Consider again the proposed water treatment plant for a city of 100,000 people w

Question

Consider again the proposed water treatment plant for a city of 100,000 people where the maximum daily water usage rate is 99 X 106 L/d. Determine (a) the number of kilograms of chlorine needed per day and (b) the capacity of the contact tank. You may assume that the chlorine demand is 1.0 mg/L and that the desired chlorine residual is 0.2 mg/L. Furthermore, you may assume that the minimum contact time should be 30 minutes. (The contact time in water treatment plants is usually provided by a large storage chamber called a clear well or storage reservoir. The primary function of the clear well is to isolate the plant from the hourly fluctuation of municipal water demand, but it also serves the purpose of allowing sufficient time so that the concentration of free available chlorine will stabilize at the proper value before being pumped to users.)

Explanation / Answer

(a).

Given :

chlorine demand is 1.0 mg/L and that the desired chlorine residual is 0.2 mg/L.

then total required chlorine for treatment plant,C = 1.0 + 0.2 = 1.2 mg/L

for the proposed water treatment plant for a city of 100,000 people has a maximum daily water usage rate, Q = 99*106 L/d.

then, total amount of chlorine needed per day = Q*C =99*106*1.2 = 118.8*106 mg = (118.8*106)*(10-6) = 118.8 kg

therefore, total amount of chlorine needed per day = 118.8 kg

(b).

maximum daily water usage rate = 99*106 L/d

and minimum contact time assumed, t= 30 minutes.

or t = 0.0208333 day

then, capacity of contact tank = Q*t = (99*106)*(0.0208333) = 2062500 L = 2062.5 m3

therefore, capacity of contact tank is 2062.5 m3

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