Optimization of surface water treatment using indigenous biocoagulants ( Opuntia ficus-indica and Moringa stenopetala ): a central composite design approach

ABSTRACT The growing concerns over the economic, health, and environmental impacts of traditional chemical coagulants have sparked a global interest in finding more sustainable alternatives for water treatment. This study explores and compares two locally available Ethiopian plant-based coagulants, cactus (Opuntia ficus-indica) and Moringa stenopetala, to determine their effectiveness in purifying contaminated surface water. Using response surface methodology with a face-centered central composite design, we examined how four critical operational factors – pH, coagulant dosage, stirring speed, and stirring time – influence the removal of color, chemical oxygen demand, and turbidity from water. The findings highlight a clear difference in the optimal conditions for each coagulant: cactus showed the best performance in highly acidic environments (pH 2.5), while M. stenopetala was most effective in alkaline conditions (pH 10). More notably, M. stenopetala outperformed cactus across all key metrics, achieving M. stenopetala achieved high physical clarification (color: 99.4%, turbidity: 96.6%) but moderate dissolved organic removal (chemical oxygen demand (COD): 56.1%), which is consistent with the known limitations of natural coagulants for treating dissolved organic matter. The study's statistical models proved both reliable and experimentally validated. It highlights M. stenopetala as a natural, eco-friendly, and affordable bio-coagulant, an effective water purification solution, especially for low-resource communities.