5.7.5 Problems caused by large numbers of microorganisms

Large numbers of aerobic heterotrophic bacteria in treated water can interfere with the interpretation of test results for the coliform group. The method employed for coliform group measurement and enumeration influences the type of interferences that may occur, for example:

• where membrane filtration methods are employed, non-target organisms may overgrow target organisms, masking their presence and yielding false-negative results.

• where defined enzyme substrate methods are employed, large numbers of non-target organisms may give false positive reactions (e.g. the presence of high levels of Aeromonas spp. in water samples, which may have a low level of galactosidase production, can lead to false positive results).

Most of these organisms can be controlled relatively easily by water treatment processes, including disinfection. Nutrient-rich raw water should be avoided if water treatment cannot be applied.