Disinfectant Byproducts (THM) in Drinking Water:

Past Present, and Future



DBPs: The Past

Chlorine has been used widely for the purification of drinking water since 1906.





1974 - The Dark Side of Chlorine



DBPs From Chlorination

HOCl + NOM → chlorinated DBPs

HOCl + Br → HOBr + Cl- (Fast Reaction)

HOBr + NOM → brominated DBPs


HOCl + Br + NOM → chlorinated, brominated, and mixed bromo-chloro disinfection byproducts




Disinfection Byproducts From Chlorination




Disinfection Byproducts From Chlorination



DBP Regulations: Part 1

Nov 1979 THM Rule



Control of Halogenated DBPs

Most common techniques have been:






Stage 1 DBP Rule (published 1998, 2002 compliance)



Stage 2 DBP Rule (published 2006, compliance 2013)



What Are Utilities Doing to Comply With Stage 1 and In Anticipation of Stage 2?



Combined Chlorine

Use of monochloramine as terminal disinfectant in distribution systems







Nitrification Can Be Controlled By:






Mobilization of Lead?

Recent literature suggests that the higher pH values and lower redox potentials associated with the use of chloramines compared to free chlorine can solubilize lead deposits (precipitates) that have accumulated on pipe walls.

With free chlorine - Pb(IV)O2 deposits

With combined chlorine - Pb(II) deposits

PB(II) is more soluble than Pb(IV)



Emerging DBPs of Potential Health Concern



Perhaps it is better to remove precursors, i.e. NOM/TOC, before disinfection; continue to use free Cl2 in the distribution system


UV is an attractive alternative primary disinfectant, although it does not remove precursors.






DBPs: The Future




Disinfectant Byproducts

All chemical disinfectants are poisons of some kind, and the most important are oxidizers like chlorine and ozone. Whenever oxidizers are used, they "attack" all sorts of other things in the water besides micro-organisms, producing a variety of unwanted chemical byproducts. Many of them are carcinogenic or otherwise toxic, and their nearly universal occurrence has made them very prominent in recent news reports. This is in spite of the fact that most of them are obscure chemicals with unpronounceable names, and they occur in such faint traces (parts per billion, ppb, and even less) that their importance as contaminants is difficult for many people to understand.



Byproducts of Chlorine Disinfection

Chlorine is the most commonly used water disinfectant, and organic compounds are the most common oxidizable contaminants, so most of the disinfection by-products (DBPs) identified so far are chlorinated organics. Ozone is powerful enough to produce free chlorine from chloride ion, and if the water contains bromide ion, either free chlorine or ozone can oxidize it to free bromine, and both can then participate in the reactions with organics. Most organic material in water has a biological origin-it is either cellular fragments of recently-dead micro-organisms and plants, or "color" molecules of the brownish tannin-lignin-humus complex-like tea-that leaches out of dead vegetation. The latter is almost pure "carbohydrate," containing only carbon, hydrogen, and oxygen. But the recently-dead matter includes proteins, amino acids and nucleic acids, which contain lots of nitrogen atoms that react freely with chlorine. Many surface water supplies may also have significant levels of nitrogenous fish and animal waste materials such as ammonia and urea. These contaminants cause a lot of trouble when chlorinated, because, at best, they smell and taste terrible, and at worst, they may cause cancer or miscarriages.

CHLORAMINES: When ammonia (NH3) or nitrogen-containing organics are in contact with aqueous chlorine (HOCl, hypochlorous acid), "chloramines" are produced. If the chlorine level is maintained, all amine-nitrogens will have their hydrogen atoms replaced with chlorine, as long as there are any to replace. The last reaction in the sequence produces harmless nitrogen gas, which bubbles off to join the other 80% of our atmosphere. This is shown below, in steps, using the simplest example, ammonia:


Even though all of these chloramines are toxic and offensive to some degree, the first reaction is intentionally produced in about 25% of U.S. water supplies. This is usually done by injecting ammonia as the finished water leaves the water-works, converting the free chlorine residual that accomplished disinfection earlier into monochloramine. They do that because monochloramine still retains about 5% of free chlorine's disinfection power, which is enough to inhibit bacterial regrowth in the distribution system, but it is not powerful enough to continue producing toxic chlorinated organics, which free chlorine would do if it were still present. But monochloramine lasts a lot longer in the mains than free chlorine, so it is often a good trade-off. The surface water supplies used by many of the largest cities contain so much organic matter that it is more economical to meet the THM standard (which see, below) by limiting the extent of chlorination than by using more thorough treatment to remove the organic precursors first. And these largest municipal systems are exactly the ones that need a disinfectant residual to last the longest in the mains, so "chloramination" makes sense for them.

