Environmental Engineering

2. The design flow for a water treatment plant (WTP) is 1 MGD (3,800 m3/d). The rapid

mixing tank will have a mechanical mixer and the average alum dosage will be 35 mg/L. The

theoretical mean hydraulic detention time of the tank will be 1 minute. Determine the

following:

a) the quantity of alum needed on a daily basis in kg/d.

b) the dimensions of the tank in meters for a tank with equal length, width, and depth.

c) the power input required for a G of 700 s-1 for a water temperature of 5 °C –express the answer in kW (viscosity of 5 °C=1.52×10-3 N·s/m2)

3. Flocculation tanks are to be designed for a total flow rate of 40,000 m3/d. The following

conditions apply to the design: water temperature of 10 °C, total mean detention time of 40

min, basin depth of 3.5 m, 3 parallel trains of flocculators (each train receives one third of the

total flow), 3 flocculation stages of the same dimensions for each train (so a total of 9

basin), the first stage G is 50 s-1, the second stage G is 30 s-1, and the third stage G

is 20 s-1. (viscosity of 8 °C=1.39×10-3 N·s/m2)

Determine:

a) The dimensions of each basin, in meters,

b) the power required for each stage, in kW.

4. A rapid-mix tank is designed for a new water treatment plant. The design flow rate is 0.050 m3/s. The average water temperature is 10 °C. The following design assumptions for a rapid-mix tank have been made:

a. Number of tanks = 1

b. Tank configuration: circular with liquid depth = 1.0 m

c. Detention time = 5 s

d. Velocity gradient = 700 S-1

e. Impeller type: Radial turbine, 6 flat blades, Np = 3.6

f. Available impeller diameters: 0.25, 0.50, and 1.0 m

g. Assume B = 1/3 H

h. viscosity of 10 °C=1.31×10-3 N·s/m2

Design the rapid-mix system by providing the following:

a. Water power input in kW

b. Tank dimension in m

c. Diameter of the impeller in m

d. Rotational speed of impeller in rpm

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