AWWA ACE99473

The design for the Seattle Public Utilities' 120 mgd Tolt Treatment Facility, now under construction, incorporates an innovative combined ozone contactor and flocculation basin designed to achieve 3-log Cryptosporidium inactivation. Subsequent direct filtration (i.e., no sedimentation process) is designed to achieve an additional 2-log Cryptosporidium removal. By combining the ozonation and flocculation processes, only one basin instead of two is necessary which reduces construction costs considerably. Prior to and during design, pilot tests were conducted to verify that the combined ozone/flocculation process would provide both effective disinfection and flocculation. Key findings from this pilot study were: that ozone demand and decay were not affected significantly by the presence of an organic polymer coagulant; and, that the required polymer dose to achieve optimum filtration performance was roughly 50 percent greater with the combined ozone/flocculation process than with separate ozone and flocculation processes.