, and health issues during its final disposal. The possibility of integrating zinc oxide nanoflakes (ZnO nanoflakes) during the ultraviolet (UV)-Fenton process for the efficient treatment of rubber wastewater was evaluated in this study. The reaction rates of various treatment processes, including coagulation-flocculation (CF), UV alone, ZnO-assisted UV-Fenton, and CF followed by ZnO-assisted UV-Fenton, were estimated using different kinetic models. Additionally, the synergistic effects among these processes were analyzed. CF pretreatment achieved 35% COD and 91% turbidity removal at 2 g/L alum dosage and pH 5.5. Meanwhile, the UV-Fenton treatment resulted in 32.6% COD removal at 5.5 mol H 2 O 2 : g Fe 2+ after 60 min of contact time at pH 3.5. On the other hand, ZnO nanoflakes-assisted UV-Fenton was efficient in removing 55% of COD at a ZnO nanoflakes dosage of 500 mg/L and Fenton reagent dosage of 5.5 mol H 2 O 2 : g Fe 2+ . Moreover, ZnO nanoflakes-assisted UV-Fenton achieved an overall COD removal of 75% from the CF pretreated sample at the same ZnO nanoflakes and Fenton reagent dose. The overall cost of treatment was found to be USD 11.7 /kg for CF, followed by ZnO nanoflakes-assisted UV-Fenton, and the electrical energy order was estimated to be 723 kWh/m 3 /order. The combined treatment system demonstrates efficiency, synergy, energy sustainability, and cost-effectiveness in rubber wastewater treatment.