Coliforms are used as indicators of faecal pollution in water. Therefore, the presence of coliforms in drinking water causes concern as it indicates the potential presence of other bacteria. Coliforms have been seen in water within the Vivendi Water UK area during the summer months and their presence has previously been explained by localised pipe renovation. In this study, the influence that the algal bloom has on the presence of coliforms has been assessed. It has been shown that there is a strong link between the end of the algal bloom and coliforms being found in the distribution system. The algal bloom does not allow coliforms to pass through the treatment works. However, the high level of total organic carbon (TOC) in the treated water, made up of algal breakdown products, provides a good nutritional source for regrowth in the distribution system. Where there are high TOC levels, coliforms can grow even with high chlorine concentrations. However, where there is little TOC, then even a very low chlorine residual is adequate to prevent coliform growth.

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