Can agriculture residue help clean water?

–by Jonathan Eisenthal

Since the beginning of agriculture, farmers have prided themselves on leaving nothing to waste.

A research project funded by AURI, Minnesota Corn Research & Promotion Council and the U.S. Department of Agriculture’s Agricultural Research Service (USDA-ARS) could lead to farmers making use of agricultural residue in a device called a denitrifying bioreactor, which reduces runoff of nitrate from farm fields.

Bioreactor technology has been developing for a few decades, using wood chips as a medium. The Minnesota research project tested materials even closer to hand for the average farmer. Initial results show materials like corn cobs can be even more effective than wood chips in reducing nitrate run-off.

Nitrate is a vital food for plants. Manufactured nitrogen fertilizer has revolutionized modern agriculture, helping farmers to grow five times more corn, wheat, potatoes and other produce on the same amount of land.

The problem with nitrate comes when it escapes from farm fields. Scientists believe that high nitrogen levels in the surface waters of the Mississippi River Basin are a major contributing factor in the occurrence of a seasonal dead zone in the Gulf of Mexico, which harms marine life as well as the fishing and tourism industries that depend on it.

Since 2011, USDA-ARS agricultural engineer Gary Feyereisen has built a number of prototype bioreactors to test the denitrifying potential and flow characteristics of barley straw, corn stover and corn cobs, alongside the more conventional choice of wood chips.

“We found that these materials, corn cobs specifically, performed better than wood chips,” said Feyereisen.“Ag residues offer carbon in a more labile form than wood chips, a form that’s more easily broken down and provided to the denitrifying microbes. With wood chips, the carbon is tied up in lignins and longer molecular chains, which are harder to break down.

“Unfortunately, the ag residues are going to be used up quicker. We saw that particularly with barley straw and corn stover. Even after five months they were starting to be used up, and the rates of denitrification were dropping. The results suggest that if we could design a modular bioreactor—one that could easily be emptied of the exhausted medium and then refilled—we could achieve the kind of nitrate reduction we are hoping for.”

Another key finding of the research is that these agricultural residues perform better than wood chips under cold temperature conditions.

“We wanted to test [ag residues] at colder temperatures,” explains Feyereisen. “There hasn’t been much work done on that. We know that in the Upper Midwest, especially in Minnesota, much of the farm field drainage occurs during April and May when the ground temperatures are cold and the water is cold. In places south of here, they are not quite as cold, and they do quite well with wood chip-filled bioreactors. We wanted to see if there is any advantage to using these agriculture residues under the cold temperature regime.”

Feyereisen and his graduate student assistants set up bioreactor columns in a room that could be chilled to 35 degrees Fahrenheit.

The reason temperature is a critical factor for bioreactor performance is that the chemical mechanism for denitrification is, as the name suggests, a biological one. Microbes establish colonies on the media inside the bioreactor. Water containing nitrate drains from the farm field and flows through the bioreactor where the microbes use the nitrate for respiration and convert it to gaseous nitrogen. This nitrogen gas is released harmlessly into the atmosphere, which is 78 percent nitrogen.

“The good news is that agricultural residue works better than wood chips under those cold weather conditions,” says Alan Doering, senior associate scientist at AURI, who played a supporting role in Feyereisen’s project—sourcing and preparing the ag residues so that they would match what a farmer would use.

“The other critical performance parameter for bioreactors is the hydraulic properties of the materials,” says Feyereisen. “Wood chips work well in terms of flow, so we used that as a standard for comparison to these other materials. We don’t want to have things that are too tight, restricting the flow of water. On the other hand, we don’t want spaces that are too big. That wouldn’t provide the intimate contact between the biofilm and the water passing through the bioreactor bed. So we built a ‘permeameter’ specifically to test flow.”

“We thought corn stover and corn cobs might stop up the bioreactor, but we were pleasantly surprised to see that both of these media have good flow characteristics,” says Doering, who noted that the cobs remained whole, while they put the stover through a chopper that any farmer might use, to create particle lengths of several inches for the stover and straw media.”

Feyereisen’s project now moves into a field testing phase. “We are hoping that by using a new modular design for bioreactors, they will be more user-friendly for the farmer,” explains Becky Philipp, who serves as project manager for AURI. “Once the material has been depleted, it can be taken out and placed back on the land. We won’t know how well that works until we conduct the field trials, but we’re hopeful that the spent media will provide organic matter and additional fertility to the soil.” Experts evaluating the adverse effect of nitrate runoff have set a goal of 45 percent reduction in nitrate loss from farm fields, Feyereisen notes.

Philipp noted that projects like this are an ideal form of public-private partnership. AURI served as the fiscal agent, funding partner and project partner, providing technical and laboratory assistance as well as project management resources. The Minnesota Corn Research & Promotion Council provided funding and USDA-ARS provided research expertise and in-kind funding. The University of Minnesota provided space and materials at its
St. Paul campus.

“As the project developed, conference calls brought this diverse team together,” Philipps says. “The result can be a benefit for farm producers and the general public.”

“Nitrate fits the farmer’s definition of a weed. As they say, it’s like a plant out of place. When it’s in the right place it plays an essential role, but outside of that context, nitrate becomes a problem,” concludes Feyereisen. “We’re hoping bioreactors can be part of the solution.”