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Ballasted Biological Process Achieves Low Nitrogen and Phosphorus without Tertiary Filtration

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The Winebrenner Wastewater Treatment Plant (WWTP) is located in Cascade, Maryland and is operated by Washington County. The plant is an aging rotating biological contactor (RBC) facility that needs to be upgraded to achieve compliance with the Maryland Department of the Environment's (MDE) enhanced nutrient removal (ENR) permit limits, specifically total nitrogen (TN) less than 3 mg/l and total phosphorus (TP) less than 0.3 mg/l. The County and its consulting engineers, Buchart Horn and RK&K, had considered several potential upgrades to address these requirements. In particular, they previously recommended using sequencing batch reactors, followed by denitrification filters. This proposed new treatment scheme has demonstrated compliance with Maryland's ENR limits at other facilities, but carried a relatively high capital cost and footprint. BioMag was identified by the (MDE) as a potential cost effective alternative technology.

The consulting engineers worked with Cambridge Water Technology (CWT) and Washington County to conduct a full scale BioMag trial by using existing tankage to create a Four-Stage Bardenpho system. The anoxic and aerobic stages were installed in an aerobic sludge digester. One of the plant's existing primary clarifiers was converted into a secondary clarifier. The purpose of the trial was to demonstrate that BioMag can be used to cost effectively meet the proposed permit limits. BioMag is an emerging technology for use with the activated sludge process that can more than double a facility 's biological treatment capacity and free up tankage for biological nutrient removal, utilizing existing bioreactor tanks and clarifiers.

BioMag is a ballasted treatment technology that uses finely divided magnetite to increase the specific gravity of biological floc. Magnetite is Fe3O4, a readily-available, inexpensive, inert iron ore that is not magnetic itself, but is strongly drawn to a magnet. Magnetite has a specific gravity of 5.2, approximately twice that of sand. Increasing the specific gravity and settling rate of the biological floc provides the opportunity to appreciably increase the mixed liquor suspended solids (MLSS) concentration, while still maintaining adequate settling and thickening in the secondary clarifiers. The process works by impregnating biological floc with magnetite. The dark magnetite particles can be easily seen with a microscope as black specs within the floc. The magnetite is recovered from the waste activated sludge (WAS) using high shear mixing and magnetic drum separation, and is returned to the aeration tank.

The primary objective of this full scale trial was to demonstrate that converting the RBC system to a Four-Stage Bardenpho BioMag system would allow the plant to effectively and reliably achieve compliance with the MDE's proposed nutrient removal limits, including the TN and TP limits of 3 mg/l and 0.3 mg/l, respectively. A secondary objective was to demonstrate that BioMag can minimize the need for additional tankage to treat present and future loadings. A third objective was to demonstrate the peak flow handling capability of the BioMag system. Because the mixed liquor is weighted down with magnetite, the BioMag clarifiers are relatively less sensitive to variations in flow and loading.

The full scale BioMag demonstration was conducted in a small footprint, utilizing one existing clarifier and one half of the existing aerobic sludge digester tankage to facilitate the suspended growth process. The magnetite feed and storage equipment was housed in a portion of an equipment storage building. The trial began in February 2010, when the wastewater temperature was 6 degrees C. The Four-Stage Bardenpho process was seeded with mixed liquor from a neighboring publicly owned treatment plant (POTW) and magnetite was added to achieve a magnetite-to-MLSS ratio of approximately 1-to-1. The BioMag process operated over a wide range of mixed liquor and temperature conditions. MLSS concentrations ranged from 6,000 to 13,000 mg/l, not including the weight of magnetite. Mixed liquor volatile suspended solids (MLVSS) concentrations ranged from 4,500 to 9,000 mg/l.

The BioMag full scale trial demonstrated significant process benefits relative to reduced footprint requirement; maintenance of high MLSS; low effluent ammonia, consistently less than 0.2 mg/l; low effluent TN, consistently less than 3 mg/l; low effluent TP, averaging less than 0.1 mg/l; low effluent total suspended solids (TSS), consistently less than 5 mg/l; and low clarifier sludge blanket depth, consistently less than 2 feet. All of these were achieved at wastewater temperatures as low as 6 degrees C.

The results of this successful full scale demonstration show that constructing a four-stage BioMag system will allow the Winebrenner WWTP to effectively and reliably achieve compliance with the State's proposed ENR permit limits, without the need for filtration or membrane separation. BioMag can also effectively operate at elevated MLSS concentrations, minimizing the need for additional tankage to treat present and future loadings, including wet weather peak flows. The elevated MLSS and MLVSS concentrations also facilitate complete nitrification and nearly complete denitrification under cold weather conditions.

The County concluded that the demonstration was a success and, pending confirmation of funding availability, plans to move forward with the design and installation of BioMag systems at multiple WWTPs. Present worth costs for the BioMag upgrade are lower than those of upgrades previously contemplated for the WWTPs.
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Keywords: Activated Sludge; Ballasted Flocculation; Enhanced Nutrient Removal; Nitrogen Removal; Phosphorous Removal

Document Type: Research Article

Publication date: 2011-01-01

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