Dynamic Modeling of Wet Weather Operations at Blue Plains Advanced Wastewater Treatment Plant

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ABSTRACT
The District of Columbia’s Advanced Wastewater Treatment Plant at Blue Plains is a 370-mgd facility that provides wastewater treatment for over 2 million people in Washington, DC and surrounding jurisdictions in Maryland and Virginia. Blue Plains receives combined sewer flows that originate in the District’s combined sewer system. Two separate activated sludge processes provide removal of BOD and nitrogen. The DC Water and Sewer Authority (WASA) is evaluating enhanced nutrient removal options for Blue Plains to meet the future more stringent nutrient limits of the Chesapeake Bay Program. Providing ENR at a combined sewer plant is challenging given the operational changes implemented during wet weather events.

This paper focuses on the impact of the wet weather operation on nitrogen removal performance. The plant’s biological processes are routinely operated in plug flow mode. When wet weather events are anticipated, the plant converts the secondary and the nitrification/denitrification reactors to step feed modes to avoid solids washout from the clarifiers. A wet weather operation model, using the BIOWIN process model, was developed to simulate the impact of the wet weather operations on total nitrogen removal during a severe storm. Dynamic simulations were used to evaluate the impact of various peaking factors and treatment options for wet weather flows on plant discharge of total nitrogen. The treatment options for combined sewer system storage tunnel pump-out included processing through the complete treatment system and through a new wet weather treatment system.

INTRODUCTION
The District of Columbia Water and Sewer Authority (WASA) owns and operates the Advanced Wastewater Treatment Plant at Blue Plains in Washington, D.C. Blue Plains provides treatment to combined sewer and sanitary flows from the District of Columbia and sanitary flows from Fairfax County and Loudoun County in Northern Virginia, and Montgomery County and Prince Georges County in Maryland. Blue Plains is designed to treat an average daily flow of 370 mgd, a peak flow to the advanced treatment system of 740 mgd, and a peak plant flow of 1,076 mgd.

The plant, which has a two-sludge process, was designed for nitrification of wastewater flows. In 2000, WASA met the requirements of the Chesapeake Bay Program to remove nitrogen to meet a total nitrogen goal of not greater than 8,467,200 pounds per year (equivalent to a concentration of 7.5 mg/l at 370 mgd) by utilizing a portion of the nitrification reactors for biological nutrient removal (BNR). The last 2 of the 5 stages in each nitrification reactor were converted to anoxic stages and methanol was added ahead of Stage 4. This conversion to BNR was successful but it significantly reduced the factor of safety in meeting the plant’s NPDES permit.

WASA is now faced with complying with two regulatory initiatives that will require significant capital investment to provide a treatment facility that can routinely meet its permit. WASA has completed a Long Term Control Plan that calls for construction of a tunnel system to capture 193 million gallons of combined sewer flows. The plan calls for treating this captured flow at Blue Plains after the storm flows to the plant subside. WASA also must comply with the new requirements of the Chesapeake Bay Program that call for a total annual nitrogen loading limit from Blue Plains of 4,766,000 pounds (4.2 mg/l). To meet the combined requirements of these initiatives in a cost-effective manner, WASA has conducted strategic process engineering planning to identify process alternatives that can meet the reduced TN requirements while at the same time treating increased wet weather flows.

As part of the strategic planning, WASA developed a process simulation model using BIOWIN to evaluate process options. This model was developed and calibrated with the assistance of Envirosim, the model’s developer. The model was used in the steady-state mode to predict monthly effluent TN discharge levels for the various process options. These were used to predict annual TN discharge performance. However the process development showed two wet weather impacts that must be accounted for when developing a strategy for ENR at a CSO plant. The first impact is infiltration during wet hydrologic years. Historically, infiltration from the plant’s entire service area has increased plant influent flows by as much as 50 mgd for a significant period. The second impact is plant performance during and after a significant wet weather event. Blue Plains has the ability to operate the two biological processes in various step-feed modes during a storm to prevent solids washouts. These modes are used to store solids in portions of the reactors until the plant flow subsides and then the stored solids are gradually fed back to the main process. This paper describes the dynamic process modeling used to evaluate the impact of a wet weather event and the use of these solids storage modes on effluent TN performance for various process alternatives.

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