<i>Opuntia ficus-indica</i> (L.) as a natural coagulant and alum aid for surface water clarification in tropical Uganda

ABSTRACT Diagrammatic representation of the use of alum, Opuntia ficus-indica extract, and their combination for water purification. The extraction, coagulation-flocculation, settling, and enhanced water quality results are summarised in the figure. Plant-based coagulants could reduce reliance on traditional chemicals for water clarification; however, their performance requires validation using actual, very turbid waters. This study assessed the efficacy of Opuntia ficus-indica (L.) extract, aluminium sulphate (alum), and a 1:1 Opuntia–alum formulation for the clarity of raw water in Uganda through jar tests, varying coagulant concentrations from 10 to 60 mg/L and specific pH conditions. Untreated water had a turbidity of 135 ± 5 NTU and a colour measurement of 884 ± 8 Pt–Co units. Opuntia eliminated 54–82% of turbidity and 56–76% of colour, while alum removed 78–83% and 76–89%, respectively. The hybrid formulation had the highest observed removals, attaining 96% for turbidity at 30 mg/L and 95% for colour at 60 mg/L. Nonetheless, trends in pH, alkalinity, and conductivity indicated that treatment chemistry was responsive to dosage and the alkaline extraction method. The findings offer region-specific evidence that Opuntia can serve as a coagulant aid for heavily contaminated tropical surface water. The study did not assess post-treatment microbial elimination, sludge characteristics, lifecycle effects, or full-scale expenses; hence, findings are restricted to clarifying efficacy and laboratory-scale operational behaviour.