Report No FR0286

C J Cable and M Fielding

April 1992



To review the occurrence of transformation products in water supplies and make recommendations on analytical and monitoring needs in relation to substances of importance.


Contaminants in drinking water can arise due to their presence in the water source. However, water constituents, natural or otherwise, can be transformed in the water source, or during water treatment to new substances. In terms of potential impact on water utility business, transformation of natural and man-made chemicals in water sources by the chemical action of oxidants, such as chlorine and ozone, added during water treatment to achieve disinfection, or in the case of ozone, to destroy chemical contaminants in the water source are of major importance. While such oxidants are known to react freely with constituents of the water source to form transformation products (or disinfection by-products), reactions could also take place with other water treatment chemicals- particularly polymeric organic treatment aids. The generation of chlorination by-products that need to be regulated, along with many other by-products, the significance of which is still being investigated around the world, has led to much enthusiasm for other options, such as ozone and chlorine dioxide. Recent research has shown that alternative disinfectants also produce by-products which may be just as (or more) unacceptable as chlorination by-products. The World Health Organisation (WHO) is currently reviewing its drinking water guidelines (DWG) for a variety of contaminants and it is likely that new WHO DWGs for disinfection by-products (DBP) will be published in the near future; currently they only have a DWG for chloroform. It is also possible that the EC will revise its drinking water directive in the near future and in so doing include standards for a variety of DBP.

As a consequence of this activity, it is timely to review DBP and consider the need for improved analytical methods, monitoring requirements and the urgency for any specific research.


The use of chemical oxidants such as chlorine and ozone inevitably involves the generation of a wide variety of by-products in low concentrations - that is, new chemicals synthesised by the action of the oxidant on mainly naturally occurring chemicals in the water source.

The risks to health of these by-products, although almost certainly low, are difficult to assess accurately in most cases due to insufficient information on their toxicity.

New legislation in the US, the prospect of WHO drinking water guidelines on several disinfection by-products and the likelihood of the EC including new standards in a revised drinking water directive, mean that the issue of disinfection by-products is likely to be highlighted in the coming years - with potentially serious implications for the water utilities.

Information on the occurrence of some potentially important by-products in water supplies is poor, specifically in the case of bromate, chlorate/chlorite, halogenated acetic acids, halogenated acetonitriles and aldehydes. For some by-products, especially bromate, the development and testing of suitable analytical methods are needed. In other areas, more research is needed to identify by-products, for example, aldehydes and carboxylic acids, generated by ozone; and the brominated analogues of, already characterised, chlorinated species. Appropriate monitoring methods can then be developed, and an assessment of any risks to health can be attempted.

Some water treatment chemicals, mainly organic polyelectolytes, can react with chlorine and ozone, either directly or via their impurities, to form by-products. However, there are insufficient data at present to satisfactorily evaluate the overall importance of the phenomenon.


A sensitive and reliable method for measuring bromate, especially in ozonated water, needs to be developed, followed by production of data on the occurrence of bromate in typical treated water.

A similar recommendation is made for chlorate in chlorinated drinking water, and for for chlorate/chlorite where chlorine dioxide is used.

More information on the occurrence in drinking water of certain other disinfection by-products is needed: halogenated acetonitriles, halogenated aldehydes and carboxylic acids in general, MX and brominated MX, and with lower priority, formaldehyde, chloral hydrate, trichloronitromethane and cyanogen chloride. In some cases this involves the development of improved analytical methods.

More research should be carried out on chloroacetic acids, brominated MX, trichloroacetonitrile and on chlorinated and non-chlorinated aldehydes and carboxylic acids to provide a better understanding of the conditions under which they are formed and to enable improved assessment of their significance to consumers. More information is needed on the reactions between water treatment chemicals and oxidants/disinfectants and the by-products produced.


The report reviews the occurrence of transformation products in water supplies. The emphasis is put on oxidation/disinfection by-products. Due to possible changes in standards and legislation, these are of considerable significance, with important implications for the water industry. The review complements a related review on the significance of disinfection by-products (FR 0288, March 1992).

Copies of the Report are available from FWR, price 25.00 less 20% to FWR Members