South Cary Water Reclamation Facility's Nutrient Removal Modifications and Reduction Success, Town of Cary, North Carolina

To accommodate anticipated growth and associated wastewater increases, the Town of Cary recognized the need to expand their South Cary Water Reclamation Facility (WRF) from 6.4 million gallons per day (MGD) to 12.8 MGD. The 6.4 MGD plant was designed to meet effluent limits of 5.0 mg/l BOD and 1.0 mg/l ammonia nitrogen using a completely mixed activated sludge (CMAS) process. A phosphorus limit of 2.0 mg/l was accomplished by chemical precipitation. A critical component of the design was the capability to comply with nutrient reduction strategies that were being developed for the Neuse River Basin by the North Carolina Division of Water Quality.ARCADIS G&M, along with key members of the Town staff, evaluated several process options that would meet the design objectives. The main area of concern was the re-design of existing aeration basins and the design of a new aeration basin for biological nutrient removal. The existing facilities consisted of two single-cell circular aeration basins with fixed aeration equipment. The recommended alternative was to re-configure the existing aeration basins to accommodate multiple modes of operation for biological nutrient removal and to provide additional capacity. A new aeration basin was designed to be identical to the modified existing aeration basins.After beginning operation of the facilities, ARCADIS and Town staff collaborated on experiments with multiple flowsheets, return activated sludge (RAS) rates, dissolved oxygen (D.O.) levels, and physical modifications to optimize process performance. Several biological nutrient removal processes may be operated within the modified basins by varying the flow patterns, feed points and application of aeration. Both mainstream and sidestream phosphorus removal processes can be operated, along with multiple processes for nitrogen removal. Performance data from the modified facilities indicates over 90 percent reduction in plant effluent total nitrogen. The plant effluent phosphorus has been reduced an additional 80 percent over that previously achieved by chemical precipitation.