Leopold Center studies denitrifying bioreactors - possible help for Gulf 'dead zone'

Alok Bhandari hopes that the water from his denitrifying bioreactor in Story County, Iowa will make the northern Gulf of Mexico safer for shrimp than what is coming out of other Midwestern tile-drained fields.

Nitrates from Iowa’s tile-drained landscape have been contributing to an increasingly large “dead zone” in the Gulf of Mexico since the 1980s. As much as 39 percent of the nitrogen buildup has been traced back to the Upper Mississippi River Basin, including Iowa. That’s why the Leopold Center Ecology Initiative is supporting a new multi-year research project led by Bhandari, an associate professor in the Iowa State University Department of Agricultural and Biosystems Engineering.

Bhandari’s project will investigate the value of denitrifying bioreactors as a nitrogen management option for Iowa farmers.

“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,” said 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 hole or trench is filled with organic matter that is high in carbon, in this case a mix of chips from various hardwoods, that act as a strainer for 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.

Three key “actors” contribute to the process: a source of carbon, (wood chips), bacteria (which grow on and around the carbon matter) and nitrate (present in the runoff), which is food for the bacteria. In addition, oxygen cannot be present, otherwise bacteria will feed on the oxygen and not the nitrate.

Bioreactors capture nitrate in water from underground tile drainage lines at the edge of fields, yielding water that is much cleaner when it enters Iowa streams and rivers.

However, little is known about this technology, such as how effective it can be at removing nitrates. Bhandari hopes to perfect a bioreactor design that can remove a high percentage of nitrate and optimize its performance under Iowa field conditions. He will monitor three small-scale and three full-scale bioreactors over two full growing seasons beginning in 2009.

Three pilot bioreactors were installed at the Iowa State University Agronomy/Agricultural Biosystems Engineering Research Farm west of Ames. They are identical in volume but vary in shape so researchers can evaluate the effectiveness of each model. The reactors are 2 feet deep, filled with wood chips, then covered with geofabric and 3 inches of top soil.

Bhandari is monitoring two full-scale bioreactors installed by the Iowa Soybean Association in the West Butrick Creek watershed on the Mike Bravard farm in Greene County and on the Arlo Van Diest farm in Hamilton County. The third reactor is at Iowa State University’s Northeast Iowa Research Farm near Nashua.

While the pilot bioreactors are only about 12 square feet in surface area, full-scale bioreactors require about 25 square feet per acre of farmland drained and a depth of about 4 feet depending on the location of the tile line. A 100-acre field would require about 2,500 square feet of bioreactor space covered by a grass buffer. 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 nitrogen in drainage water can still exceed approved levels.

Bhandari said he became interested in water quality work several years ago and began to look at what happens in groundwater systems when pesticide levels increase. When asked why this project would matter to Iowans, he said, “This is one more way to protect our surface water, and that is important.”

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 corn cobs with the wood chips as the carbon source.

Bhandari is working with co-investigator Matt Helmers, Agricultural and Biosystems Engineering; graduate research assistant Laura Christianson; and research associates Carl Pederson, Loren Shiers and Reid Christianson.