Tertiary Denitrification and very low Phosphorus Limits: a Practical Look at Phosphorus Limitations on Denitrification Filters

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ABSTRACT
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.

INTRODUCTION
Deep-bed denitrification filters have been successfully used for over 25 years to meet effluent total nitrogen (TN) limits of 3 mg/L and even lower. More recently, some facilities are required to meet very low TN and total phosphorus (TP) limits simultaneously. For example, as the result of water quality concerns for the Chesapeake Bay, many wastewater treatment plants (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, and some WWTPs have even more stringent phosphorus limits of 0.18 mg/L monthly average. This has raised practical operating concerns about potential limitation of biological growth in the filters if too much phosphorus is removed in upstream processes.

To examine the impact on denitrification performance, simultaneous phosphorus precipitation and denitrification was tested using upflow filters in the Stockholm area (Hultman et al, 1994; Jonsson et al, 1997; Jonsson, 1998). It was estimated that under the conditions tested, a phosphorus concentration of 0.1 mg/L was sufficient for denitrification. At reduced concentrations of 0.03 mg/L PO4-P, denitrification performance was impacted. Similar concerns of phosphorus limitation have been discussed for separate stage nitrification processes that follow chemically enhanced primary settling. For example, testing conducted by Nordeidet et al (1994) showed that tertiary nitrification in rotating biological contactors was impacted at phosphorus concentrations below 0.15 mg/L PO4-P. These results illustrate the potential difficulty of operating denitrification filters to meet low nitrogen and phosphorus limits simultaneously.

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