The last reaction in the sequence given above is critically important. It is called the "break-point reaction" because it represents the destruction or "breaking" of the last of the oxidizable material in the water, called the oxidant demand or chlorine demand, leaving behind a "residual" of "free available chlorine" (FAC) to do the actual disinfection. Disinfection cannot proceed until the oxidant demand has been destroyed. Thus, when testing for chlorine with a chemical test kit, it is critically important to be able to distinguish between FAC (which disinfects) and "combined chlorine" (which does not disinfect). If the purpose of the test is to assess taste & odor, use the "Total Chlorine Test," which includes FAC and all of the many kinds of combined chlorine that may be present. If the purpose of the test is to assess disinfection and safety, test specifically for FAC, or free chlorine.




Halogenated Organics

When organic matter is in contact with free chlorine, a large variety of oxidized and chlorinated organic compounds is produced. The most common are one- and two-carbon fragments of larger molecules with a number of chlorine and/or bromine atoms attached, exemplified by tri-chloro-methane or chloroform, CHCl3, the predominant THM or trihalomethane. A generation ago, when these contaminants were first discovered in drinking water, concentrations of several hundred ppb were common. Now, after many years of actively avoiding THM production, most water-works supply tap water with less than 20 ppb. THMs have been considered probable human carcinogens (kidney, liver, bladder cancers) and have been regulated in drinking water for many years with a mandatory Maximum Contaminant Level (MCL) of 0.10 ppm. However, the U.S. EPA recently proposed to relax that, to 0.30 ppm, because new data has finally established that there is a carcinogenic threshold level for these compounds, and 0.30 ppm is safe. On the other hand, other new data from other sources has now implicated THM levels as low as 0.08 ppm (80 ppb) as a cause of miscarriages.

Next in prominence after THMs are the halogenated acetic acids or HAAs, the halogenated aceto-nitriles or HANs, and the halogenated ketones or HKs, most of which are made from two-carbon fragments. They are found in chlorinated waters at levels one-third to one-half of the THM level. If and when they are ever regulated explicitly, it is expected that they will receive MCLs in the 30-80 ppb range.

Probably the most recognizable chlorinated organics are the chlorophenols, a chemical family with 6-carbon rings (benzene rings) with one or more -OH groups attached, plus one or more chlorine atoms. They have moderate toxicity, but their main offense is an extremely powerful "medicinal" or "iodine-like" taste & odor. The worst is 2,4-dichlorophenol, with a taste & odor threshold of only a few parts per trillion (ppt). Pentachlorophenol is probably the most well known of the group. It is used as an insecticide to kill termites and as a fungicide to prevent wood rot in telephone poles, etc. All of the chlorophenols are easy to oxidize to oblivion-just add more chlorine-and that signals their true importance: their characteristic stink occurs only when disinfection has been lacking and/or the remaining chlorine residual is too weak to do anything. The presence of chlorophenol T&O indicates that the water is unprotected and that more chlorine is needed. This problem actually occurs more often in systems using ozone or chlorine dioxide for disinfection than in those using plain chlorine. Their source in water is usually chlorination of the brownish "color" molecules, which produces many "phenolic" fragments in addition to the many one- and two-carbon fragments mentioned earlier. Reactive phenols can also leach out of certain plastics that may improperly find their way into the beverage circuit of commercial equipment such as coffee makers and vending machines. If the equipment is protected with a carbon filter to control T&O, excessive flow rate or continuous use might occasionally be able to strain the filter's chlorine-reducing efficiency. If only a few parts per quadrillion (ppq!) of chlorine get past the filter and are available to react with a few ppq of a phenol, they can produce a few ppt of a chlorophenol that tastes and smells terrible. The remedy for this kind of T&O problem is to avoid using phenolic plastics in water-using equipment. The remedy for chlorophenol odor in an ordinary water supply is to enhance the disinfection process so that any phenols are destroyed, along with the rest of the oxidant demand.

