The fate of Escherichia coli through water treatment and in distribution.

The removal of E. coli bacteria at each stage of water treatment is presented, showing how the filtration stages contribute most to reduction of bacterial numbers in the water. At treatment works without filtration stages, the emphasis is put on catchment management to limit contamination of the raw water and ensure that the numbers of viable E. coli in the source water remain low. Routine monitoring of the raw water provides data on seasonal trends in numbers of E. coli bacteria and allows effective management of supply. In the UK there is no evidence that E. coli grows in the water distribution system, whether in the planktonic stage or within biofilms (O'Neill et al. 1997). The detection of E. coli in the distribution system is rare and prompts a thorough investigation. Repeat samples are taken from the point which originally failed, along with a number of hydraulically linked samples including samples from hydrants. The response to the detection of E. coli is discussed. A series of experiments carried out on a pilot pipe system is briefly described and the results discussed in relation to the routine samples taken in the Thames Water Supply area.

The fate of environmental coliforms in a model water distribution system.

A series of experiments to determine the survival characteristics of environmental and faecal coliforms in a 1.3 km long experimental pipe distribution system is described. In the first experiments, tertiary treated effluent (10(3)-10(4) coliforms ml-1) was inoculated directly into the distribution system. Coliform organisms were not detected in any samples taken downstream of the inoculation point. By comparison, laboratory jar tests showed low level survival for coliforms at the same chloramine residuals (0.3 mg l-1) for up to 48 h. In the second series of experiments, a by-pass pipe in the experimental distribution system was isolated, drained, and filled with tertiary treated effluent (10(3)-10(4) coliforms ml-1) to simulate the conditions in a dead-end. Coliform numbers were monitored and found to decrease rapidly, but they were still detectable at low levels after 7 weeks. The water in the by-pass section was then released into the main pipe-rig and sampled downstream. No coliforms were detected in water samples or in samples swabbed from the pipe walls. Finally, the flow in the main pipe-rig and in the by-pass was increased to dislodge any deposits and biofilm into the water. The absence of coliforms in any water samples taken during or after the flow increases is contrary to the widely accepted understanding that coliforms survive and grow in biofilms on pipe walls.