Addressing the nutrient crisis: evaluation of a biological filter to meet low total nitrogen discharge limits
New regulations aiming to reduce total pollutant loads entering receiving waters are emphasizing more stringent nutrient discharge limits for wastewater treatment plant (WWTP) effluents. Numerous state Departments of Environmental Quality and Protection have proposed regulations to lower nitrogen pollutant discharge load, especially in environmentally sensitive regions such as the Chesapeake Bay and Long Island Sound watersheds. When the new regulations go into effect, the most stringent tier will limit WWTP effluent total nitrogen (TN) concentrations to 3 mg/L, while current regulations have typically established 7 mg/L TN as the upper limit. Such new regulations pose a challenging task for existing WWTPs, which have not generally been designed to address such low nutrient discharge limits.
This paper will describe research conducted to evaluate the feasibility of Biological Nutrient Removal (BNR) technology, specifically an upflow biological anoxic filter (BAF) configured to provide enhanced solids capture for use in denitrification (DN) applications to meet a TN discharge limit of 3 mg/L.
The modified BAF-DN pilot was operated at nominal wastewater flow rates ranging from 5 to 11.5 gpm/ft2, demonstrating an average NOx-N removal efficiency of >85% while consistently eliminating more than 105 lbs/ft3/day of nitrate-nitrite load. The study objective of 3 mg/L effluent TN was consistently met over the course of the study.
During peak operation, the modified upflow BAF-DN pilot ran at a maximum hydraulic loading rate of 11.5 gpm/ft2 and a nitrate-nitrite load in excess of 195 lbs/ft3/day at 15.6°C. Results demonstrated that effluent TSS levels below 5 mg/L were consistently achieved. The research results clearly demonstrate that denitrification is achievable with the modified upflow BAFDN configuration with process performance yielding effluent TN levels below 3 mg/L without inhibiting normal operation and backwash criteria.