Report No DWI0738


Report to the Department of Environment


Aug 1995


This is a stand-alone report commissioned under the Department of the Environment's contract Effects of Disinfectants on Organic Substances in Water (Reference No. PECD 7/7/137). It specifically examines the USEPA's recent approaches to modelling the formation of disinfection by-products in drinking water and attempts to balance the risks of infectious disease with those of cancer from by-products.

The first part of the report reviews the features of microbial disease and infectivity, the kinetics of the disinfection process, the formation of by-products by different disinfectants, the features of carcinogenicity and derivation of standards for by -products and the frameworks for legislation and guidelines adopted by the USEPA, the UK, the European Union and the World Health Organisation.

The features of the modelling carried out by the USEPA are then considered in detail, the results described and their validity critically assessed. The successive interacting parts of the model calculate reduction in microbial risks from treatment and di sinfection, the generation of by-products from precursors in raw water, having selected the cheaper option of disinfectant and provide risk estimates. Unfortunately, the underlying relationships between dose and risk, the quality of source water and the assumptions made are all variable or uncertain so that the output is both imprecise and inaccurate and cannot be calibrated. The value of the model is therefore qualitative and is set against there being little convincing evidence that drinking-water contaminants are a significant cause of human cancer. The modelling activity is therefore seen to be regulation-driven. No consistent balancing of chemical and microbiological risks can be achieved, except for individual pairs of pathogen and by-product.

The approaches used by the USEPA are seen to be irrelevant to the needs of the UKs regulators (i.e. the Department). Alternative approaches are suggested. One study, already underway will attempt to find more reliable baseline data, relevant to UK conditions and use uncertainty analysis with statistical distributions appropriate to water supply. A prudent course, is to consider the reduction of by-products by attention to treatment prior to terminal disinfection, thereby improving the efficiency of disinfection, as well as reducing the formation of by-products. Other recommendations are to investigate the safety factor inherent in the requirement for absence of Escherichia coli in drinking water, examining the cost-benefit relationships of disinfection and other barriers to transmission of infection and exploring the public's perception of microbial and chemical risks from drinking water against other environmental hazards.

The views expressed in this report are those of the author and not necessarily those of the Department of the Environment or any other government department or organisation.

Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.