Tertiary Denitrification and Very Low Phosphorus Limits: A Practical Look at Phosphorus Limitations on Denitrification Filters

Deep-bed denitrification filters have been successfully used for over 25 years to meet total nitrogen (TN) limits of 3 mg/L and even lower. In some cases, the wastewater treatment plants (WWTPs) also have had moderate total phosphorus (TP) limits, in the range of 0.5 to 2 mg/L. More recently, some WWTPs are required to meet very low TN and TP limits simultaneously. For example, as the result of the development of total maximum daily loads (TMDL) for the Chesapeake Bay, many WWTPs in Maryland and Virginia will be required to meet limits of 3 mg/L TN and 0.3 mg/L TP on a 12-month average basis. A number of these plants have even more stringent phosphorus limits of 0.18 mg/L monthly average. This has raised practical operating concerns with respect to the ability of these WWTPs to meet low TP limits without additional treatment downstream of the filters. In addition, a certain amount of phosphorus is required to support the growth of denitrifying bacteria and it may be difficult to ensure there is sufficient phosphorus to promote denitrification if too much is removed in upstream processes. This paper will examine phosphorus requirements for denitrification, and expands upon earlier information presented at the International Water Association Biofilm Systems VI Conference in September 2006. Operating data from several full-scale and pilot denitrification filters operating for low TN and TP simultaneously will be presented and evaluated to determine when low phosphorus concentrations actually impact denitrification, and identify strategies for ensuring reliable performance.