PENETRATION OF COLIFORMS THROUGH MUNICIPAL DRINKING-WATER FILTERS

The penetration of coliform bacteria through municipal drinking water was studied under normal operating conditions of water treatment plants. Samples of filtered water were taken during the 5 h immediately following the backwash cycle and analyzed for turbidity and coliform bacteria. Coliforms were enumerated using both m-Endo and m-T7 media to evaluate coliform passage and injury. Elevated turbidity and intermittent peaks of coliforms (as high as 60/100 ml) were observed during the post backwash period. Highest numbers of coliforms were observed during the flow through time for the interface of the backwash remnants and the influent water after backwash. In addition, most of the coliforms passing through the filters during this period were injured because they failed to grow on m-Endo but were enumerated on m-T7 medium. These results indicate that the post backwash period can be characterized by comparatively higher levels of turbidity as well as coliforms that cannot be detected using accepted enumeration methods. Furthermore, such undetected bacteria may seed the distribution system. Therefore, improved bacteriological surveillance for injured coliforms in filter effluents could guide the establishment and design of filter-to-waste periods. This would reduce the incidence of coliforms in some municipal potable water systems.