In 1974 trihalomethanes (chloroform, bromodichloromethane, dibromochloromethane, and bromoform) were discovered to be formed during the disinfection step of drinking water if free chlorine was the disinfectant. This, coupled with the perceived hazard to the consumer’s health, led the U.S. Environmental Protection Agency to amend the National Interim Primary Drinking Water Regulations to include a maximum contaminant level of 0.10 mg/L tor total trihalomethanes.
For trihalomethane removal, aeration – either by diffused-air or with towers – and adsorption – either by powdered activated carbon or granular activated carbon – is effective. The major disadvantage of this approach is that trihalomethane precursors are not removed by aeration. For trihalomethane precursor control, effective processes are: (1) oxidation by ozone or chlorine dioxide; (2) clarification by coagulation, settling and filtration, precipitative softening, or direct filtration; or (3) adsorption by powdered activated carbon or granular activated carbon.
In addition, some modest removal or destruction of trihalomethane precursors can be achieved by oxidation with potassium permanganate lowering the pH, or moving the point of chlorination to the clarified water. Lowering of trihalomethane precursor concentrations has the additional advantage of reducing overall disinfectant demand, thereby reducing the possibility of the formation of all disinfection byproducts. Neither chlorine dioxide, nor ozone, nor chloramines produce trihalomethanes at significant concentrations when used alone as disinfectants. Furthermore, the cost of any of these unit processes is very low. The major disadvantage of using alternate disinfectants for trihalomethane control relates to the lack of any precursor removal.