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Corn+Soybean Digest

Nitrate Neutralizer

As a county ditch inspector, Eric Schrader has worked hard to promote conservation drainage measures, such as grass buffer strips. Now, the Rice County, MN, farmer is trying out a new type of “drain cleaner.” It's a buried trench filled with wood chips.

Wood chip biofilters, or bioreactors, remove nitrates from tile drainage water without removing cropland from production. “They can be used in areas where wetlands might not be feasible,” says Richard Cooke, an ag engineer at the University of Illinois and a leading bioreactor designer.

Biofilters have other advantages for farmers, too, he says. They don't restrict drainage, require little maintenance and can be installed in a few hours with a backhoe. And they work with any drainage system.

Biofilters are among several new drainage-management technologies that are being field-tested by Corn Belt farmers and conservation groups.

“We don't currently manage drainage in modern agriculture,” says Dan Jaynes, a soil scientist at the USDA-ARS National Soil Tilth Laboratory in Ames, IA. “We manage everything else, yet with drainage, we put a pipe in the ground and leave it.” But now, Jaynes says, there's growing interest in managing agricultural drainage, both to “improve yields and reduce the impact on water quality.”

A NETWORK OF underground pipes laces the Corn Belt, draining more than 50 million acres of cropland. “Drainage is a great success story,” Jaynes says. “We wouldn't have the agriculture we have without drainage.”

Artificial drainage rapidly removes excess water from farm fields. But rapid draining also leaches nitrogen (N) from fertilized cropland. Even with careful nutrient management, Jaynes says, some nitrate loss is unavoidable in row-crop production — “no way around it.”

Nitrates move along with drain water to tile outlets, entering streams, rivers and, ultimately, the ocean. There they feed aquatic plant growth. As plants die and decompose, they deplete the oxygen in the water, which kills fish and shellfish.

Nitrates from tile drain water are blamed for poor stream water quality and hypoxia — or low oxygen — in the northern Gulf of Mexico. Nitrates in drainage water can also contaminate drinking water. A 2007 report by the Environmental Protection Agency Hypoxia Advisory Panel calls for a 45% cut in the amount of N and phosphorus entering the Gulf of Mexico from the Mississippi and Ohio River watersheds.

WOOD CHIP BIOREACTORS remove nitrates from drainage water by the same natural process that works in wastewater treatment ponds and wetlands. Tile water flows through an underground trench filled with wood chips. Anaerobic soil microbes feed on the carbon in the wood chips and convert the nitrates in the water to N gas, which escapes harmlessly into the atmosphere.

Last spring, Ray Cerise volunteered to have a bioreactor installed on his farm in southeast Minnesota, a heavily tiled region. “I'm concerned about contaminants in drainage water,” says Cerise, who has farmed in Mower County since 1973. “We've all got to be more concerned about the environment, especially water quality.”

The Cerise bioreactor is buried on the edge of a recently tiled field, which has conventional-, controlled- and shallow-pattern drainage. The water drains into a judicial ditch, which flows into the South Branch of the Root River. Portions of the river are on the Minnesota Pollution Control Agency's list of impaired waters. The Nature Conservancy helped pay for the demonstration project.

Cerise's bioreactor treats drainage water from 27 acres. It consists of a trench 6 ft. deep, 30 in. wide and 155 ft. long, filled with 75 cu. yd. of hardwood chips. A diversion structure channels drainage water through the wood chips, and a capacity control structure regulates how fast the water flows through the biofilter. During high flows, water is allowed to bypass the bioreactor, so there is no decrease in drainage effectiveness, says Cooke, who designed the system. Denitrification typically takes about five hours, he says.

Similar bioreactors in Illinois have cut nitrate flows significantly, Cooke says. “During ordinary flow periods, more than 60% of the nitrate is removed from tile drains.”

In Iowa, Dan Jaynes is testing a slightly different bioreactor design. Six-foot-deep trenches run alongside existing drainage pipes out in the field. The trenches are filled with about 4 ft. of wood chips and covered with 2 ft. of topsoil, so the field can be farmed normally. Water is denitrified as it passes through the wood chip wall and into the tile line.

