How deep is your BAF? Evaluation of different bed depths for high rate biological denitrification filters with limit of technology pollutant removal

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Downflow Denitrification Filters are tertiary water and waste water treatment systems that allow denitrification and suspended solids removal to occur in a single step and allow limit of technology pollutant removal. With more and more stringent nutrient removal regulations coming up, downflow denitrification filters have increasingly become the solution of choice for plants faced with the dilemma of installing a robust system to tackle the new treatment goals without breaking the bank.

A high rate downflow denitrification filter employing high surface area media was tested with different bed depths at a waste water treatment facility in Ohio to test its pollutant removal capabilities in cold winter conditions. The expanded clay media used in the filter was theorized to offer much higher surface area than sand used in traditional deep bed denitrification filters, thereby allowing application of significantly higher loading rates to the filter. Testing was carried to treat increasingly higher hydraulic and pollutant loadings while achieving effluent NOx-N < 1 mg/l and effluent TSS < 5 mg/l. Four different bed depths (9ft, 6ft, 4ft and 2.5ft) were tested during this study and the loading rates and pollutant removal capabilities tested to compare the same with traditional deep bed filters. Influent flow rates varied from 2 gpm/ft2 to 6 gpm/ft2 depending on the bed depth, with influent NOx-N varying between 8.6 mg/l to 37 mg/l throughout the study.

The filter showed excellent results for all four bed depths as even at influent NOx-N loadings of 250 to 300 lbs/kcf/d and influent temperatures as low as 8 °C, the filter was able to achieve greater than 95% NOx-N removal with effluent NOx-N < 1 mg/l and effluent TSS < 5 mg/l. Utilization of high surface area media was hence demonstrated to have a pronounced impact on the pollutant removal capabilities of a downflow denitrification filter with filter performance remaining consistent even at loading rates twice that of conventional filters. The system hence offers a flexible, cost effective upgrade option for utilities to meet stringent effluent limits in a small footprint with a low capital and long term O&M investment.

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