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Disinfection

Many factors in disinfection by-products removal

October 13, 2010
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Disinfection by-products (DBPs) are formed when a disinfectant such as chlorine or ozone or chloramine is added to water in order to kill pathogens (disease-causing microorganisms). In addition to killing the pathogens, the disinfectants react with naturally-occurring organic and inorganic materials known as DBP precursors to form disinfection by-products.

The types and levels of by-products formed (see chart) are specific to the water that is being treated, and there are literally hundreds of different disinfection by-products that have been identified in drinking water systems.

They typically form within a water treatment plant and then continue to form because the water distribution system will take the disinfectant out into the system; the chemical reaction continues in the distribution.

Disinfection still required
EPA now has several rules that limit the formation of disinfection by-products and that public water systems are required to follow. There’s a new rule that has not yet gone into effect that will tighten up the requirements a little more.

In January 2006 the EPA finalized the new rule, the Stage 2 Disinfection By-products Rule. It didn’t change the Stage 1 limits, but changed the way compliance was calculated, and the net effect is that individuals throughout the distribution system will receive better quality water, with a reduction in the number of bladder cancer cases that are associated with exposure to disinfection by-products.

However, it is important to remember that disinfection itself is an essential part of the treatment process because it kills pathogens which cause cholera, acute gastrointestinal diseases, giardiasis (from Giardia lamblia), and cryptosporidiosis (from Cryptosporidium) and other diseases. Many of these diseases especially affect those with lowered immunities.

POU/POE can help
NSF International, the third-party certification organization, does certify point-of-use/point-of-entry (POU/POE) devices that remove disinfection by-products.

EPA believes that a public water system in compliance with its rules is delivering water that doesn’t require any additional treatment.

EPA encourages (and in some cases requires) systems to lower the levels of those constituents that react with disinfectants to form DBPs, believing that the best first step is not to form DBPs or to lower the levels that are formed. That can be done through enhanced coagulation or activated carbon adsorption.

Some public water systems use a powerful disinfectant in the treatment plant and then, as a residual disinfectant in the distribution system, a less-reactive but longer lasting one such as chloramine. That forms lower levels of DBPs.

There are various strategies for disinfection and DBP minimization, depending on what is already in place, the particular water quality conditions, and the nature of the distribution system. There are many different factors, and there’s no one answer to fit everyone and no way to say what is the right answer for a system without looking at it in some detail.

Levels of DBPs vary by season, and their formation is affected by water temperature, pH, level and type of DBP precursors, and other water quality parameters. Many chemical reactions occur more rapidly under warmer conditions.

Many chemical reactions occur more rapidly under warmer conditions. Although DBP levels tend to increase the longer the water stays in the distribution system, temperature effects are sometimes mitigated. For instance, sometimes water stays in a distribution system a shorter time in the summer, when water temperature tends to be higher, because people are filling their pools and watering their lawns.

This is just one example of the complexity of factors involved in assessing DBP formation.

DBP raw materials
Typically, systems that use surface water as a source have higher levels of precursor materials, the organic and inorganic substances that react with disinfectants. These systems also typically have a larger, more complex treatment system to address that; because of that these systems have a greater capacity to control the levels of precursors.

Portions of the new Stage 2 Disinfection By-products Rule are in effect because large systems are already monitoring to identify those locations within their distribution system have higher levels of DBP.

Although full compliance by all public water systems won’t begin officially for a few years, most systems are already in compliance. They won’t have to make any treatment upgrades. However, a limited number of systems will have to make changes in the ways they operate.

DBPs is a very complex subject, because systems must balance the treatment needs and the risk from pathogens and from DBPs, and the nature of those risks is not perfectly understood. So there is an ongoing effort to learn more.

Public water systems, state and local regulatory and public health officials, and consumer advocates are all part of developing the standards included in the new rule.

EPA holds public meetings, sends out press releases and involves stakeholders in all of its processes to make the water industry aware of DBPs.


Tom Grubbs is an environmental engineer with the US Environmental Protection Agency (EPA).
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