September 2010, Vol. 22, No.9
Algae recover nutrients in runoff and show promise as fertilizer
Beneficial uses for algae just keep growing. In addition to being a potential source for biofuel, algae can remove nitrogen and phosphorus from runoff, according to a study conducted by the U.S. Department of Agriculture’s Agricultural Research Service (ARS).
In 2003, ARS microbiologist Walter Mulbry set up four algal turf scrubber (ATS) raceways — like shallow ponds — outside of dairy barns at the ARS’ Henry A. Wallace Beltsville (Md.) Agricultural Research Center. The 30-m (100-ft) raceways were covered with nylon netting that provided a platform for algae to grow. For 3 years, from April to December, a submerged water pump circulated a mix of fresh water and untreated or anaerobically digested dairy manure effluent over the algae.
Mulbry and his colleagues harvested wet algae every 4 to 12 days, dried this material, and analyzed the dried biomass for nitrogen and phosphorus levels. Results indicate that the ATS system recovered 60% to 90% of nitrogen and 70% to 100% of phosphorus from the manure effluents, according to an ARS news release.
The researchers also determined that the cost for this capture was comparable to costs for other manure management practices, at approximately $11 to $13/kg ($5 to $6/lb) for nitrogen and $55/kg ($25/lb) for phosphorus. These estimates include the ATS system’s high capital costs and the requirement for workers onsite to harvest algae, the news release says.
Mulbry thinks that algae can be used to capture nitrogen and phosphorus in livestock manure before it reaches waterways; then, the captured nutrients can be dried and sold as a slow-release fertilizer. “Algae: A Mean, Green Cleaning Machine,” published in the May/June issue of the ARS magazine Agricultural Research, says Mulbry found that corn and cucumber seedlings grown in algae-amended potting mixes performed as well as those grown with commercial fertilizers.
Mulbry is now working with University of Maryland (College Park) scientist Patrick Kangas and State of Maryland and Caroline County, Md., researchers on a study of Maryland’s Eastern Shore to see if farmers who use poultry litter for fertilizer also can use ATS systems to clean up nitrogen and phosphorus runoff in field ditches, the article says.
In addition, Mulbry is contributing to a study examining different ATS systems to identify the most important factors in designing a cost-effective approach to their animal-feed supplement.
Solids present a potential source for biofuel
Producing biodiesel fuel from municipal solids has become more cost-effective. The report “Biodiesel Production From Municipal Sewage Sludges,” published in the American Chemical Society (ACS; Washington, D.C.) journal Energy and Fuels, details how existing technology can produce fuel from wastewater solids at a cost within a few cents a gallon of being competitive with conventional diesel refined from petroleum.
Demand for biodiesel has led to the search for cost-effective biodiesel feedstock. Soybeans, sunflower seeds, and other food crops can be used to create biodiesel but are expensive. But solids are abundant — the United States produces about 6 million Mg/yr (7 million ton/yr). Municipal solids contain significant concentrations of lipids that can make production of biodiesel from the feedstock profitable. Solids are a good source for biodiesel production, the report says.
According to David M. Kargbo, author of the report, treatment plants could use microorganisms that produce higher amounts of oil to boost biodiesel production to 38 billion L (10 billion gal), which is more than triple current U.S. biodiesel production capacity, an ACS news release says. However, using solids for biodiesel poses challenges that include determining the most effective and least costly way to collect and treat solids for maximum lipid extraction, handling solids that contain pharmaceutical chemicals, and responding to regulatory concerns, the report says.
Still, the report says that biodiesel production from solids could be profitable in the long run. Currently, the estimated cost to produce biodiesel from solids is $0.82/L ($3.11/gal). To be competitive, this cost must be at or below petrodiesel costs, which have recently been at $0.79/L ($3/gal), the news release says. n
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