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United States Department of Agriculture

Agricultural Research Service

Research Project: BIOLOGICAL TREATMENT OF MANURE AND ORGANIC RESIDUALS TO CAPTURE NUTRIENTS AND TRANSFORM CONTAMINANTS

Location: Sustainable Agricultural Systems Laboratory

Title: Nutrient removal of agricultural drainage water using algal turf scrubbers and solar power

Authors
item Kangas, Patrick
item Mulbry, Walter

Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 13, 2013
Publication Date: November 19, 2013
Citation: Kangas, P., Mulbry III, W.W. 2013. Nutrient removal of agricultural drainage water using algal turf scrubbers and solar power. Bioresource Technology. 152:484-489.

Interpretive Summary: Restoration of the Chesapeake Bay poses significant challenges because of increasing population pressure, conversion of farmland to urban/suburban development, and the expense of infrastructure needed to achieve significant and sustained nutrient reductions from agricultural and urban sources. One approach that is being evaluated as for removing non-point source nutrients before they reach the bay is to deploy algal turf scrubbers (ATS) along its tributaries and/or drainage canals. In ATS systems, algae grow on nutrients in the water and are harvested for potential use as a fertilizer or soil amendment. The objective of this study was to determine rates of nutrient removal and costs using ATS raceways located on a primary drainage canal (Mason Branch), on Maryland’s Eastern Shore. Since drainage canals are typically remote from conventional power connections, we also evaluated solar-powered pumping systems for off-grid ATS use. Replicate ATS raceways were operated continuously for five month periods in 2010 and 2011. Results showed that projected nutrient removal costs for a one acre ATS unit are, depending on the source of electricity and capital cost, $40-$50 per pound of nitrogen or $230 to $450 per pound of phosphorus. The cost of electricity for powering water pumps accounts for approximately 80% of these costs. These nutrient removal costs are more than 10-fold higher than previous estimates (roughly $5 per pound of nitrogen, $24 per pound of phosphorus) for ATS units used to remove nutrients from manure effluents. These much higher cost estimates are due to lower rates of algal growth, lower algal nutrient content, and higher electricity rates compared to those used in previous studies. Algal based water treatment systems are being evaluated for possible inclusion as a best management practice within the Chesapeake Bay Program Watershed Model. These results are important for this evaluation because they are derived from field-scale experience on agricultural drainage water.

Technical Abstract: Restoration of the Chesapeake Bay poses significant challenges because of increasing population pressure, conversion of farmland to urban/suburban development, and the expense of infrastructure needed to achieve significant and sustained nutrient reductions from agricultural and urban sources. One approach for removing non-point source nutrients before they reach the bay is to deploy algal turf scrubbers (ATS) along its tributaries and/or drainage canals. The objective of this study was to determine rates of nutrient removal and costs using pilot scale ATS raceways located on a primary drainage canal on Maryland’s eastern shore. Additional objectives were to evaluate solar-powered pumping systems for off-grid ATS deployment, and to determine the effect of water flow rate on algal growth and nutrient removal. ATS raceways (each containing 50 square meters of growing area) were operated continuously for five month periods (June to November) in 2010 and 2011. Nitrogen (N) and phosphorus (P) removal rates were calculated by multiplying harvested algal mass and algal N or P content. Removal rates fluctuated considerably but averaged 125 mg N, 25 mg P per square meter per day at the highest flow rates. Spreading the combined flow over more raceways (and therefore decreasing the flow rate over each raceway) yielded the same amount of removed N and P, but decreased the calculated areal removal rate. The composition of the harvested material (composed of algae and trapped sediment) was relatively consistent, with mean values of 2% N, 0.25% P, and 70% ash. Extrapolated nutrient removal rates (including the nutrients from the sediment in the ATS sump) are equivalent to removal rates of approximately 230 to 350 lb N and 40 lb P per acre over a 7-month operating season. Projected nutrient removal costs for a one acre ATS unit are, depending on the source of electricity and capital cost, $40-$50 per pound of N or $230 to $450 per pound of P. The cost of electricity for powering water pumps accounts for approximately 80% of these costs. These nutrient removal costs are more than 10-fold higher than previous estimates (roughly $5 per pound of N, $24 per pound of P) for ATS units for removing nutrients from manure effluents. These much higher cost estimates are due to lower rates of algal growth, lower algal N and P content, and higher electricity rates compared to those used in previous studies.

Last Modified: 12/22/2014
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