Trihalomethanes are a group of volatile organic chemicals that are formed when chlorine, added to drinking water during the treatment process for disinfection, reacts with naturally occurring organic matter in the water. These compounds are present at various levels in all drinking water systems that treat surface water and use chlorine in the treatment process. Disinfection byproducts for which regulations have been established include trihalomethanes (chloroform, bromoform, bromodichloromethane, and dibromochloromethane), haloacetic acids, bromate, and chlorite.
Disinfection is a chemical process used in drinking water treatment to kill disease causing organisms found in the source water. This process usually involves the use of disinfectants such as chlorine, ozone, chlorine dioxide, or a combination of chlorine and ammonia (chloramines).
As is the case with many environmental contaminants, some studies have shown that high dosages of these compounds given to laboratory animals over long periods of time may be correlated with the development of certain types of cancer in these animals. Accordingly, the EPA has sought to regulate and limit the amount of THMs in drinking water.
| Year | Maximum Contaminant Level | Measurement |
| 2001 | Not to exceed 100 parts per billion(ppb) | Annual system wide average |
| 2002 | Not to exceed 80 ppb | Annual system wide average |
| 2006 | Not to exceed 80 ppb | Annual system wide average |
| Not to exceed 120 ppb | Annual average at any site | |
| 2009 | Not to exceed 80 ppb | Annual system |
Quarterly by the State. Samples are collected at the plant and at various locations throughout the system.
The measured levels of THMs in WCID 17 drinking water do not present a significant health risk. District 17 has reduced the average amount of THMs in the water to acceptable levels, and is trying for further reductions every year.
Reducing the amount of THMs in the finished water while still maintaining enough disinfection residual in the system to prevent disease is a challenge faced by many water systems. As requirements become more stringent, adjustments must be made to stop the formation of these compounds. Several techniques which WCID 17 is using are:
Use of chloramines for disinfection – District 17 has used chloramines since the early 90s to aid in disinfection as well as reduce THMs.
Use of membrane microfiltration technology – This year, the District will finish construction on a 12 million gallon per day ultrafiltration membrane plant. Membranes do not require prechlorination, and do not require additives such as alum or polymer to assist coagulation. These types of plants generate less THMs.
Addition of Chlorine Dioxide as a pretreatment to reduce organic precursors of THMs – A complete system to use this technology has been installed and is in operation.
Water systems are required by the state to have a disinfectant residual in the distribution system (piping and tanks) to prevent bacterial contamination. Although they can disinfect water at the plant, neither ozone nor ultraviolet light can provide this residual disinfectant property. In addition, each type of disinfectant has its own set of inherent disadvantages. Each approach has advantages and disadvantages as well as different degrees of effectiveness and unit costs.
