Water pollution, exacerbated by dye-intensive industries such as textiles, seriously threatens human health and aquatic ecosystems by disrupting photosynthesis and degrading habitats. Eliminating dyes from wastewater is crucial to environmental sustainability. Traditional chemical coagulants used in water treatment are often toxic and expensive. This study examines watermelon rind powder as a natural and safer alternative for removing toxic synthetic dyes such as methylene blue and crystal violet from wastewater, using response surface methodology to optimize the process. The watermelon rind powder was prepared and characterized through various techniques including X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, and zeta potential analysis. The coagulation-flocculation process was optimized using a Box-Behnken design, examining the influence of coagulant dose (200–500 mg L−1), particle size (60–120 μm), and initial solution pH (3–9) on the removal of methylene blue and crystal violet. A quadratic model was developed using analysis of variance to evaluate the statistical significance of the input parameters, describing the relationship between these parameters and pollutant removal. The results showed that watermelon rind powder achieved removal rates of 97.86 % for crystal violet and 92.53 % for methylene blue. The effectiveness of watermelon rind was further supported by zeta potential measurements, indicating that its active compounds contribute to dye removal through mechanisms like charge neutralization and adsorption.