BNR RATING OF THE ARLINGTON COUNTY WATER POLLUTION CONTROL PLANT THROUGH FULL-SCALE TESTING AND CALIBRATED MODELING

The Arlington County Water Pollution Control Plant (ACWPCP) is a tertiary treatment plant currently rated at 30 mgd, designed to meet monthly average discharge standards of CBOD5 ≤ 5 mg/L, TSS ≤ 6 mg/L and TP ≤ 0.18 mg/L. The plant was recently upgraded to meet a new ammonia standard (NH3-N ≤ 1.0 mg/L NH3-N/L in summer, and NH3-N ≤ 2.0 mg/L in winter). The plant is also required to remove nitrogen (TN ≤ 8mg/L) on a yearly basis and to reduce the number of plant bypasses that occur during wet weather operating conditions. Significant challenges existed in the plant upgrade project; including no redundancy in the Aeration Tank and Secondary Clarifier infrastructure, low available hydraulic head across the site, a constrained site and limited budget. A decision was made to construct a new Primary Effluent Equalization Tank, retrofit the existing Aeration Tanks, construct one additional Aeration Tank and retrofit the Secondary Clarifiers as part of the BNR upgrade. The retrofitted and expanded BNR facility would be tested under normal and stressed operating conditions. Data derived from the full-scale testing would be utilized to calibrate the activated sludge process simulation model, to give a reliable plant rating and determine essential future facilities. The Arlington County WPCP is unique in the sense that it is one of the first multiple-pass, step-feed BNR processes to be constructed in the eastern USA. The practical experience with this plant will be of interest to municipalities considering BNR upgrades to their existing facilities.