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