Clean nitrates from tile water
Denitrifying bioreactors — underground trenches filled with wood chips in farm fields — are still few and far between.
“We’re trying to get more bioreactors installed in Iowa, so more farmers can become familiar with them,” says Keegan Kult of the Iowa Soybean Association. The ISA manages and carries out projects for Agriculture’s Clean Water Alliance, a group of 13 ag retailers organized to work with conservation partners to reduce nutrient loss from farm fields in the Raccoon River and Des Moines River watersheds in Iowa.
ACWA and the Sand County Foundation helped fund and install the first bioreactor in Iowa in 2008; Kult says he’s aware of about 10 in operation in the state and a few across Minnesota, Illinois and Pennsylvania.
• Wood chip bioreactors can remove up to 60% of nitrate load from drainage water.
• One Iowa farmer has two bioreactors on his flat land.
• Bioreactors are easier to install and take less space than wetlands.
One Iowa farmer and longtime conservationist, Arlo Van Diest, has two bioreactors on his flat land near Webster City. He’s added the bioreactors to strip-till and cover crops to improve water and soil quality. “I like these bioreactors,” Van Diest says. “I like seeing what they can do to clean the water. We can monitor and measure the nutrients in the water above and below the bioreactor, so we know how much nitrate we are removing.”
Iowa State University water resources engineer Matt Helmers says wood chip bioreactors can remove from 15% to 60% of the annual load of nitrate from drainage water in tile lines. Helmers says there’s still much to be learned about bioreactors and how to maximize their performance.
Van Diest, likely the only farmer in the country with two bioreactors on his land, was willing to put his own money into installing them, but ISA found other funding sources, including a grant. “I got payments from USDA to convert to strip till, and I’d like to use some of that money to demonstrate conservation and water-quality practices to other farmers to encourage them to use these newer management practices,” Van Diest says.
The bioreactors aren’t inexpensive. Depending on size and topography, costs for one range from $6,000 to $12,000. In some counties, USDA now offers 50% cost-share up to $4,000. Eligible landowners in the Mississippi River Basin Initiative can receive as much as 75% cost-share.
Kult says many farmers use wetlands to remove nitrates from tile and surface waters, but adds that the wetlands take up more land, making wetlands installations a difficult decision for the producer. “Wetlands don’t always fit into the landscape, either. The bioreactor is a nitrate removal option that can be installed at the edge of the field, without losing cropland. It has a much smaller footprint on the ground than a wetland,” he adds.
How bioreactors work
Sizes vary, but most bioreactors are about 100 feet long and up to 30 feet wide. Since they’re underground, bioreactors do their job of reducing nitrates in tile drainage waters out of sight. Most bioreactors now in use clean water from 6- to 8-inch tile lines that drain from 40 to 80 acres.
A water control structure installed at the upper end of the trench intercepts and diverts tile water into the bioreactor, Kult says. “Stop logs in the structure control flow of tile water into the bioreactor and can allow tile water to bypass the bioreactor in times of high flow,” he adds. “That bypass feature keeps water from backing up into the field.”
Stop logs in a second structure at the lower end of the bioreactor control how fast water moves. Microbes that occur naturally in the soil colonize saturated wood chips. “The carbon in the wood chips is their food source, and in underground, anaerobic conditions, they break down nitrates in the water and convert them to harmless nitrite gas,” Kult explains.
The life of a wood chip bioreactor is estimated at 10 to 15 years, Kult notes, but adds that early research installations from 12 to 14 years ago are still working. One key to their lifespan is keeping them saturated, he says. If the wood chips dry out, the lifespan of a bioreactor is shortened.
Betts writes from Johnston.
This article published in the January, 2012 edition of WALLACES FARMER.