Pilot Testing of Upflow Continuous Backwash Filters for Tertiary Denitrification and Phosphorus Removal

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ABSATRACT
Pilot testing of upflow continuous backwash filter (UCB) for tertiary denitrification and phosphorus removal was conducted at the Hagerstown wastewater treatment plant. The pilot testing was conducted to determine denitrification performance capabilities of the upflow continuous backwash filters under cold weather operation and low effluent total nitrogen (TN) and total phosphorus limits. As part of a state-wide strategy for meeting the nutrient reduction goals of the Chesapeake 2000 Agreement with the U.S. Environmental Protection Agency and the other Chesapeake Bay watershed states, the State of Maryland is requiring all WWTPs > 0.5 mgd be upgraded to meet “limit of technology” (LOT) effluent standards of 3 mg/L TN and 0.3 mg/L TP on an annual average basis.

The testing program included operation over a range of hydraulic and nitrate mass loading rates, moderate and cold weather operation, with and without chemical phosphorus removal; and testing the response of the filters to nitrate, solids spikes and wet weather flows. At hydraulic loading rates up to 4.0 gpm/ft2, the filter consistently reduced NO3-N from 5-8 mg/L to <1 mg/L at wastewater temperatures ranging from 13 oC to 19 oC. The methanol dosing ratio was consistently between 2.5 and 3.0 mg MeOH / mg NO3-N applied. The filter consistently reduced nitrate to < 1 mg/L under diurnal hydraulic loading conditions varying from 2.2 to 4.4 gpm/ft2.

The pilot test was conducted with mass loading rates which typically ranged from 20 to 40 lbs NOx-N/1,000 ft3/day with peak loading conditions up to 120 lbs NOx-N/1,000 ft3/day. The pilot filter was also used to test chemical phosphorus removal polishing. A nutrient analyzer was used to control ferric chloride dosing for chemical phosphorus precipitation to maintain an effluent phosphorus concentration below 0.3 mg/L. Phosphorus removal was tested both with and without a reaction tank upstream of the filters with similar results experienced under both conditions.

INTRODUCTION
Downflow, deep bed denitrification filters have been successfully used for 25 years to meet total nitrogen (TN) limits to as low as 3 mg/L. Most of this experience has been in the southeastern US, with relatively warm wastewater temperatures and with adequate phosphorus in the filter influent wastewater to support the growth of denitrifying bacteria. Upflow continuous backwash (UCB) filters have been used for just over 10 years in a limited number of denitrification applications in the US and Europe, but currently do not have operating history for meeting very low TN limits. With renewed interest in UCB filters for tertiary denitrification, limited cold weather operating data, and the requirement in some areas to meet very low TP limits simultaneously, there was a need to further assess UCB filter denitrification performance capabilities.

As part of a state-wide strategy for meeting the nutrient reduction goals of the Chesapeake 2000 Agreement with the U.S. Environmental Protection Agency and the other Chesapeake Bay watershed states, the State of Maryland is requiring all WWTPs > 0.5 mgd be upgraded to meet “limit of technology” (LOT) effluent standards of 3 mg/L TN and 0.3 mg/L TP on an annual basis. The City of Hagerstown pilot tested UCB filters for tertiary denitrification and phosphorus removal to meet these limits. Hagerstown is interested in UCB filters because of the simplicity of operation, small footprint, elimination of a clearwell and mudwell, and the ability to tolerate high solids and flow spikes during wet weather events. Testing was conducted through the winter months to develop design criteria for cold weather operating conditions in western Maryland.

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