In addition to the above "families" of related chloro-organic compounds, there are also several isolated, individual byproducts that bear mentioning. These include chloropicrin, also called nitrochloroform, and cyanogen chloride, both of which were used in chemical warfare; and chloral hydrate, once known as "Mickey Finn" knockout drops. Of course, their concentrations in chlorinated water are much too low to cause those effects, but they certainly illustrate the variety and notoriety of the group.




Byproducts of Chlorine Dioxide Disinfection

Surprise, there are hardly any chloro-organics produced by the action of chlorine dioxide on the organics in drinking water. However, its main breakdown product, chlorite ion, ClO2-, and its main precursor, chlorate ion, ClO3-, are both toxic, causing oxidation of the iron atoms in hemoglobin so that the red blood cells fail to distribute oxygen to the tissues effectively. This is a potentially deadly condition called "methemoglobinemia" (from meta, Greek for "beside" or "other"), and it is also the toxicologic mechanism of poisoning by cyanide, carbon monoxide, and nitrite salts.




Byproducts of Ozone Disinfection

First, ozonation can produce all of the chlorinated organics that chlorination produces, because ozone oxidizes chloride and bromide ions to free chlorine and bromine. That includes the bromate ion, BrOM3-, (similar to chlorate, above), which forms when bromine is involved and the water is alkaline. In addition, a host of other oxidation products (alcohols, aldehydes, ketones, and organic acids) are readily formed, both with and without any added chlorine or bromine atoms. Formaldehyde (H2C=O) and glyoxal (O=CH-HC=O) are the main ones, produced by chlorination as well as ozonation. [Oxidation of organic compounds often proceeds in steps, first producing an alcohol (-C-OH) and then oxidizing the alcohol to a ketone or aldehyde (-C=O), and then oxidizing that to an organic acid (-C=O-OH).] A couple of complex organic pesticides are only partially oxidized by ozone, producing new compounds that are more toxic to humans than the original chemical. These are heptachlor epoxide (from heptachlor) and aldicarb sulfoxide and aldicarb sulfone (from aldicarb).

Another unusual ozonation byproduct to consider is the huge variety of larger fragments of natural organics that don't have names, but simply provide ready nutrition to micro-organisms. The tannin-lignin-humus complex that creates "color" in water normally takes many centuries to be broken down by bacteria, but "pre-digestion" by ozone often leads to runaway bacterial re-growth in the water mains if and when the disinfectant residual dissipates. Therefore, water systems using ozone for disinfection often include large granular activated carbon beds, where intentional bacterial growth consumes so much of the organic "soup" that subsequent regrowth in the mains is greatly reduced. This is called Bacteriological Activated Carbon or BAC treatment.


Disinfection Byproducts (DBPs)



  • Chloramines
  • Dichloramine
  • Ammonia
  • Bromate ion
  • Chlorite ion
  • Monolchloramine
  • Chlorate ion
  • Trichloramine

Haloketones (HKs)

  • 1,1-Dichloropropanone
  • 1,1,1-Trichloropropanone




Haloacetic acids (HAAs)

  • Monochloroacetic acid
  • Trichloroacetic acid
  • Dibromoacetic acid
  • Monobromoacetic acid
  • Dichloroacetic acid
  • Bromochloroacetic acid

Trihalomethanes (THMs)

  • Chloroform
  • Dibromochloromethane
  • Bromoform
  • Bromodichloromethane


Haloacetonitriles (HANs)

  • Monochloroacetonitrile
  • Trichloroacetonitrile
  • Dibromoacetonitrile
  • Monobromoacetonitrile
  • Dichloroacetonitrile
  • Bromochloroacetonitrile


  • 2,4-Dichlorophenol
  • 2,6-Dichlorophenol
  • 2,4,6-Trichlorophenol

Haloacetonitriles (HANs)

  • Monochloroacetonitrile
  • Trichloroacetonitrile
  • Dibromoacetonitrile
  • Monobromoacetonitrile
  • Dichloroacetonitrile
  • Bromochloroacetonitrile


