Comparing the effect of various pipe materials on biofilm formation in chlorinated and combined chlorine-chloraminated water systems

To compare the effect of various pipe materials on biofilm formation, surface water was used as the test water source; plastic-based pipe materials (polyvinyl chloride-PVC, unplasticised polyvinyl chloride-UPVC, medium density polyethylene-MDPE) and cement-based pipe materials (cement and asbestos cement) are commonly used in drinking water distribution systems in South Africa as test pipe materials. Disinfection was performed using ca 2.5 mg·l initial chlorine followed by ca 1.5mg·l monochloramine. The evaluation of the process relied on attached coliforms and heterotrophic plate count bacteria. General data indicated the colonisation of all test pipe materials by micro-organisms under the chlorination process within the first 20 min and over the remainder of the study period. The addition of monochloramine to the chlorinated water system (24 h after chlorination) resulted in the removal of coliforms and heterotrophic bacteria attached to pipe materials. Less than 1 cfu·cm viable bacterium (except for PVC) was observed on the surface of test pipes between 48 and 168 h. However, the factor of time cannot be ignored in determining the effect of pipe materials on biofilm formation in potable water systems. Bacterial regrowth occurred on the surface of all pipe materials between 168 and 672 h. The capability of bacterial regrowth occurring on the surface of test pipe materials during this period was linked to the depletion of the concentration of monochloramine residual. Statistical evidence showed that the generic type of pipe materials greatly influenced the density of bacteria in laboratory-scale systems. Cement-based materials significantly supported less fixed bacteria than plastic-based materials (at p<0.05 and p<0.01). No significant difference in attached bacterial counts was found between the same generic types of pipe materials. This study suggests the use of cement and asbestos cement pipes for the distribution of chlorine-monochloramine treated water. Statistical evidence also showed that all physico-chemical parameters (temperature, pH, turbidity, dissolved organic carbon, total nitrogen, sulphate) had no significant effect on bacterial number at p<0.05, implying that the presence of an effective monochloramine residual in the chlorinated water system remains one of the most important factors in controlling the effect of pipe materials on biofilm formation.