During a regular cleaning cycle for a cylindrical drinking water storage tank (diameter = 70 feet; depth = 12 feet), a technician accidently overdoses the tank with chlorine. The resulting chlorine concentration in the tank is 18 mg/L and the EPA limit for chlorine in drinking water is 4.0 mg/L. Now, the tank is completely full of water that has a higher chlorine concentration than allowed by law. The operator decides to flush the tank with water from the distribution system which has a chlorine concentration of 1.8 mg/L and waste the water to the sewer until the chlorine level reaches the EPA limit. You may assume that for the timeframes considered here that chlorine is conservative (i.e., non-reactive) and the tank is completely mixed. 

 

a. If the tank is flushed at a rate of 4,000 gpm, how long (in hours) will it take to achieve the 4.0 mg/L concentration? 

b. The operator is impatient and wants to get this done in 30 minutes. What flushing flowrate (in gpm and cfs) would be required to achieve the 4.0 mg/L concentration?

 c. Calculate the volume (in gallons) of water used (i.e., wasted) for Part A and Part B. Is there

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Given data Initial chlorine concentration in the tank 18 mgL EPA limit for chlorine concentration 40 mgL Chlorine concentration in the water used for ... View the full answer