Optimising alum-based turbidity removal using quartz sand as a coagulant aid

Abstract

Locally available coagulant aids can significantly enhance water treatment while reducing chemical requirements and costs. This study investigated quartz sand, a low-cost, inert material, as a coagulant aid to improve turbidity removal by alum in highly turbid river water used for irrigation. Quartz sand was selected for its ballast properties, which provide nucleation sites for floc formation and produce larger, faster-settling flocs. A Box–Behnken design under Response Surface Methodology was employed to optimize pH, alum dose, quartz dose, agitation speed, and agitation time. Standard 500 mL jar tests were conducted at varied conditions. Alum alone achieved a 94.95% turbidity reduction under optimal conditions (alum 5 mg/L, pH 6.5, 150 rpm, 32.5 min). Combining alum with fine quartz sand increased removal efficiency to 96.70% at a much lower coagulant dosage (alum, 1 mg/L; quartz, 0.0135 g/L; pH 7.6; 85 rpm; 20 min). The regression model (ANOVA, p < 0.0001) for the alum–quartz demonstrated excellent predictive power (R2 = 0.9826). Quartz sand enabled an 80% reduction in alum usage, resulting in substantial cost savings without introducing chemical contaminants due to its inert nature. Combining quartz with alum is operationally feasible and environmentally beneficial, making it a promising approach for improving water treatment.