The effect of biofilm on THM formation in chlorinated drinking water of a PVC pipe distribution system

This research investigated the interaction between chlorine residual in drinking water and the interior surface of polyvinyl chloride (PVC) distribution system pipes. The major objectives were to establish evidence of a biofilm effect on THM production, and to examine the effectiveness of chlorine on inactivating biofilm bacteria associated with PVC surfaces. This project employed a modified SDS test, known as Material Specific Simulated Distribution System (MS-SDS) test (Brereton, 1998), to evaluate the chloroform formation in pipe-incubated water samples, collected at a treatment pilot plant in Vancouver, British Columbia. Although the ultrafiltration membrane system installed produced an excellent finished water quality in terms of turbidity and some reduction in THM formation, the process did not selectively remove chloroform precursors. However, statistical analysis on MS-SDS test results showed that whatever impact the treatment process might have on chloroform formation in the bulk water was "drowned out" by the pipe environment. Pipe environment becomes the dominant effect on chloroform production, with residence time being the main factor of interest. Under chlorine-limited conditions, pipe wall contribution increases THM levels in the bulk water phase during the short incubation period (5 hours), but has a negative overall effect on chloroform production during the long incubation period (4 days). Adsorption was identified as the surface mechanism responsible for the removal of THM precursors from the bulk water phase. Heterotrophic plate counts (HPCs) carried out on biofilm scrapings from the pipe surfaces revealed that exposure to chlorine stress could lead to bacterial inactivation or biofilm regrowth. Under stress, biofilm bacteria can take advantage of some form of biological resistance that resulted in additional chloroform formation in the bulk water. Humic substances previously adsorbed onto biofilm surface are released following cell damage repair; these additional substances are n ow available to participate in THM formation in the bulk water phase. A pictorial model summarizes the major mechanisms interacting in a PVC pipe environment. Experiments on re-chlorination effect showed that the practice of re-chlorination created more favorable reaction conditions of slow-reacting THM formation. Although booster disinfection has the potential of becoming an effective prevention to biofilm regrowth, it may also be out-weighed easily by other in-situ factors, such as the organic (and inorganic) composition of the water and the dynamic state o f the biofilm.Applied Science, Faculty ofCivil Engineering, Department ofGraduat