OPTIMIZATION OF BNR PROCESSES TO ACHIEVE LOT AT WSSC PLANTS

The Washington Suburban Sanitary Commission (WSSC) owns, operates and maintains several award winning wastewater treatment plants in Prince George's and Montgomery Counties in Maryland. Over the past several years, four of these plants were upgraded with single sludge BNR processes to meet a seasonal goal of Total Nitrogen in the range of 7–8 mg/l. In accordance with WSSC's mission and core strategy of protecting and enhancing the environment, the Commission continued its efforts to optimize these processes from the startup to reduce the discharge of nutrient to the Chesapeake Bay. These efforts were intensified as it became evident that the plants have to meet the more stringent Limits of Technology (LOT) when Maryland enacted regulations requiring wastewater plants to upgrade from Biological Nutrient Removal (BNR) processes to the more advanced Enhance Nutrient Removal (ENR) processes several years ago. The challenge became how to achieve this goal and how to minimize future O&M costs associated with the operating the ENR processes. These plants are:Piscataway WWTP: A 30 MGD single sludge BNR plant utilizing Step-Feed processes since 2000Parkway WWTP: A 7.5 MGD single sludge BNR plant utilizing Bardenpho processes since 1992Seneca WWTP: A 20 MGD single sludge BNR plant utilizing MLE process with internal recycle since 2003Damascus WWTP: A 1.5 MGD single sludge BNR plant utilizing MLE process with no internal recycle since 1999, with the flexibility to be converted to a Bardenpho process.The O&M staff at these BNR plants realized that the conversion to the ENR processes more than likely would necessitate a supplemental carbon source that is fed to a Denitrification process downstream of the BOD removal stage. They also realized that this carbon feed could be reduced if the BNR process is optimized by reducing the Nitrate portion of the Total Nitrogen (TN) to a minimum after full Nitrification. The objective for each plant became to achieve further reduction in TN to levels well below those targeted by the BNR design in anticipation of the ENR upgrades.