Algae removes nitrogen and phosphorus from manure

Algae – already being eyed for biofuel production – could be put to use to remove nitrogen and phosphorus from livestock manure runoff, according to a US Agricultural Research Service (ARS) scientist.

This could give resource managers a new eco-friendly option for reducing the level of agricultural pollutants that contaminate water.

In 2003, microbiologist Walter Mulbry from the ARS in Beltsville, Maryland, set up four algal turf scrubber (ATS) raceways outside dairy barns in Beltsville. The shallow 100ft (30m) raceways were covered with nylon netting that created a scaffold where the algae could grow.

For the next three years, from April until December, a submerged water pump at one end of the raceways circulated a mix of fresh water and raw or anaerobically digested dairy manure effluent over the algae. Within two to three weeks after the ATS system was started up every spring, the raceways supported colonies of green filamentous algae.

Mulbry and his partners harvested wet algae every four to 12 days, dried it and then analysed the dried biomass for nitrogen and phosphorus levels. His results indicate that the ATS system recovered 60-90 per cent of the nitrogen and 70-100 per cent of the phosphorus from the manure effluents.

They also calculated that the cost was around $5 (£3) to $6 for each pound of nitrogen and around $25 for each pound of phosphorus that was recovered.