Coliform bacteria, isolated from treated drinking water supplies, can be derived from a range of sources (e.g. infiltration, breakthrough at the treatment works or from the biofilm established within the pipework). The sensitivity of these bacteria to chlorine may be related to their source and metabolic status. Strains of coliforms were isolated from sewage works effluents, river and reservoir waters as well as from the bulk water and biofilms from distribution systems. These were assayed for sensitivity to free and total chlorine using two assay procedures. For E. coli, the isolates from the distribution system bulk water showed greater resistance to free chlorine than those from sewage effluents and equivalence to those from river waters. For non-E. coli coliforms (mainly strains of Klebsiella, Enterobacter and Citrobacter), those from distribution system biofilms showed the greatest sensitivity to free and total chlorine whilst those from river water had the greatest resistance.

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