Different sets of glass flocculation experiments were performed at predetermined pH values to determine the effect of pH changes on coagulation ( Figure 1 ). Before starting each glass test, the pH of the raw water was adjusted to a predetermined pH value in the range of 6.0 to 8.5 using 0.10 N HCl or NaOH. For the experiments, the cups were first mixed at high speed to ensure rapid mixing of the coagulant, and then mixed relatively slowly to achieve flocculation – the different mixing speeds form a pair of mixing speeds. Three pairs of different mixing speeds (i.e. 40/180, 25/180, 25/150 rpm) were used in the experimental group.
In the first experiment, the pH of the raw water was adjusted to 6. Each of six 1-L flocculant vessels ( Figure 1 ) was filled with raw water and five were dosed with PAC to achieve concentrations of 15, 20, 25, 30, and 35 mg/L. The sixth vial was kept empty (i.e., PAC was not dosed). The mixing time adopted was 5 minutes fast and 10 minutes slow. In other words, the stirrers for each jar were set to 180 rpm for 5 min (rapid mixing) and then to 40 rpm for 10 min. After the specified mixing time has elapsed, the samples are allowed to stand for 15 minutes.
Then, the liquid on top of the cups was sampled and its turbidity and color were determined using a HANNA turbidimeter and a HACH spectrophotometer (model DR 3900), respectively. To ensure high accuracy, the experiment was repeated five times and average values of water quality parameters were determined. Other sets of experiments were performed at the same pH using different mixing speed pairs – 25/180 and 25/150 rpm. This experimental set was repeated for sample sets with the pH adjusted to 6.5, 7.0, 7.5, 8.0, and 8.5, respectively.