Algae Effects: Positive Role Algae Plays with Livestock Manure
It sounds like a late-night infomercial pitch: a plant that grows at a monstrous pace in polluted water, soaks up CO2 and cleans up manure runoff. Agricultural Research Service microbiologist Walter Mulbry has discovered there’s more to algae than stringy masses that muck up waterways. Though some scientists believe algae effects might be the next best thing in biofuel production, Mulbry thinks it can be used right now to reduce the nitrogen and phosphorus in livestock manure – then dried and sold as a slow-release fertilizer.
Livestock manure is used as a cheap and abundant field-crop fertilizer because it contains large amounts of nitrogen and phosphorus, both essential plant nutrients. But some of the nitrogen and phosphorus that isn’t used by the crops can be washed into nearby streams and rivers and end up in sensitive waterways such as the Chesapeake Bay.
Like cultivated field crops, aquatic plants grow and flourish when nitrogen and phosphorus are plentiful. When these plants die, their decomposition robs the bay of oxygen, which in turn harms other aquatic life. A 2005 report by the Chesapeake Bay Foundation, whose motto is “Save the Bay,” estimated that animal manure contributes 18 percent of the nitrogen and 27 percent of the phosphorus that drain into the bay every year.
“Other researchers have looked at using algae-based systems to clean up municipal wastewater and other types of polluted water,” says Mulbry, who is with the ARS Environmental Management and Byproduct Utilization Research Laboratory in Beltsville, Maryland. “So we built on that concept and focused on tracking manure nitrogen and phosphorus through the treatment system.
“Nutrient recovery values – the amounts of manure nitrogen and phosphorus that are added to the system compared with the amounts of algal nitrogen and phosphorus removed from the system – are surprisingly difficult to measure in the field because of frequent changes in manure characteristics.”
In 2003, Mulbry set up four algal turf scrubber (ATS) raceways outside dairy barns at the ARS Henry A. Wallace Beltsville Agricultural Research Center. The shallow raceways were covered with nylon netting on which the algae grew. Then for the next three years, from April until December, submerged water pumps at one end of the raceways circulated a mix of fresh water and raw or anaerobically digested dairy manure over the algae.
Just add water and stir
Within two to three weeks after the ATS system was started up each spring, the raceways supported thriving colonies of green filamentous algae, including Rhizoclonium hieroglyphicum, the most abundant algae species, and Microspora willeana, Ulothrix ozonata and Oedogonium sp. Algae growth was highest in the spring and declined during the summer months in part because of the higher water temperatures and also because the raceways provided snails and midge larvae ample opportunity to graze on the algae.
Working with postdoctoral researcher Elizabeth Kedebe-Westhead and others, Mulbry harvested wet algae every four to 12 days, dried it, then analyzed it for nitrogen and phosphorus levels. Results indicate that the ATS system recovered 60 to 90 percent of the nitrogen and 70 to 100 percent of the phosphorus from the manure.
The potential benefits don’t stop with nutrient capture. Mulbry also studied whether the dried algae had potential as an organic fertilizer. He found that corn and cucumber seedlings grown in algae-amended potting mixes performed as well as those grown with commercial fertilizers.
“It’s possible that farmers using ATS management could sell the dried algae as an organic fertilizer for use by urban and suburban residents. It’s a ready-made, ‘Save the Bay’ fertilizer,” Mulbry says.
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