Microbial barriers in treatment processes are the major block to the passage of waterborne pathogens from raw source waters. Many of the processes utilized in water supply treatment have some impact on microbial densities and survival. For instance, raw water storage for 24 to 48 hours can generally reduce the bacterial load by 50%, coagulation-sedimentation by 60% and filtration by 99.9 percent under favorable conditions. Combining these processes in a series of successive treatments can provide a cumulative reduction in waterborne organisms so that the burden on final disinfection to achieve a 6 log reduction of bacteria, 4 log reduction in virus and 3 log reduction in protozoan cysts is possible on a continuous basis. The key variable in the case is fluctuating source water quality that can impact treatment barrier effectiveness. Treatment barrier effectiveness can also change as a consequence of operational changes at the plant.

"Presented at Proceedings: Water Quality for the New Decade, Annual Conference, Philadelphia PA, 6/23-27/91, p:107-137." Includes bibliographical references. Microfiche.