Consider a tank of drinking water (tank capacity = 10,000 liters ) that is conti

Question

Consider a tank of drinking water (tank capacity = 10,000 liters ) that is continuously stirred (well - mixed). Initially, 100 liters per minute of water enter the tank and 100 liters per minute of water leave the tank (the tank is in steady state). At some point, the source becomes contaminated with ethylbenzene. The concentration of ethylbenzene in the source is 10mg/L. The contamination in the source goes unnoticed long enough for the tank to become contaminated, and the concentration of ethylbenzene in the tank becomes 10mg/L . At this point, the mixer is turned off. Assume the outflow has a constant concentration of 10 mg/L and the source no longer contains ethylbenzene. The EPA's established Maximum Contaminant Level (MCL) for ethylbenzene in drinking water is 0.7 mg/L. How long would you have to wait to turn the mixer back on to obtain a concentration of 0.7 mg/L?

Explanation / Answer

If 10 mg / lit means 10,000 liters having = 10000 x10 /1000 = 100 Kg ethylbenzene

if we reduce 100 kg to 7 kg it will become = 7 x 1000/10000 = 0.7 mg /Lit

If we leave water flow for 93 min total water will go out = 93 x 100 = 9300 Liter /10 mg ethyl benzoate

rest 7 kg of ethyl benzoate will give 0.7 mg/Lit

Hence we have to wait for 93 min

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