The fate of environmental coliforms in a model water distribution system

A series of experiments to determine the survival characteristics of enviro nmental and faecal coliforms in a 1.3 km long experimental pipe distributio n system is described. In the first experiments, tertiary treated effluent (10(3)-10(4) coliforms ml(-1)) was inoculated directly into the distributio n system. Coliform organisms were not detected in any samples taken downstr eam of the inoculation point. By comparison, laboratory jar tests showed lo w 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 c onditions in a dead-end. Coliform numbers were monitored and found to decre ase rapidly, but they were still detectable at low levels after 7 weeks. Th e 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 samp les 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 c oliforms survive and grow in biofilms on pipe walls.