Report No DWI0736

Aug 1995

The reaction of oxidants used in drinking water treatment with naturally-occurring organic matter (e.g. humic material), inorganic ions (e.g. bromide) or man-made chemicals (e.g. pesticides) can result in the formation of disinfection by-products (DBPs). Information on the formation and identities of DBPs is required to enable a full evaluation of their potential risk to health.

An investigation of the reaction of chlorine and ozone with selected organic compounds likely to occur in water sources found that chlorine and ozone reacted with unsaturated compounds (e.g. alkenes). Chlorohydrins and aldehydes/carboxylic acids were the major products identified from the reaction of chlorine and ozone respectively. Ozone was found to react with a number of aromatic hydrocarbons (e.g. fluoranthene) and pesticides. Polyelectrolytes did not appear to yield significant concentrations of lower molecular weight compounds after reaction with ozone or chlorine. Evidence was obtained that the structure of the polymer was altered during ozonation; the implications, in terms of any health risk associated with the modified polymer, are unclear.

Dichloroacetic and trichloroacetic acids are commonly found in UK chlorinated drinking waters. Both compounds are included in the revision of the WHO Guidelines for drinking water, and although neither currently appear in the proposed revision of the EC Drinking Water Directive, they remain of potential concern and the implications of any new toxicological data will need to be considered.

Bromate can readily be formed during the ozonation of waters containing bromide. Bromate was also found in both commercial and on-site generated sodium hypochlorite solutions, as well as in some of the corresponding treated waters. Data obtained during this study suggest that a standard of 10 µg/l could be exceeded at certain treatment works in the UK, either from the use of ozone or sodium hypochlorite or a combination of the two.

The extraction and concentration techniques typically used to assess the mutagenicity of chlorinated drinking water extracts may not be suitable for polar, highly water soluble, ozonation by-products. Attempts to balance the risks to health from the presence of DBPs in drinking water against those arising from microbial risks due to using less, or a different, disinfectant are underway in the US. This is a potentially important area and developments in the US and elsewhere should continue to be monitored. Preliminary analysis of data for a range of DBPs in chlorinated water suggested that correlations do exist between individual DBPs and classes of DBPs (e.g. THMs) or general parameters (e.g. AOX).

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