For the past eight years, in a corn-soybean rotation, the bioreactor has consistently cut nitrate concentrations in the tile line flow by about two-thirds, Jaynes says. It's not yet known how long wood chip bioreactors will last, but they could function for 20 years or more, he says.

THE BEST USE of wood chip bioreactors will likely be in environmentally sensitive areas or “hot spots,” drainage experts say. “I see this as a targeted practice,” Jaynes says. For example, intensively drained cropland or fields that regularly receive manure applications could benefit from bioreactors, he says.

Bioreactors would be a good fit for farms near pristine waters such as trout streams and other places “where there are high public expectations for the environment,” says Mark Dittrich, conservation drainage senior planner for the Minnesota Department of Agriculture.

That's Eric Schrader's situation. He raises corn, soybeans and wheat just 40 minutes south of Minneapolis-St. Paul, MN. The region is rapidly urbanizing, and the influx of city folks has sparked a rural housing boom and raised land-use questions, says Schrader, whose family has farmed in Rice County since 1857.

“People are very concerned about environmental issues here,” he says. That's one reason he volunteered for a demonstration bioreactor.

The 100-ft. bioreactor was installed in April 2007 on the edge of a 24-acre field with dark, heavy, clay soil and conventional-pattern tiling. Last fall, 10,000 gal./acre of dairy manure were knifed into the soil. The field's drainage water ends up in nearby Spring Brook, a nice little stream with 50-ft. DNR fishing easements on both banks. “I hope these small experiments can show people the benefits,” Schrader says.

BIOREACTORS COULD also be useful for cleaning up polluted waters, Dittrich says. The technology is now being used to help protect the municipal water supply in St. Peter, a city of 11,000 on the Minnesota River in south-central Minnesota.

St. Peter had to close two of its three shallow drinking water wells in the late 1980s because of nitrate contamination coming from intensively tiled cropland above the town, says Public Works Director Lewis Giesking. Last year, two wood chip bioreactors were installed at drain tile outlets located within the city's 4,500-acre wellhead protection area.

Wood chip bioreactors are still in the development and testing stage. For example, one thing that is not yet known is whether bioreactors release nitrous oxide, an active greenhouse gas that is produced as nitrates are reduced to nitrogen gas. “When you have dentrification in the soil under natural conditions, you'll get some nitrous oxide emissions to the atmosphere,” Jaynes says. There is some concern that “we may be replacing a water quality problem with a global warming problem,” he says. So “more research is needed,” Cooke says.

But already, farmers are keenly interested in these and other drainage innovations, Dittrich says. He tells about visiting with a group near the Minnesota-Iowa border about some of the new conservation drainage methods being tested. “Their response was, ‘Where have you been the last 20 years?’?”

Still, says Cerise, the Mower County grower, bioreactors are not the kind of improvement “that's going to put money on the bottom line.”

The costs for materials and installation of a 200-ft. bioreactor range from about $2,900 to $4,300, depending on whether you get the Chevy or the Cadillac model, says Dittrich. He has helped organize five wood chip bioreactor demonstrations in Minnesota — all with local sponsors. If you can get the wood chips donated by a city or county partner, he adds, the cost falls to about $1,600.

Advocates hope that eventually, the technology will qualify for federal cost-sharing funds, Jaynes says, just as other conservation measures do now.

“These types of projects will need to be cost-shared,” Cerise agrees. Because the benefits are primarily public, he says, “we should all share in the cost.”


Corn Belt researchers are working on a variety of conservation measures to improve water quality. For example, the Agricultural Drainage Management Coalition, Owatonna, MN, is testing controlled drainage on Midwest farms. Floats or stacked flashboard risers in drainage control structures raise and lower a field's water table.

Water levels can be set high during the winter and low during planting and harvesting. During the growing season, the water table can be maintained at about 2 ft. from the surface and adjusted to crop needs. The practice reduces nitrate loads in tile effluent and also conserves water during drought, says Craig Schrader, a University of Minnesota Extension drainage specialist.

Shallow tile drainage is another new practice being tested. Tile is installed at about 3 ft., instead of the conventional 4 ft., Schrader says, reducing drainage intensity and lowering the nitrates coming off the field.

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