Study of Coagulant Dosage Variation According to the Design G Value of Coagulation and Flocculation Processes in a Water Treatment Plant

There are several treatment processes in conventional water treatment plants (WTP). The coagulation and flocculation processes (CFP) are conducted to remove the turbidity of raw water (RW). CFP has become critical in the water treatment process due to impacts from overdosing and underdosing of coagulant on the public health and treatment efficiency. Most of the WTPs in Sri Lanka perform the jar test considering only the RW pH and turbidity in order to determine the optimum coagulant dose. However, there is no concern over the velocity gradient (G) value in WTP design when conducting the jar test. Therefore, this study was conducted to compare the optimum coagulant doses obtained from a general jar test method (M1) and a WTP design G value considered method (M2) for the same RW sample. The rapid mixing, slow mixing speeds, and settling times for M1; 2 min in 150 rpm, 20 min in 30 rpm, and 30 min respectively. The rapid mixing, slow mixing speeds, and settling times for M2; 3 min in 71 rpm, 30 min in 40 rpm, and 30 min respectively. Two trials were carried out for each method using Poly aluminium chloride as the coagulant (PAC). The optimum coagulant dose of M1 is 0.62 mg/L in trial one and 3.95 mg/L in trial two. The optimum coagulant dose of M2 is 0.70 mg/L in trial one and 4.05 mg/L in trial two. Hence the optimum coagulant dose of M1 is greater than that of M2. Therefore, it is recommended to consider the design G value of the WTP to obtain a more accurate optimum coagulant dose.