  • Formaldehyde
  • C2-C14 Aldehydes
  • Glyoxal
  • Methyl glyoxal



  • Chloropicrin
  • Cyanogen chloride
  • Chloral
  • Chloral hydrate
  • Chlorinated furanones (MX)
  • 2,2-Dichloropropanoic acid
  • 3,3-Dichloropropanoic acid
  • 2,3,3-Trichloropropenoic acid
  • Dichloropropanedioic acid




Disinfection Byproducts

The protection of our nation's drinking water supply has been a priority for many years. In fact, a major accomplishment in public health during this century has been the chlorination of public drinking water supplies. This practice has greatly reduced serious illness and death associated with many waterborne diseases, such as cholera and typhoid.

We are often reminded of the important role that chlorination plays in protecting the public each time we hear about an outbreak of waterborne disease resulting from inadequate disinfection. But, as with many issues, it sometimes becomes necessary to weigh the benefits against the potential risks. The presence of disinfection byproducts in drinking water supplies, formed when chlorine reacts with natural organic materials in water, has raised concerns about the overall safety of chlorination.

As a result, many members of the drinking water protection community have been actively working to more clearly understand the possible health problems from exposure to disinfection byproducts, while at the same time ensuring a high level of protection against waterborne diseases.

Meanwhile, federal and state governments have taken steps to protect the public from the potential health risks from disinfection byproducts by conducting health effects research, strengthening drinking water regulations, and supporting improvements in water treatment technology.



What Are Disinfection Byproducts (DBPs)?

Disinfection byproducts (DBPs) consist of a wide variety of chemicals that form when chlorine is added to drinking water during the treatment process. Chlorine is added to drinking water for disinfection purposes.

DBPs include:


Of these chemicals, THMs and HAAs are most often found in chlorinated drinking water. Others, such as HANs and MX, are formed in smaller amounts during the chlorination process. Still other DBPs have not yet been chemically identified.

Some water treatment plants use other types of drinking water disinfectants, such as ozone, chlorine dioxide, and monochloramine, usually in combination with chlorine. Each of these disinfectants produce their own group of byproducts during the treatment process.




Why Is Chlorine Added to Drinking Water Supplies?

Chlorine is widely used during the water treatment process because it is very effective in destroying harmful bacteria and viruses. Disinfection of drinking water is one of the most important accomplishments of public health practice because it has resulted in a major reduction in cholera, typhoid, and other waterborne diseases.




How Can DBPs Get Into Your Drinking Water?

DBPs are formed when chlorine reacts with the natural, organic materials found in water, such as algae and decaying plants.

Chlorine + Decaying Plants = DBPs


Organic materials can wash into surface water from surrounding lands, such as farms and wooded areas. Urban runoff also carries organic material into surface water when it rains. During the warmer months, surface water often contains a lot of organic material. As a result, DBP levels are generally higher in the summer and fall than other times of the year.

THMs and HAAs are two families of related chemicals that contain different amounts of chlorine and bromine. During the water treatment process, bromine is formed when chlorine reacts with naturally-occurring bromide in the water. Bromine, like chlorine, can combine with organic material naturally found in water to form THMs and HAAs.




What About Private Well Water?

Groundwater is unlikely to contain the organic materials needed to form DBPs as long as it comes from a protected underground supply. Also, well water does not usually need to be chlorinated on a regular basis.

Although DBPs are not commonly found in private well water, there are certain conditions under which they may be present:




How Can DBPs Get Into Your Body?

There are several ways that DBPs can get into your body:




Are DBPs Harmful to Your Health

The health risks from exposure to low levels of DBPs in drinking water are not well understood.


Additional research is in progress to further study if exposure to low levels of chlorinated water can cause harmful health effects in people.



How Can You Find Out if DBPs Are In Your Drinking Water?

Your public water supplier is required to test for THMs in your drinking water every 3 months. At least 4 samples collected over a 12 month period are used to determine an average THM level for the year. You can find out the results of these tests by contacting your water supplier or the New Jersey Department of Environmental Protection, Bureau of Safe Drinking Water. Also, public water suppliers are now required to send information about the quality of their water to customers each year.

In 1999, certain large public water suppliers must start testing regularly for HAAs. Currently, there are no monitoring requirements for other DBPs in water supplies.




Is There a Safe Level of DBPs in Your Drinking Water?

