Varying Alum
In the experiment, to investigate alum dosage at a given turbidity and length, we first planned to vary the alum dose over a large range, incrementing the dose in set intervals while holding the plant flow rate (shear), influent turbidity, and residence time constant.For all replicates, the flow rate was held constant at 5 ml/s. This corresponded to an average velocity of 7.01 cm/s, a shear rate (G) of 57.56 1/s, and an energy dissipation rate of 0.00177 m2/s3.
In this experiment, we focused on influent turbidities of 5, 100 and 500 NTU on three different flocculator lengths (28 m, 56 m, 84 m). The flocculators had residence times of 398.71 s, 797.42 s, and 1196.13 s, respectively. This corresponded to Gθ values of 22951, 45902, and 68853 respectively. We varied alum across a range from 0-90 mg/L. This was accomplished using one of two increment functions. To study low alum dosages, we used a power increment function with a base of 1.5 mg/L. For standard tests, we used a linear increment function with a slope of 10 mg/L
Additionally, the flow rate was held constant
The alum dose was varied in process controller using the increment function shown in Figure 1:
FIGURE 1: Process Controller setpoints used to vary the alum dose.
The following are characteristic Process Controller Files were used in our experiments. We provide one example of a low alum dosage (power increment) test, and one example of standard alum dosage (linear increment) test: