LARGEST PLANT IN THE EASTERN UNITED STATES ACHIEVING LIMITS OF TECHNOLOGY UTILIZING A CONVENTIONAL SUSPENDED GROWTH BNR ACTIVATED SLUDGE PROCESS

The Neuse River Wastewater Treatment Plant is a 60-mgd advanced wastewater treatment plant serving the City of Raleigh and surrounding municipalities in eastern Wake County, North Carolina. The facility is the largest facility in the eastern United States that has consistently achieved effluent total nitrogen concentrations approaching the limits of technology. The plant discharges to the nutrient-sensitive Neuse River. As a result of North Carolina's Neuse River Basin Nutrient Sensitive Waters (NSW) Management Strategy rules, promulgated in 1997, the plant must comply with a stringent total nitrogen (TN) limit of approximately 676,000 lbs/year, or 3.7 mg/L at 60 mgd. Annual average effluent TN concentrations over the last four calendar years (2003–2006) were 3.28, 2.88, 2.55 and 2.30 mg/L, respectively. Average effluent TN concentration has averaged 1.98 mg/L over the first five months of 2007.From 1997 through 2000, a two-phased construction program, which included conversion of the conventional activated sludge process to a biological nitrogen removal (BNR) process and conversion of existing deep-bed filters to denitrification filters, was implemented. The majority of TN removal is achieved in the BNR activated sludge process, with only a ±1 mg/L trim of TN occurring on the denitrification filters.This paper will present design information on the TN removal processes implemented at the Neuse River Plant, which include a four-stage process featuring two anoxic zones, internal nitrified recycle and methanol addition. The major focus of the paper will be a discussion of the optimization of the TN removal process, including performance and operations issues associated with:Control of dissolved oxygen within anoxic and aerobic zonesInternal nitrified recycle (NRCY)Methanol feedUse of mixed liquor distribution channel for reaerationPlant staff has continued to optimize both the TN removal processes and operating costs since startup of the BNR process. These efforts have included strict zone DO control to minimize DO in the NRCY and to minimize carryover of DO into the post anoxic zone. Plant staff has also manipulated the existing basins to provide a larger post anoxic zone by using the aerated mixed liquor distribution channel for the reaeration stage and operating the reaeration stage unaerated. The paper will also present design and operational information on the conversion of the plant's conventional dual media deep bed filters (30-inches of media) to denitrification filters, which included increasing media depth (48-inches of media), providing air scour, media changeout, and methanol feed and storage and control system modifications. This conversion was unique and provided a non-conventional, cost effective means of achieving denitrification filters, which have been used successfully to provide nitrate trimming when needed to ensure the effluent TN limit is met.The paper will also review the results of on-going evaluations and recommendations for expanding the nitrogen removal facilities to a 75-mgd capacity. This includes development of a calibrated BioWin™ model of the plant in conjunction with a site-specific sampling and monitoring program to obtain the information required to develop, calibrate and verify the model. Detailed influent and effluent wastewater characterization, BNR basin nutrient profiles, nitrification kinetics evaluation, and evaluation of full-scale denitrification kinetics with methanol will be included.