DEVELOPMENTS IN MICROBIOLOGICAL RISK ASSESSMENT MODELS FOR DRINKING-WATER - A SHORT REVIEW

Microbiological risk assessment (MRA) is the emerging method to predic t the risks of infection from waterborne pathogens (e.g. rotavirus and Cryptosporidium parvum) in the drinking water supply. The objectives of this paper are to review the appropriateness of current models, wit h emphasis on pathogen exposures through drinking water, and to consid er the information necessary to further their development. Calculating pathogen exposures in MRA is currently limited by the fact that patho gen density data for drinking water supplies are only available for ve ry large volume samples-much larger than imbibed daily by any consumer . To develop MRA, information is needed on how pathogens are dispersed within those large volumes at the resolution of volumes typically con sumed daily by individuals. Available evidence suggests that micro-org anisms, including pathogens, are clustered to some degree, even within small volumes, exposing some drinking water consumers to much higher doses than others. By assuming pathogens are randomly dispersed, curre nt models overestimate the risk from the more infectious agents (e.g. rotaviruses) but underestimate the risk from less infectious pathogens (e.g. C. parvum). Approaches to modelling pathogen densities in drink ing water from source water data and treatment removal efficiencies re quire additional information on the degree to which treatment processe s (e.g. filtration and coagulation) increase pathogen clustering. The missing information could be obtained from large-scale pilot plant stu dies.