Dose–Turbidity Response of Poly Aluminium Chloride in Raw-Water Treatment: Jar Tests and Conditional Feed-Rate Estimation

Abstract

Poly aluminium chloride (PAC) is used to destabilize suspended and colloidal particles during raw-water clarification. This study evaluates the relationship between PAC dose and residual turbidity for five raw-water samples and estimates a corresponding chemical feed rate for a reported raw-water flow of 200 L/s. Jar tests were conducted using 1,000 mL samples, six PAC doses (30, 32.5, 35, 37.5, 40, and 42.5 mg/L), rapid mixing at 140 rpm for 5 min, slow mixing at 50 rpm for 10 min, and settling for 20 min. Initial turbidity ranged from 25.2 to 51.7 NTU, while final turbidity across all tested conditions ranged from 0.567 to 3.73 NTU. The lowest observed final turbidity for each raw-water sample occurred at 37.5 mg/L for initial turbidities of 25.2, 31.8, and 51.7 NTU, and at 42.5 mg/L for initial turbidities of 27.3 and 30.6 NTU. Thus, the dataset does not support a universal turbidity-minimizing optimum of 30 mg/L. However, 30 mg/L was the lowest tested dose and produced final turbidity values of 1.16–3.73 NTU across the five samples. A feed-rate estimate of approximately 56.07 mL/s at 30 mg/L is obtained only if the reported 10.7% PAC concentration is interpreted as 107 mg/mL (w/v); this concentration basis must be confirmed before operational use. The results provide a site-specific dose-response assessment, but replication, water-chemistry measurements, and field validation are required before the dose can be generalized or described as economically optimal.