Cold Weather Design and Operational Considerations for Deep-bed Denitrification Filters to Achieve Limit-of-Technology Nutrient Removal

For several decades deep bed downflow denitrification filters have proven reliable as tertiary treatment for achieving low effluent nitrogen levels. The majority of these facilities are operating in the southeastern region where wastewater temperatures are generally warmer. In recent years, a number of facilities have also been installed in the mid-Atlantic region to meet nutrient reduction requirements. While wastewater temperatures are comparably colder in this region most of these facilities are either operating in seasonal denitrification mode (low flow, warm period), or operating in filtration mode only (no external carbon addition) where the plant's upstream process is capable of meeting current total nitrogen requirements. Also, it is found that many of these facilities are operating at lower loading conditions and therefore reported performance data may not be representative of design conditions. In an effort to confirm the design criteria for typical mid-Atlantic cold weather operation and year-round performance to meet limit-of-technology (LOT) levels (TN ≤ 3 mg/L, TP ≤ 0.3 mg/L) required for treatment plants in Maryland, as well as other jurisdictions within the Chesapeake Bay watershed, the Maryland Department of the Environment (MDE) and the City of Baltimore collaborated on a denitrification filter pilot testing program. Testing was conducted at the Back River WWTP from January through July of 2009 and the results are presented in this paper, and compared with performance observed at full-scale facilities.In summary, the cold weather testing demonstrated the system's ability to achieve the effluent objectives (for nitrate removal) at loading rates from 40-50 lbs nitrate/1,000 ft3/day. The average hydraulic loading rates were up to 3.0 gpm/ft2, with 80-90% removal efficiency at average wastewater temperatures of about 13 °C. During warmer weather testing the nitrate mass removal capacity increased and the system was able to achieve lower effluent concentrations at loadings similar to or higher than those for cold weather testing. The filter system was also hydraulically tested during warmer weather at peak-day loading rates up to 9 gpm/ft2 (at a loading near 100 lbs nitrate/1,000 ft3/day) while still achieving about 80% nitrate removal, demonstrating the system's ability to handle peak flows and loads without significant reduction in effluent quality.