How Well Do Bioreactors Remove Nitrates from Tile Drainage Water?

Iowa farmers could have another practice in their nitrogen management toolbox if research sponsored by Leopold Center shows positive results.

Published on: Jul 21, 2009
Iowa farmers could have another practice in their nitrogen management toolbox if research sponsored by the Leopold Center for Sustainable Agriculture produces positive results. The Leopold Center is headquartered at Iowa State University.

The Leopold Center's Ecology Initiative is supporting a new study to test how well bioreactors remove nitrates from the water that comes from tile-drained fields. Alok Bhandari, an associate professor in the ISU Department of Ag and Biosystems Engineering, is leading the multiyear project.

Nitrates, a form of nitrogen from fertilizers applied to crops, have been contributing to an increasingly large "dead zone" in the Gulf of Mexico since the 1980s. As much as 39% of the nitrogen buildup has been traced back to the Upper Mississippi River basin, including Iowa.

Need to keep fertilizer N from causing "dead zone"

"The goal of this project is to facilitate adoption of nitrogen management practices in Iowa and the Upper Mississippi River Basin and thus promote a more sustainable agriculture," says Bhandari. "This goal depends on several factors, including cost-effectiveness, social acceptance and performance of practices such as bioreactors."

A bioreactor is a large trench through which water from underground drainage tiles passes before leaving the field. This trench is filled with organic matter high in carbon, in this case a mix of chips from various hardwoods that act as a strainer for the water coming from the tile. The wood chips "strain-off" nitrogen (appearing as nitrates) in the water by growing bacteria that digest the nitrates before the water flows out of the field and into nearby streams.

Exactly how effective bioreactors are at removing nitrates is not yet known. Bhandari hopes to perfect a bioreactor design that can remove high percentages of nitrates and optimize its performance under Iowa field conditions. Over two full growing seasons beginning in 2009, Bhandari will monitor three small-scale bioreactors at ISU research farms and three full-scale bioreactors, two of which were built by the Iowa Soybean Association on private farms.

Testing being conducted in cooperation with ISA

The pilot bioreactors cover only about 12 square feet of surface area and are about 2 feet deep. Full-scale bioreactors require about 25 square feet per acre of farmland drained and a depth of about 4 feet, depending on location of the tile line. Grass is planted over the entire area so that the only thing visible aboveground is a grassy buffer strip.

Several things can help prevent nitrate loss through groundwater runoff besides denitrifying bioreactors; among them are tillage, cropping systems, type of fertilizer and tile-drain spacing. However, even with good management strategies, the levels of N in drainage water can still exceed approved levels.

Bhandari and his team also are looking at the length of time bioreactors will function effectively before the filtering material must be replaced. He predicts that the bioreactors will last at least a decade. Next year, they will compare corncobs with the wood chips as the carbon source.

To learn more about this project, check out the report in the new online video section, On the Ground with the Leopold Center, at: www.leopold.iastate.edu/research/eco_files/ground.html.