Disinfection by-products.

What Are Disinfection By-products?

Chlorine is the most widely used water disinfectant because of its effectiveness and cost. The use of chlorine as a drinking-water disinfectant has prevented millions of waterborne diseases such as typhoid, cholera, dysentery, and diarrhea. Most states require that community water systems use chlorination.

Nevertheless, research shows that chlorine has side effects. It reacts with organic matter present in water and forms a series of compounds that have been linked to cancer in animals. These compounds are called disinfection by-products (DBPs).

Disinfectants form DBPs in one of two reactions: 1) Chlorine and chlorine-based compounds (halogens) react with organics in water, causing the chlorine atom to substitute for other atoms and resulting in halogenated by-products. 2) In oxidation reactions, chlorine oxidizes compounds present in water. When multiple disinfectants are used, secondary by-products are also formed.

Where Does the Organic Matter Originate?

All living organisms have carbon as an essential element in their cells. When trees shed their leaves, they start decomposing and are ultimately broken down by bacteria into carbon-containing compounds. Similarly, dead animals on land, and fish and other aquatic life decompose and disintegrate into compounds that contain carbon as an essential element. Hence, all surface water and groundwater contain varying amounts of carbon-containing compounds called organic matter (primarily humic and fulvic acids).

What Are the Types of DBPs?

Disinfection can produce hundreds of by-products when chlorine reacts with organic matter. Two major classes, trihalomethanes (THMs) and haloacetic acids (HAA), make up the bulk of these by-products. The four THMs are chloroform, bromoform, bromodichloromethane, and dibromochloromethane. The five HAAs--chloroacetic acid, dichloroacetic acid, trichloroacetic acid, bromoacetic acid, and dibromoacetic acid--are commonly abbreviated HAA5. In addition, there are a variety of other compounds, such as haloacetonitriles, haloketones, haloaldehydes, chloropicrin, cyanogens chloride, and chlorophenols. Alternative disinfectants, such as chloramines, chlorine dioxide, and ozone, can also react with organics to form organic by-products.

Temperature, time, and water pH, along with the disinfection process and other source water characteristics, determine which DBPs will form. Most reactions that form DBPs occur in the first 24 hours. Because the pH determines in part which DBP will be formed, setting the pH entails risks and risk tradeoffs. For example, lowering pH to control for trihalomethane (THM) formation can result in the increased formation of trihaloacetic acids. Reaction time is also an important variable. For example, chloral hydrate is unstable at high pH levels, and over time, it degrades to chloroform, which results in increased THMs.

What Does the DBP Rule Say?

In 1979, the U.S. Environmental Protection Agency (U.S. EPA) adopted regulations that established a maximum contaminant level (MCL) of 0.1 mg/L for total trihalomethane (THM). In 1998, the Stage 1 Disinfection By-Products Rule (DBPR) was established, updating and superseding the original THM limits. A Stage 2 DBPR has been proposed that would supplement the Stage 1 rule and require systems to comply with by-product MCLs based on locations in the distribution system. U.S. EPA conducted its first meeting about the Stage 2 rule in March 1999. The last public hearing on the Stage 2 Rule was in January 2005, with a final rule expected soon.

What Are the Health Effects of DBPs?

DBPs have been linked to bladder and rectum cancer, and may also have reproductive and developmental effects. Chloroform affects liver and kidney function in humans in both acute and long-term exposures. In lab studies on mice and rats, three THMs (bromoform, bromodichloromethane, and dibromochloromethane) caused changes in kidney, liver, and serum enzyme levels and decreased body weight. Dichloroacetic acid (DCA) and trichloroacetic acid (TCA), the HAA5s most commonly found, are also of concern. U.S. EPA has classified DCA as a human carcinogen. A DCA exposure for six to seven days of 43 to 57 mg/kg/day has been shown to result in mild sedation, reduced blood glucose, reduced plasma lactate, reduced plasma cholesterol, and reduced triglyceride levels. Studies in mice and rats have shown that DCA causes liver tumors. TCA has been shown to produce developmental malformations in rats, particularly in cardiovascular systems.

How Can DBPs Be Minimized?

There are four ways to deal with DBPs: 1) minimize precursors, 2) reduce disinfectant doses, 3) remove DBPs after they form, and 4) use alternative disinfectants.

Minimizing Precursors

One way to prevent DBPs is to prevent the occurrence of natural organic matter (precursors) in the source water. The precursor content of raw water can be reduced by blending source waters, removing precursors through treatment in the plant, disinfecting the water after all other treatment has been completed, or combining the three methods. Natural organic-matter levels can be reduced by adsorption with granular activated carbon and through coagulation with alum and ferric salts.

Reducing Disinfectant Dosages

Reducing the primary and secondary disinfection dosages and having booster chlorination later in the distribution system can help reduce disinfection loads. Avoiding prechlorination altogether means the organic matter won't come in contact with chlorine. Incorporating an anthracite layer in the filter or feeding activated carbon before the filtration step will adsorb organic matter before filtration. Chlorination can then be conducted later.

Removing DBPs

U.S. EPA has specified best available technologies for removing THMs and HAA5s: enhanced coagulation, enhanced softening, or granular activated carbon. These methods are expensive, however, and must be used only after other methods have been tried.

Alternative Disinfectants

Alternative disinfectants are ultraviolet light, potassium permanganate, ozone, or a combination of chlorine dioxide and chloramines. Some states do not recognize alternative disinfectants and will not approve their use unless they are combined with chlorine or chloramines that provide a residual. Ozone is a powerful disinfectant that does not produce chlorinated organics, but it does result in other by-products. Ozone does not have a residual, so it is used along with chloramines that will provide a residual. UV disinfection also has the same problem of no residual, so chloramines or chlorine are used to provide the required disinfection residual in the distribution system. UV light is not effective for turbid waters, and its effectiveness decreases with increasing turbidity. Unfortunately, none of the other disinfectants are as economical as chlorine. Systems should check with their primacy agency before selecting alternative disinfectants.

(Adapted, with permission, from On Tap, a publication of the National Environmental Services Center, Winter 2006, Volume 5, Number 4. For a complete copy of the original article and its references, visit www.nesc.wvu.edu/ndwc/ndwc_ontap.htm.)