What are disinfection byproducts?

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).

According to news stories of the time, 19th-century Denver residents rated their tap water by its low minnow count. Denver Water was among the first water providers in America to introduce chlorine-based disinfection and carbon-based filtering. Incidence of typhoid dropped dramatically between 1890 and 1940 as treatment methods became more effective.

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

Is our tap water safe?

Our water is treated so that it meets all present health standards, including the new rules. 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.

Why do you disinfect the water?

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 diptheria killed many thousands of people.

Should we get a filter?

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.

Should we drink bottled water?

The EPA encourages people to drink and use tap water provided by a water service that meets EPA standards. Denver Water has met all regulatory standards throughout its history. Bottled water is not regulated by the EPA. Check with the bottling company to determine what safety standards apply to bottled water.

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

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.

If DBPs are harmful, why do utilities have six years to comply with the regulations?

Additional study is needed to determine the amount of risk and what might be causing it. There is sufficient concern to warrant changes, and the six-year period is to help utilities better identify any problem areas within their systems and to make the necessary capital improvements to mitigate them. The new regulations require that monitoring be done differently, and in some cases (distributors who purchase treated water for various suburban accounts) monitoring must be put in place that was not required previously. It will take some time to make the necessary changes. The results of the survey phase could require significant system modifications for water providers that are not in compliance. Denver Water is in compliance with both the old Stage I and new Stage II regulations, and we anticipate that the monitoring requirements for Stage II will not change that.

What has changed about the monitoring required by the EPA?

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. Additionally, the new rules require monitoring by distributors ("consecutive users") who deliver water to metro customers not part of the City and County of Denver, such as Lakewood and Littleton. Details of addressing this challenge are being worked out cooperatively with our distributors.

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, which is added in the final phase of Denver Water's process, 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.