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Disinfection Byproducts

Disinfection, sometimes referred to loosely as chlorination, is a necessary part of the water treatment process. It kills pathogens, and it produces chemical byproducts. Disinfection is typically done by adding small amounts of a chlorine-based disinfectant to water. It destroys water-borne microbes, bacteria, and viruses — organisms that can cause serious illnesses or death. Typhoid and cholera, which have killed hundreds of thousands of people in global epidemics, have been controlled in the United States through the addition of disinfectant to drinking water.

Denver Water has to be diligent to keep bacteria out of our water. There is a possibility that microorganisms might get into treated water after it leaves the treatment plant. This is why public health regulations require that tiny but detectable amounts of disinfectant must remain in the water all the way to the tap. Disinfectant ensures that the water coming from your tap is free of pathogens.

So small amounts of disinfectant are added to the water before it leaves the plant. And as the disinfectant reacts with organic substances in the water, byproducts are created.


The term "disinfection byproducts" covers a host of compounds that may be formed after water is treated. Depending on the substances present in the water, a wide range of byproducts may be created. But the only reliable approach to protecting against bacteria in drinking water is to add a disinfectant, which must travel with the water, in minute amounts, all the way to your tap. And any disinfectant creates disinfection byproducts. There is little evidence that these chemicals are dangerous at the level they occur in treated water, but research on the potential effects of specific compounds raised concerns with the Environmental Protection Agency.

Ongoing research is attempting to determine what health risks these compounds may pose. In the meantime, public water providers must follow regulations that keep the disinfectant, the microorganisms and the byproducts at safe levels.

What are disinfection byproducts? Why disinfect water?

The technical name for purifying water of pathogens (bacteria, microbes, and viruses) is "disinfection." Typically, this is done by applying small amounts of a chlorine-based disinfectant to the water, which kills the pathogens. However, the disinfectant also interacts chemically with organic substances to produce byproducts with technical acronyms like TTHM (total trihalomethanes) and HAA5 (haloacetic acids).

Disinfection of drinking water is identified by the U.S. Centers for Disease Control and Prevention as one of the most significant public health achievements of the 20th century. In the early 1900s, before disinfection of water was widely adopted, water-borne diseases such as typhoid, cholera and diphtheria killed many thousands of people.

What are "chlorine-based disinfectants," exactly?

Chlorine is familiar to most people as a household disinfectant and swimming pool conditioner. In very small quantities it kills a host of pathogens common in untreated water. Chloramine is formed by mixing chlorine and ammonia. It kills microbes effectively, does not dissipate as easily as chlorine, and does not produce as many DBPs. Note that unlike chlorine, chloramine cannot be removed by letting water set for a few days (as aquarists do before adding fish). In our service area, aquarium water must be treated to remove chloramine.

Why not change to disinfection methods that don't create these byproducts?

All chemical disinfectants have byproducts. Public health regulations require that water be disinfected at the treatment plant and that some disinfectant remain with the water until it arrives at your taps. Some utilities have adopted non-chemical disinfection methods, such as ultraviolet light. Not only is this method very expensive both to install and maintain, but it does not solve the problem of disinfecting the water after it leaves the plant. Utilities with ultraviolet systems add small amounts of disinfectant to the water as a final treatment step before release, so the problem of byproducts is not eliminated, though it may be reduced.

Are there regulations on these byproducts?

The EPA has regulated levels of microorganisms, the use of disinfectants, and their resulting byproducts since 1979 to ensure a proper balance of safety. Regulations predate the EPA by many years, however. Prior to the founding of the EPA, the Public Health Service regulated microorganisms in drinking water.

EPA regulations cover acceptable, safe levels of microorganisms, disinfectant and disinfection byproducts. In December 2005, after a lengthy review process which included water providers and environmental groups, the EPA added two new rules to the Safe Drinking Water Act: one modifies the old rules for measuring byproducts of disinfecting potable water and the other regulates protection against disease-causing microorganisms (primarily Cryptosporidium).

What has changed about the EPA-required monitoring?

The new standards do not change the maximum contaminant level (MCL) for the byproducts. What they change is the way the water system is monitored to collect data. Before, a water provider could average readings from all checkpoints, and compliance meant keeping that average below the limit. Under the new rules, each checkpoint must remain below the limit.

How dangerous are disinfection byproducts?

Some medical research suggests that specific DBPs (total trihalomethanes and haloacetic acids, referred to as TTHMs and HAA5 may cause a variety of medical conditions, including some forms of cancer and miscarriage. Thirty years of research have not provided definitive results, only sparse and inconsistent findings. But even the possibility of very minor effects is a concern to water systems and the public, which is why the EPA has established maximum contaminant levels (MCLs) for the suspect DBPs. To see details on the EPA's research that resulted in the new regulations, go to their SafeWater site.

As shown in our annual water quality reports, Denver Water has been in compliance with the standards, based on testing methods then in place, for some years. Keeping your water safe is a key part of Denver Water's mission, just as it is the EPA's and that of the Colorado Department of Public Health and Environment.

Should I get a filter for disinfection byproducts?

All treated water contains some disinfectant (as required by public health regulations) and therefore also contains some DBPs. Home water filters can reduce the levels of DBPs in tap water if they are models designed to do so. If you want to use a filter, be sure you get one that provides the specific filtration you want (chlorine, lead, etc.) and that it is approved by NSF International. And be sure to follow maintenance instructions provided with your point-of-use device.

How does Denver Water address these byproducts?

Our water is treated so that it meets all present health standards, including the new rules. Denver Water has spent millions of dollars to upgrade our treatment plants to specifically reduce the concentration of disinfection byproducts in the drinking water.

Because Denver’s water is collected high upstream, above major agricultural or municipal developments, organic materials are much less of a problem than they are for treatment facilities farther away from the origin point of the water. Organic content is a naturally occurring part of fresh ("raw") water, and it must be controlled. Our process ensures that the water is safe to drink. Testing to verify the quality of that raw water and the effectiveness of the treatment are important parts of the water quality process. Because the concentration of organic material is low, the amount of disinfection byproducts created is also low.