High Rate Treatment of CSO/SSO Flows Using a High Density Solids Contact Clarifier/Thickener - Results from a Pilot Study



A two-month pilot study was conducted at the Village Creek Wastewater Treatment Plant in Birmingham, Alabama to test the technical feasibility of utilizing a high-rate, highdensity solids contact clarification process for the treatment of sanitary sewer overflow (SSO). Five phases of testing were performed over the two-month period. During Phase I testing, optimum steady state operating parameters were determined and clarifier performance was investigated at surface overflow rates ranging from 29 to 60 gpmlft2. A ferric chloride dose of 45 mgll and high molecular weight polymer dose of 1.5 mgll provided the best treatment at all surface overload rates tested during Phase I. COD and TSS removal efficiencies were 35-61% and 80-95%, respectively.  During phases 11, Ill, and IV various start-up methods were tested to determine the time necessary to reach steady-state operation. Three methods were investigated: dry startup from a completely empty tank, wet start-up at gradually increasing surface overflow rates, and seeded start-up using mixed liquor from the plant's secondary clarifier. Dry start-up was determined to be the most efficient method. Steady-state operation was reached within 20 minutes, and this time was independent of the surface overflow rate. TSS removal rates ranged from 70-91% after 30 minutes of clarifier operation at surface overflow rates as high as 60 gpm/ft2. In the final phase of the study, an alternative coagulant, aluminum sulfate (alum) was tested. Also during this phase, the clarifier's operational requirements were evaluated.



Solids contact, sludge thickening, DensaDeg 4D, lamellar settling, coagulation, flocculation



Combined Sewer Overflows (CSO) are a discharge of a mixture of storm water and untreated domestic wastewater that occurs when the flow capacity of a sewer system is exceeded during a rainstorm. CSOs are excess flows from a combined sewer system that are discharged into a receiving water without being treated. Combined sewer systems are found primarily in older metropolitan communities of the northeast, Mid- Atlantic, and Great Lakes regions of the U.S.

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