STEP-FEED BNR PROCESS ACHIEVING TN < 4 mg/LA Case Study of WSSC-Piscataway WWTP, Prince George's County, Maryland

摘要

Piscataway WWTP is a 30 MGD advanced plant with average flow of 18 MGD, operating 3 of 4 parallel basins (see attached plant schematic). It is operated by Washington Suburban Sanitary Commission (WSSC), and located in Prince George's County, Maryland. The Plant undertook one of the first large-scale Step-Feed BNR applications in the nation, which went on line in the fall of 2000 with the purpose of achieving a seasonal goal of 8 mg/l TN. The step-feed process was selected due to its lower construction cost compared with other BNR processes, yet meeting the State's program to get POTW's to voluntarily meet a seasonal TN goal of 8 mg/l. The lower construction cost is attributed to smaller basins, no internal recirculation pumps, and no methanol addition systems when compared with other more conventional BNR processes. The process was designed to meet a seasonal TN limit of 5 mg/l. During the first year of operation, a monthly trial was conducted on the process, varying one or more operating parameter to test the process. Based on the first year's results, the 2002 operating parameters were adjusted to maximize nitrogen reduction, achieving TN monthly averages less than 4 mg/L from June to November 2002. The retrofit converted two activated sludge processes (Secondary Activated Sludge Process and Nitrification Activated Sludge Process) that were in series into two parallel trains of Step-feed BNR process. Raw wastewater is screened and degritted before Primary clarification. The Primary Effluent is step fed into the BNR basins. Each train is comprised of two identical basins, however the two trains differ significantly from each other: Train 1 basins are 2.65 MG each and comprised of 5 passes each, while Train 2 basins are 2.76 MG each and comprised of 4 passes each. The following table summarizes the characteristics of the basins.