Blended natural and synthetic coagulants for the COD and BOD removal from surface water; optimization by response surface methodology: The case of Gibe river

A novel wastewater treatment method is presented in this study. It combines natural coagulants derived from watermelon seeds with the commonly used synthetic coagulant alum. This research demonstrates a remarkable synergy between these two coagulants in removing nutrients from Gibe River wastewater. Combining natural and chemical coagulants often improves water treatment by enhancing particle destabilization, accelerating floc formation, and broadening the range of removable contaminants, resulting in lower chemical dosage requirements. The optimal mixing ratio, found to be 1 part watermelon seed coagulant to 3 parts alum, leads to improved treatment efficiency. At this ratio, the process achieves impressive removal rates: 98.26 % for total dissolved solids (TDS), 96.10 % for biochemical oxygen demand (BOD), and 95.26 % for chemical oxygen demand (COD). These findings not only validate the use of watermelon seeds as a coagulant but also highlight the combined approach's environmental and economic benefits. This integrated method offers a more sustainable and cost-effective solution for wastewater treatment. •Wastewater treatment by natural coagulant derived from watermelon seeds with synthetic coagulant is a novel approach.•Treating Gibe River wastewater with natural coagulant (watermelon seed) and synthetic alum proves to be more effective.•The maximum removal efficiencies obtained at watermelon seed to alum a ratio of 1:3 were 98.26 % (TDS), 96.10 % (BOD), and 95.26 % (COD).•The combination strategy proves to be a superior wastewater treatment method, economically and environmentally advantageous.