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Achieving Limit-of-Technology Treatment for Total Nitrogen and Total Phosphorus Removal at the Alexandria Sanitation Authority AWTF

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The Alexandria Sanitation Authority (ASA) Advanced Wastewater Treatment Facility (AWTF) is located in Alexandria, Virginia. The plant is rated at an annual average flow of 54 MGD and currently receives about 36 MGD. Located in the sensitive Chesapeake Bay watershed, the ASA facility's current nutrient limits are to meet a monthly average total phosphorus (TP) concentration limit of 0.18 mg/L and an annual average total nitrogen (TN) concentration goal of 8 mg/L. The ASA AWTP is consistently achieving an annual average TP concentration of less than 0.1 mg/L and in preparation for future nitrogen limits, has achieved an annual average TN concentration of 3 mg/L. This level of performance has been achieved for a combined sewer plant operating in cold temperatures.

Phosphorus removal at ASA consists of dual-point precipitation using chemicals. The first point for removal is the secondary settling tanks where ferric chloride is added to the mixed liquor coming from the biological reactor basins. The second point of removal is the tertiary settling tanks where alum is added to the chemical mixing zone, which is followed by a baffled flocculation tank containing slow-moving paddle mixers and, finally, inclined-plate settlers for solids removal. The effluent is then filtered through 5-ft deep sand bed filters prior to UV disinfection. These technologies have effectively achieved low effluent TP concentrations with an annual average TP at or below 0.06 mg/L in the past 5 years.

The nitrogen removal system consists of five biological reactor basins and six secondary settling tanks. Each biological reactor basin has a total volume of 4.2 million gallons (MG), divided into an anoxic zone (0.8 MG), an anoxic/aerobic swing zone (0.5 MG) and an aerobic zone (2.9 MG). The system was designed so that the basins could be operated in parallel (MLE mode) or in series (step-feed configuration). The plant has met the annual average effluent TN concentration goal of 8 mg/L consistently since 2003. Starting in 2004, the plant began operating in step-feed mode and adding methanol for TN reduction. The plant is currently operating with four out of the five reactor basins in service. The last tank in service is operated as a post-anoxic zone with methanol addition for denitrification. In 2007 the plant implemented upgrades to the system including installing submersible mixers in the post-anoxic denitrification tank to maintain the volume well mixed and re-configuring the methanol pump piping to allow dosing of all the methanol to the last tank in service.

In 2004, Virginia's Secretary of Natural Resources began the process of enacting new regulations to reduce nutrient levels in the Chesapeake Bay in order to meet the goals of the Chesapeake 2000 Agreement. The regulations included lower nutrient limits on wastewater treatment plant discharges that would require limit-of-technology treatment for nutrient removal. Starting in 2011, the ASA facility will have to meet an annual waste load allocation of 493,381 pounds per year, which equates to a TN discharge concentration of 3.0 mg/l at the AWTF's current design capacity of 54 million gallons per day (MGD), and continue to meet a monthly average TP concentration limit of 0.18 mg/L. ASA is currently designing a facility upgrade to meet the upcoming regulations at the design flows and loads.
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Keywords: Dual-point chemical phosphorus precipitation; activated sludge; alum; denitrification; ferric chloride; limit of technology; methanol; step-feed

Document Type: Research Article

Publication date: 2009-01-01

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