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Author: Andre van Niekerk, Janice Ruhl, and Paul Pitt (Hazen and Sawyer); Denny Parker (Brown and Caldwell); Salil Kharkar and Aklile Tesfaye (District of Columbus Water and Sewer Authority)
Date: 10/03
WEFTEC Conference 2003

The District of Columbia Water and Sewer Authority's Blue Plains Advanced Wastewater Treatment Plant, which has a design average daily flow of 370 mgd and a design peak flow of 1,076 mgd, discharges to the Potomac River, a major tributary to the Chesapeake Bay. In order to satisfy the NPDES permit and the nutrient load reduction goals of the Chesapeake Bay Agreement (i.e. 40% load reduction from 1985 levels), strict seasonal nitrogen and phosphorus effluent discharge limits, shown in Table 1 below, must be met. Table 1: NPDES Permit Requirements. Summer (May 1 – October 31) Winter (November 1 – April 30) Monthly Maximum Weekly Maximum Monthly Maximum Weekly Maximum NH3-N (mg/L) 1.0 1.5 6.5 9.8 TN (mg/L) 5.5 - 8.58 - TP (mg/L) 0.18 0.27 0.18 0.27 The Nitrification/Denitrification facilities are currently being upgraded to reliably achieve strict Nitrogen and Phosphorus limits, and to provide for future centrate and increased influent loads. This paper presents the evaluation of alternative Nitrification/Denitrification (N/DN) processes, for both dry weather and wet weather operations, using the Biowin™ model. Results are presented in terms of solids inventory concentrations, ammonia and nitrate concentrations, and sedimentation basin factor of safety. Step feed with a dedicated stage for centrate treatment was determined to be the most beneficial process configuration for the N/DN reactors. Additional process modifications including automated control of the Secondary Effluent feed to the reactors, automated process air control, polymer addition to the mixed liquor channels and an increase in RAS pumping capacity are recommended to provide reliable process performance under wet weather scenarios.