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Water Disinfectants & disinfectant by-products

1. What disinfectants and by-products are we talking about?

  • 1.1 Why is there concern about water disinfectants?
  • 1.2 How are disinfectant by-products formed?
  • 1.3 What are the most relevant disinfectant by-products?
  • 1.4 How can the amount of disinfectant by-products be reduced?

1.1 Why is there concern about water disinfectants?

The source document for this Digest states:

Chlorine (Cl2) has been widely used throughout the world as a chemical disinfectant, serving as the principal barrier to microbial contaminants in drinking water. The noteworthy biocidal attributes of chlorine have been somewhat offset by the formation of disinfectant by-products (DBPs) of public health concern during the chlorination process. As a consequence, alternative chemical disinfectants, such as ozone (O3), chlorine dioxide (ClO2-) and chloramines (NH2Cl, monochloramine), are increasingly being used; however, each has been shown to form its own set of DBPs. Although the microbiological quality of drinking water cannot be compromised, there is a need to better understand the chemistry, toxicology and epidemiology of chemical disinfectants and their associated DBPs in order to develop a better understanding of the health risks (microbial and chemical) associated with drinking water and to seek a balance between microbial and chemical risks. It is possible to decrease the chemical risk due to DBPs without compromising microbiological quality.

The most widely used chemical disinfectants are chlorine, ozone, chlorine dioxide and chloramine. The physical and chemical properties of disinfectants and DBPs can affect their behaviour in drinking-water, as well as their toxicology and epidemiology. The chemical disinfectants discussed here are all water-soluble oxidants, which are produced either on-site (e.g., ozone) or off-site (e.g., chlorine). They are administered as a gas (e.g., ozone) or liquid (e.g., hypochlorite) at typical doses of several milligrams per litre, either alone or in combination. The DBPs discussed here are measurable by gas or liquid chromatography and can be classified as organic or inorganic, halogenated (chlorinated or brominated) or non-halogenated, and volatile or non-volatile. Upon their formation, DBPs can be stable or unstable (e.g., decomposition by hydrolysis).

Source & ©: IPCS "Environmental Health Criteria for Disinfectants and disinfectant by-products”  ,
EHC 216, Chapter 1: Summary, section 1.1

1.2 How are disinfectant by-products formed?

The source document for this Digest states:

DBPs are formed upon the reaction of chemical disinfectants with DBPs precursors. Natural organic matter (NOM), commonly measured by total organic carbon (TOC), serves as the organic precursor, whereas bromide ion (Br-) serves as the inorganic precursor. DBP formation is influenced by water quality (e.g., TOC, bromide, pH, temperature, ammonia, carbonate alkalinity) and treatment conditions (e.g., disinfectant dose, contact time, removal of NOM before the point of disinfectant application, prior addition of disinfectant).

Chlorine in the form of hypochlorous acid/hypochlorite ion (HOCl/OCl-) reacts with bromide ion, oxidizing it to hypobromous acid/hypobromite ion (HOBr/OBr-). Hypochlorous acid (a more powerful oxidant) and hypobromous acid (a more effective halogenating agent) react collectively with NOM to form chlorine DBPs, including trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HAAs), haloketones, chloral hydrate and chloropicrin. The dominance of chlorine DBP groups generally decreases in the order of THMs, HAAs and HANs. The relative amounts of TOC, bromide and chlorine will affect the species distribution of THMs (four species: chloroform, bromoform, bromodichloromethane [BDCM] and dibromochloromethane [DBCM]), HAAs (up to nine chlorinated/brominated species) and HANs (several chlorinated/brominated species). Generally, chlorinated THM, HAA and HAN species dominate over brominated species, although the opposite may be true in high-bromide waters. Although many specific chlorine DBPs have been identified, a significant percentage of the total organic halogens still remain unaccounted for. Another reaction that occurs with chlorine is the formation of chlorate (ClO3-) in concentrated hypochlorite solutions.

Ozone can directly or indirectly react with bromide to form brominated ozone DBPs, including bromate ion (BrO3-). In the presence of NOM, non-halogenated organic DBPs, such as aldehydes, ketoacids and carboxylic acids, are formed during ozonation, with aldehydes (e.g., formaldehyde) being dominant. If both NOM and bromide are present, ozonation forms hypobromous acid, which, in turn, leads to the formation of brominated organohalogen compounds (e.g., bromoform).

The major chlorine dioxide DBPs include chlorite (ClO2-) and chlorate ions, with no direct formation of organohalogen DBPs. Unlike the other disinfectants, the major chlorine dioxide DBPs are derived from decomposition of the disinfectant as opposed to reaction with precursors.

Use of chloramine as a secondary disinfectant generally leads to the formation of cyanogen chloride (CNCl), a nitrogenous compound, and significantly reduced levels of chlorine DBPs. A related issue is the presence of nitrite (NO2-) in chloraminated distribution systems.

Source & ©: IPCS "Environmental Health Criteria for disinfectants and disinfectant by-products, ,
EHC 216, Chapter 1: Summary, section 1.1

1.3 What are the most relevant disinfectant by-products?

The source document for this Digest states:

From the present knowledge of occurrence and health effects, the DBPs of most interest are THMs, HAAs, bromate and chlorite.

The predominant chlorine DBP group has been shown to be THMs, with chloroform and BDCM as the first and second most dominant THM species. HAAs are the second predominant group, with dichloroacetic acid (DCA) and trichloroacetic acid (TCA) being the first and second most dominant species.

Conversion of bromide to bromate upon ozonation is affected by NOM, pH and temperature, among other factors. Levels may range from below detection (2 µg/litre) to several tens of milligrams per litre. Chlorite levels are generally very predictable, ranging from about 50% to 70% of the chlorine dioxide dose administered.

DBPs occur in complex mixtures that are a function of the chemical disinfectant used, water quality conditions and treatment conditions; other factors include the combination/sequential use of multiple disinfectants/oxidants. Moreover, the composition of these mixtures may change seasonally. Clearly, potential chemically related health effects will be a function of exposure to DBP mixtures.

Other than chlorine DBPs (in particular THMs), there are very few data on the occurrence of DBPs in finished water and distribution systems. Based on laboratory databases, empirical models have been developed to predict concentrations of THMs (total THMs and THM species), HAAs (total HAAs and HAA species) and bromate. These models can be used in performance assessment to predict the impact of treatment changes and in exposure assessment to simulate missing or past data (e.g., to predict concentrations of HAAs from THM data).

Source & ©: IPCS "Environmental Health Criteria for Disinfectants and disinfectant by-products”  ,
EHC 216, Chapter 1: Summary, section 1.1

1.4 How can the amount of disinfectant by-products be reduced?

The source document for this Digest states:

DBPs can be controlled through DBP precursor control and removal or modified disinfection practice. Coagulation, granular activated carbon, membrane filtration and ozone biofiltration can remove NOM. Other than through the use of membranes, there is little opportunity to effectively remove bromide. Source water protection and control represent non-treatment alternatives to precursor control. Removal of DBPs after formation is not viable for organic DBPs, whereas bromate and chlorite can be removed by activated carbon or reducing agents. It is expected that the optimized use of combinations of disinfectants, functioning as primary and secondary disinfectants, can further control DBPs. There is a trend towards combination/sequential use of disinfectants; ozone is used exclusively as a primary disinfectant, chloramines exclusively as a secondary disinfectant, and both chlorine and chlorine dioxide in either role.

Source & ©: IPCS "Environmental Health Criteria for Disinfectants and disinfectant by-products”  ,
EHC 216, Chapter 1: Summary, section 1.1

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