Chemically Enhanced Primary Treatment for a Large Water Reclamation Facility on a Constricted Site – Considerations for Design, Start-Up, And Operation

To provide greater treatment capacity and to meet new water quality criteria for discharge to the Chattahoochee River, Cobb County Water System (CCWS) construction a new 60 million gallon per day (mgd) liquid treatment facility at the R.L. Sutton Water Reclamation Facility (Sutton WRF). A restricted site (25 acres) combined with an effluent limit on total phosphorus of 0.5 mg/L resulted in the decision to use chemically enhanced primary treatment (CEPT) to both minimize facility footprint and to provide phosphorus removal. Based on performance between May 2005 and April 2006, the new CEPT process facility at the Sutton WRF is removing over 50% of the influent 5-day biochemical oxygen demand (BOD5), over 60% of total suspended solids (TSS), and nearly 50% of the influent phosphorus. Jar settling tests performed during design indicated improved performance with chemical coagulation, and rapid mix and flocculation basins were provided prior to circular primary clarifiers. The primary coagulant is currently ferrous chloride, however, no polymer is used at this time. Although the facility is hydraulically under-loaded, it is meeting strict effluent limits with a low coagulant dose.