Maximum Contaminant Levels (MCLs) have been established to protect the public from exposure to DBPs in chlorinated drinking water. They are set at levels that reduce the chances that harmful health effects will occur.

MCLs, developed by the New Jersey Department of Environmental Protection and the U.S. Environmental Protection Agency, limit the amount of certain DBPs that can be present in public drinking water supplies.

The MCLs for DBPs are listed in the following table. Beginning in 2003, some MCLs will be lowered for certain large public water systems in order to be more protective of public health. MCLs have not been set for all DBPs since there is not enough health effects information currently available to do so.


Maximum Contaminant Levels (MCLs)
Contaminant MCL1
THMs 100 (802)
HAAs ---- (602)
HANs ----
MX ----

1measured in parts per billion (ppb) or micrograms per liter (µg/L)

2Public water systems using surface water or ground water under the direct influence of surface water, and serve 10,000 or more people, must meet the MCL by 12/16/03

Source: Federal and NJ Drinking Water Standards, 11/96, 12/98





What Steps Are Being Taken to Reduce DBP Levels in Public Drinking Water?

Drinking Water Regulations

Water quality information, along with the results of ongoing health effects research, will be used by federal and state government agencies to decide whether current drinking water regulations need to be strengthened or new regulations need to be developed to further protect the public from DBPs. Other regulatory approaches focus on improving drinking water quality by protecting water supplies at their source.



Water Treatment Technology

The USEPA is evaluating the effectiveness of different water treatment technologies in protecting the public from harmful organisms in drinking water while minimizing the formation of DBPs, such as through the use of chlorine alternatives (ozone, ultraviolet light) and the development of better water filtration methods.



Are There Home Water Treatment Devices for DBPs?

While scientists continue to learn more about the possible health effects from DBPs in drinking water, certain home water treatment devices can be used to reduce your exposure:

Granulated Activated Carbon (GAC) Filters are effective in lowering DBP levels in your drinking water. These filters are also capable of improving the taste and odor of your drinking water by removing chlorine.

Several types of GAC filters are available for home use. Point-of-use filters inlcude those that are attached to the faucet itself or connected to the cold water line beneath the sink. Free-standing units are separate from the water supply but can only filter small amounts of water at one time. These filters do not totally eliminate exposure to DBPs in the water and the air since treatment is limited to only a portion of the household water supply. Point-of-entry filters treat all the water coming into the home and, therefore, are effective in preventing exposure to DBPs through ingestion and inhalation.



What Else Can You Do To Protect Yourself From DBPs?

There are several simple steps you can take to reduce your exposure to DBPs:

Use Less Water

Take shorter showers and baths, and use shorter wash cycles for dishes and clothes. Some DBPs can evaporate or "volatilize" into the air in your home when you use the water.


Use Cooler Water

Reduce the amount of hot water that you use when you shower or bathe, and wash clothes. Volatile DBPs are more likely to get into the air in your home when the water is hot.


Provide Adequate Ventilation

Open windows or vent air to the outside during and afterw ater use. Spend lesst ime in the bathroom after the water has been used. Volatile DBPs can build up in the air in your home, especially in an enclosed area. And the longer you remain in the area, the more likely you will be exposed.


Flush Out Your Private Well System AFter Disinfection

Be sure to properly flush out your private well system after adding chlorine to your well for disinfection purposes. Chlorine can be quickly eliminated from your well by flushing out the system afterwards.




Where Are DBPs Most Often Found?

Surface water, such as rivers, lakes and reservoirs, are likely to contain large amounts of organic materials, especially during the warmer months of year. These materials can easily wash into the water from the surrounding land. As a result, drinking water from surface water supplies are likely to form DBPs after chlorine is added during water treatment.

Under certain conditions, groundwater can contain some organic materials. For example, organic materials may reach shallow wells that obtain water from close to the ground's surface. Likewise, wells may draw in organic materials if they are located near surface water bodies. This is referred to as groundwater under the direct influence of surface water.

Groundwater, such as well water, does not commonly contain the organic materials necessary to form DBPs. And some public water suppliers do not chlorinate groundwater when it comes from a protected underground source. Therefore, the amount of DBPs in well water is usually very low, and in many cases, is so low it cannot even be measured.