Rehabilitation of the BNR Facilities at the Greenville Utilities Commision Wastewater Treament Plant

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ABSTRACT
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.

INTRODUCTION
The Greenville (North Carolina) Utilities Commission Wastewater Treatment Plant (GUC WWTP) is a 17.5 mgd maximum month average daily flow (MMADF) treatment plant that operates two separate activated sludge trains (the north plant and south plant). The GUC WWTP discharges to the Tar River and is permitted by the State of North Carolina through the National Pollutant Discharge Elimination System (NPDES) program. Current permit limits include maximum month 5-day carbonaceous biochemical oxygen demand (CBOD5), total suspended solids (TSS) and ammonia-nitrogen (NH3-N) effluent concentrations of 8 mg/L, 30 mg/L and 4.1 mg/L, respectively. No specific total nitrogen or total phosphorus limits are included under the NPDES permit. However, the GUC WWTP is a member of the Tar-Pamlico Basin Association (TPBA), a coalition of 14 treatment plants discharging to the Tar River basin. Total collective annual nutrient loads have been developed for the TPBA plants. Plants within the TPBA can trade nutrients allocations if plant specific nutrient targets are not met. The target effluent total nitrogen and total phosphorus limits for the GUC WWTP are 6.0 mg/L and 2.0 mg/L, respectively. The GUC WWTP has historically met these targets.

Both process trains were originally designed to operate as biological nutrient removal (BNR) systems. The south plant is a biological removal system placed in operation in 1996 and is designed for biological phosphorus and nitrogen removal. The south plant is in good condition and consistently removes nitrogen and phosphorus. The north plant was constructed in 1987 and was originally designed as an innovative technology to perform both nitrification and denitrification; however, denitrification in the north plant was unsuccessful. Furthermore, the existing mechanical equipment installed in the north plant had deteriorated over time and needed replacement. Although the north plant has not operated as originally designed, the treatment plant has managed to consistently meet the target effluent limits by operating the south plant in a full BNR mode and the north plant in a nitrification mode. The flow split between the two plants is approximately 60/40, with 60% (10.5 mgd) of the flow to the south plant and 40% (7.0 mgd) of the flow to the north plant.

The necessity to replace equipment provided an opportunity to improve denitrification in the north plant aeration basins. A study was performed to determine the most cost effective approach for rehabilitation of these aeration tanks. Replacement of the aeration equipment was required, and retrofitting the tanks to successfully perform both nitrification and denitrification was of optimum importance.

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