What are they:

  • Disinfectants are not really contaminants since they have major public health protection benefits; however, they do produce varying amounts and types of chemical byproducts that are considered contaminants.
  • The major U.S. primary water disinfectants are chlorine (gas), sodium hypochlorite, chlorine dioxide, ozone and ultraviolet light (UV). They are the primary barriers to pathogens reaching consumers from their drinking water supplies.
  • Monochloramine is the major secondary disinfectant. It is produced by the reaction between chlorine/hypochlorite and ammonia. The role of a secondary disinfectant is to reduce microbial regrowth during storage and distribution.

Production and properties:

  • Chlorine, hypochlorite and chloramines are forms of chlorine used in about 90 percent of all public water supplies in the U.S.
  • Chlorine is manufactured by electrolysis of salt (sodium chloride). It is stored and delivered as compressed gas. Hypochlorite (bleach) is produced from electrolysis of basic sodium chloride. Addition of chlorine to water produces hypochlorite and hydrochloric acid by hydrolysis.
  • Water supplies have been shifting to hypochlorite because it is less hazardous in handling, even though it is purchased in 12 to 15 percent solution, so unit costs are greater.
  • When hypochlorite is stored for a period of time, especially at warmer temperatures, it spontaneously, slowly begins to produce chlorate at hundreds of ppm; chlorate slowly converts to perchlorate.
  • Chlorine is also available as sodium dichloroisocyanurate that is commonly used as a swimming pool disinfectant.
  • Chloramine (NH2Cl) is produced on site by adding chlorine/hypochlorite to ammonia. It is important to manage the reaction to maximize monochloramine production versus di- and trichloramine, because the latter are noxious gases and less effective disinfectants.
  • Chlorine dioxide (ClO2) is an unstable gas and usually produced on site by reaction between sodium chlorate and HCl and a reducing agent, or by reacting sodium chlorite with HCl or chlorine. Stabilized water solutions at about 400 ppm are available.
  • Ozone (O3) is produced on site by corona discharge in dried air or oxygen.
  • UV is generated by mercury vapor lamps. The wavelength distribution and intensity are functions of the lamp design and pressure.


  • Chemical disinfectants are oxidizing agents and reactive so they decompose rapidly in saliva and in the stomach. They react with the total organic carbon in the water.
  • Chlorine is both an oxidizing agent and a chlorinating agent, so it produces a large number of halogenated chlorinated disinfection by-products (DBPs). The predominant groups are trihalomethanes and haloacetic acids.
  • Monochloramine is a complex chlorine form that is not very chemically reactive. Chloramine formation is used to suppress halogenated DBP formation during distribution.
  • Chlorine dioxide hydrolyses in water to chlorate and chlorite.
  • Chlorine dioxide and ozone are oxidizing chemicals, so halogenated DBPs are produced indirectly in small amounts. They tend to produce oxygenated products, such as aldehydes and carboxylic acids.
  • Chlorine, chlorine dioxide and ozone will react with some synthetic organic chemicals in water, including some pharmaceuticals and possibly detoxify them if the dose, time and conditions are appropriate.
  • Chloramines have low reactivity, but they can be involved in the formation of nitrosamine DBPs with appropriate organic nitrogen precursors.
  • UV will produce some byproducts if chemicals that can absorb the UV wavelength are present. UV can directly decompose nitrosamines.


  • Disinfectants are most effective when applied to clarified low turbidity water.
  • Chlorine, hypochlorite, chlorine dioxide and ozone are all excellent and fast-acting primary disinfectants.
  • Chlorine and hypochlorite are very effective against bacteria and viruses, but not protozoa. Ozone, and to a lesser degree chlorine dioxide, are more effective than chlorine against protozoa.
  • UV’s biocidal activity is a function of the intensity, wavelengths being generated and the clarity of the water.
  • Disinfectant efficacies are quantified by the product of concentration (C) in mg/l and time (T) in minutes (CT value except for UV) required to achieve the desired logs of disinfection under the pH and temperature conditions (see below). The lower the CT, the better. Chloramine is obviously considerably less potent than the others, but it does have benefits as a secondary residual disinfectant.
  • Examples for virus inactivation are provided below:

Disinfectant                                                                    CT   Log Inactivation at 10o C                                          












Chlorine dioxide3








UV (mWs/cm2)




Source: EPA 1999. Alternative Disinfectants and Oxidants Guidance Manual EPA 815-R-99-014.

1. pH range 6 to 9, chlorine residual 0.2 to 0.5 mg/l; 2. pH 8; 3. pH range 6 to 9.


  • Chlorine and hypochlorite are measurable as free chlorine by colormetric, titrimetric tests and spectrophotometric, selective ion electrode, as are chloramines as combined chlorine. Similar chemical and spectrophotometric tests are available for chlorine dioxide residuals. Ozone can be measured by colormetric and titrimetric analyses and UV absorption.

Health effects:

  • The rapid presystemic chemical reduction of the disinfectants indicates that they are not likely to have much toxicological significance at water concentrations, but they do have taste thresholds. The by-products of their reactions with natural organics in the water have more potential for health concern. In particular, chlorination byproducts have been studied since their initial detection about 40 years ago.
  • The four trihalomethanes (THMs) that include chloroform and three mixed bromochloro THMs, and haloacetic acids (HAA5) have been regulated as chemical groups, partly due to their potential toxicity and because they make up a substantial portion of the total DBPs. Chloroform is not a low dose carcinogen and the carcinogenicity of the other THMs in water is questionable.
  • There have been more than 60 epidemiology studies of variable quality attempting to determine if there is a relationship between long-term consumption of chlorinated water and cancer risk. Results have been mixed, somewhat inconsistent and are debated.
  • The most consistent result has suggested a small incremental risk of bladder cancer, but it is difficult to differentiate those outcomes from other known causes, such as smoking. Health agencies advise that disinfection should never be compromised because of DBP formation concerns.
  • In its revision of the DBP rules in in 2006, the U.S. Environmental Protection Agency (EPA) concluded that there was not a sufficient basis to relate reproductive or developmental effects with DBPs.

Water treatment applications:

  • Being oxidizing disinfectants, the primary disinfectant chemicals will also assist in decolorizing water as from organics, manganese or iron presence.


  • The EPA has established maximum contaminant levels (MCLs) for chlorine (4 mg/l) , chlorine dioxide (0.8 mg/l) and chloramines (4 mg/l), but these values are probably of questionable toxicological relevance because they do not consider the rapid decomposition after ingestion. The MCL for chlorite, a by-product of chlorine dioxide use, is 1 mg/l.
  • The THM MCL is 0.080 mg/l for the sum of four chlorinated and mixed brominated THMs, but it is predominantly an indicator that causes use of technologies that will simultaneously reduce many of the unidentified DBPs.
  • The MCL for the five HAAs is 0.060 mg/l. It is also an indicator, but there is a better toxicological basis for their regulation.

Dr. Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.