Question

A CMFR will be used to reduce the concentration of chloroform(CHCl3) in water. Chloroform is a disinfection by-product that is formed when chlorine is used to disinfect water; it is a concern because it can cause cancer.The influent water contains 60 ppb chloroform and the Wisconsin State standard is 6 ppb. Chloroform decays in the reactor at a rate of 0.2 hr-1 and the water flow rate is 100 L/day.

Treating the water to meet the Wisconsin standard will reduce the lifetime cancer risk for ingesting the water to 10-5.Although this is within the range of the EPA risk guidelines (10-4to 10-7), the public is concerned about the presence of a carcinogenic compound in their water. Determine the size (in L) of the reactor necessary to achieve (a)a risk of 10-5(6 ppb) and (b)a risk of 10-7(0.06 ppb).How do the two reactors compare? Do you think it will be worth the extra cost to reduce the risk from 10-5to 10-7?

Answer 1

Since level of risk has to come down the size of reactor and duration also increases this leads to increase in cost of treatment ,as per Wisconsin State standard is 6 ppb so treating the water to meet the Wisconsin standard will reduce the lifetime cancer risk for ingesting the water to 10-5 since it is with the standards of EPA 10-4 to 10-7.it is considered to be safe enough and this is acceptable if further reduction in risk level good as per safety concern but extra cost is not worth enough