Drinking Water Disinfection: A public health success story
At the beginning of the 1900s, life was very different in the United States. Waterborne diseases like typhoid fever and dysentery were a common part of life—and a common cause of death, too. Many people thought that the taste of the water determined its purity, not knowing that even the best tasting water could contain disease-causing organisms.
In the early 1900s, cities started disinfecting drinking water supplies to kill bacteria, viruses, and other microorganisms that cause disease and immediate illness. Eventually, all Minnesota cities that get drinking water from lakes or rivers started to disinfect. As of 2018, approximately 725 community water systems in Minnesota provide disinfected drinking water.
Disinfection makes our water safer to drink, and we do not have to worry about the waterborne diseases of the past. Both the World Health Organization and the Centers for Disease Control regard disinfection of drinking water as one of the most important advances in public health.
How disinfection works
Public water systems play an essential role in protecting public health through treatment and disinfection processes. The most common method of disinfection is through the addition of chlorine to drinking water supplies. Chlorine effectively kills waterborne bacteria and viruses and continues to keep the water safe as it travels from the treatment plant to the consumer's tap. For more information on chlorination, visit Drinking Water Chlorination: Frequently Asked Questions.
Disinfection byproducts
Although chlorine has been a literal lifesaver with regard to drinking water, it also has the potential to form byproducts that can cause harmful health effects. Chlorine can react with organic materials in water to form disinfection byproducts (DBPs). The formation of DBPs is usually a greater concern for water systems that use surface water, such as rivers, lakes, and streams, as their source. Surface water sources are more likely to contain the organic materials that combine with chlorine to form DBPs.
Scientists have identified hundreds of DBPs. Several types of DBPs have limits set by the U.S. Environmental Protection Agency (EPA): trihalomethanes (THMs), haloacetic acids (HAAs), chlorite, and bromate. EPA set these limits by balancing the health benefits of water disinfection with the risk of exposure to disinfection byproducts. To learn more, visit EPA’s National Primary Drinking Water Regulations – Disinfection Byproducts.
All public water systems that disinfect must regularly test their treated water to determine if regulated DBPs are present and at what levels. If they are above the limits set by EPA, the water system must take action to reduce the DBPs. Actions could include adjustments to organics removal processes, disinfection dose and location, and distribution system management. The water system must also notify all of their customers of the DBP levels.
The Minnesota Department of Health sets health-based guidance values for some DBPs. These values are protective for the most sensitive and/or highly exposed populations. Minnesota’s public water systems are not required to meet health-based guidance values; they may use guidance values as goals, benchmarks, or indicators of potential concern. Guidance values are based only on potential health impacts and do not consider cost and technology of prevention and/or treatment and may be set at levels that are costly, challenging, or impossible for a water system to meet. To learn more, visit Guidance Values and Standards for Contaminants in Drinking Water.
Types of disinfection
Besides chlorine, there are several other types of disinfectants. Each has tradeoffs. Chloramines may form lower levels of regulated DBPs than chlorine, but, depending on the source water characteristics, they have the potential to form other DBPs and increase the risks of nitrate formation and corrosion in the distribution system. Ozone is effective and has no taste, but it can also create other DBPs and does not provide protection in the distribution system, so chloramines or chlorine must still be added to protect the water. Ultraviolet (UV) light is effective in clear water and does not form DBPs. But like ozone, UV light does not provide protection in the distribution system, so chloramines or chlorine must still be added to protect water from the treatment plant to the tap.
Addressing risks
We are always weighing the risks and benefits of any type of water treatment. The potential harmful effects of disinfection byproducts should be considered in light of the tremendous benefits of water disinfection. We should also consider the amount of these types of compounds that people are exposed to from other sources, such as processed foods and beverages, and the ability of water systems to reduce the level of DBPs in drinking water.
The World Health Organization states, "In all circ*mstances, disinfection efficiency should not be compromised in trying to meet guidelines for DBPs, including chlorination byproducts, or in trying to reduce concentrations of these substances.”1 The risk of not disinfecting drinking water—and exposing people to microorganisms that can cause illnesses—outweighs the long-term, low level risk of DBPs, particularly at the low levels typically found in U.S. water supplies.
Managing disinfection byproducts
Since the mid-1970s, when the threat posed by disinfection byproducts became known, water systems have been reviewing their operations to minimize THM and HAA formation without compromising public health protection from disinfection. This involves adjusting the type and amount of disinfectant used as well as where it is applied. In addition, as part of the treatment process, water systems optimize the removal of naturally occurring organic matter that can react with chlorine to produce THMs and HAAs.
There are also challenges associated with balancing multiple DBPs. Each DBP behaves differently in a distribution system. Some DBPs may be at low concentrations in areas where water has been in the distribution system for a short time. Others may be at low concentrations in areas where water has been in the distribution for a longer time. Concentrations of DBPs are also variable with temperature and seasonal water quality changes.
Drinking water treatment operations must meet competing objectives. They must provide adequate protection from bacteria, viruses, and microorganisms, while reducing levels of DBPs to meet EPA standards. It is not an easy task and requires close and continuous attention.
1Centers for Disease Control and Prevention Disinfection By-Products
