Rehabilitation of the BNR Facilites at the Greenville Utilities Commission Wastewater Treatment Plant

The Greenville Utilities Commission Wastewater Treatment Plant (GUC WWTP) is a 17.5 mgd treatment plant designed to meet effluent total nitrogen and total phosphorus limits of 6.0 mg/L and 2.0 mg/L, respectively, through biological nutrient removal. The treatment plant is divided into two separate activated sludge systems; the north and south plants. The treatment plant has consistently met effluent limits operating the south plant in a full biological nutrient removal mode and the north plant in a nitrification mode.The north plant was originally designed to operate as a two-stage biological nutrient removal system; however, consistent denitrification was never attained. In addition, the aeration basins utilized a rotating bridge assembly to provide aeration and mixing in the anoxic zones. The rotating assemblies required significant maintenance and were in need of replacement. A study was performed to determine the most cost effective approach to rehabilitating the existing tanks. The results of the study identified limitations of the existing system to provide for denitrification, and the most cost effective approach for rehabilitation of these tanks was to provide for two-stage denitrification.The existing tanks were redesigned for two-stage biological nitrogen removal utilizing an anoxic zone followed by an aeration zone with an internal nitrified recycle flow. The anoxic zone was mixed with submersible mixers and the aeration zone was retrofitted with fine bubble diffusers. The flow path through the tanks was converted to a plug flow arrangement. This paper addresses the conversion to and performance of the existing tanks as a two stage BNR process for total nitrogen and phosphorus reduction.