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In recent years, the robust growth of residential and commercial development in the Outer Banks of North Carolina put a strain on the wastewater treatment plant. The Monteray Shores wastewater treatment plant (WWTP) had a maximum hydraulic capacity of 180,000 gal per day (gpd) and was configured to achieve BOD and TSS reduction only and did not have an allowance for phosphorus or nitrogen removal.
In addition, the WWTP experienced significant swings in influent flow, which fluctuated heavily depending on the season. A dramatic rise of influent flow occurs in summer months due to an influx of travelers, while the opposite is true for winter months. Additional growth and development for the area was limited by the treatment capacity of the plant. Demand for additional WWTP capacity was so strong that Carolina Water Service, Inc. contracted to have the plant capacity increased by approximately three times to 520,000 gpd.
The original Monteray Shores WWTP consisted of a two-train extended aeration plant. The facility had difficulty meeting permit requirements during peak vacation season in the Outer Banks—May to October. Also, with a planned increase for development in the Outer Banks, developers were looking for effective treatment plant capabilities in order to build new properties.
In 2006, plans were developed to increase the capacity of the plant and design a process that was capable of achieving complete nitrification-denitrification for a total nitrogen of less than 4 mg/L and phosphorus removal below 2 mg/L. Options were limited because the footprint of the site was restricted due to an apartment complex on one side and retail stores on the other. Furthermore, there was a need for adequate land allocation for rapid infiltration basins as the North Carolina Department of Environmental and Natural Resources requires this method for effluent discharge.
The plant’s engineer, through various evaluations, determined that a membrane bioreactor (MBR) upgrade was the best option for meeting all of the plant’s requirements. The technology allowed for the plant to utilize much of its existing infrastructure and could be constructed in a phased manner so that a portion of the plant could treat the influent wastewater during the low flow off-season.
The existing extended aeration plant was converted to a four-stage biological treatment process. Each train of the extended aeration plant was transformed to an anaerobic, primary anoxic, aeration and secondary anoxic zone. The remaining tank volume is utilized for sludge holding and digestion. Having two separate trains allowed for one train to be constructed while the other remained in operation, treating the influent wastewater. Once one side was completed it could be placed online and the other side could be retrofitted.
Prior to converting the existing WWTP, a new headworks structure with rotary drum fine screens, grit removal and membrane tanks was constructed. The rotary drum screens have perforated openings of 2 mm to prevent items that can potentially damage the membranes from entering the process. Each membrane tank has five membrane modules installed into the reactors, with room for a sixth to be added if future capacity demands are needed. The membrane modules are of flat sheet configuration with an average pore size of 0.08 μm.
Each membrane tank has a dedicated permeate pump, scour aeration blower and return activated sludge pump. Having a dedicated mechanical piece of equipment for each provides flexibility for turndown. This flexibility is critical because the plant flows can vary from 520,000 gpd in the summer down to 50,000 gpd during a weekday in the winter months.
Overall, the expansion of the plant to MBR was a success, satisfying all of the objectives set forth in the planning stage. By utilizing the existing tank, digester and equalization tank, a significant cost savings was realized. The upgrade was carried out in two phases, resulting in zero interruption to the operation of the existing plant. The upgrade increased the plant hydraulic capacity to 0.52 million gal per day, while achieving the most restrictive effluent requirements and positioning the plant to serve its customers well into the future.