A novel high-rate sedimentation process has been developed for directly treating combined sewer overflows (CSOs). This was done using a test facility at an actual wastewater treatment plant in Tokyo. Pilot test results, carried out 13 times, from December 2002 to July 2003 in wet weather, suggested that the process was suitable for treatment of CSOs. The performance of the process was favorable at 50 m3/(m2 ·h) up to 880 mg/L influent TSS, removing between 78 % and 91 % of TSS at loading and between 64 % and 85 % of BOD5 also at loading. The pilot testing clarified that a decrease in the influent alkalinity due to rain water caused a drop in the pH after FeCl3 addition and thus improved coagulation, with a significant decrease in the effluent TSS. Mixing, coagulation and flocculation were carried out in a baffling type mixing tank to enable uniform mixing without any short-passes. Coagulation conditions in this mixing method were evaluated and results clarified that rapid mixing was required momentarily after each addition of FeCl3 and polymer, i.e. to diffuse these additives into the influent.

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