by Dan Lemke
Water quality and nutrient management can be vexing issues for farmers. Growers need fertilizer to achieve maximum crop yields, but they must also manage those nutrients to reduce potential run-off, which causes negative impacts on water quality.
Studies have linked nitrogen losses in the form of nitrates from tile-drained row cropland to ecological challenges in the Gulf of Mexico. Nitrates dissolve in water, so farm-applied fertilizer intended for crops can escape when there’s too much water.
“The biophysics of the system is that soil particles have a negative charge that holds cations really nicely,” says Gary Feyereisen, agricultural engineer with the USDA Agricultural Research Service in St. Paul. “For example, potassium is held in the soil for the plant to use, ammonium too, but nitrate is a negative ion, so it doesn’t stick to the soil. It goes wherever the water goes. It is very challenging to raise crops at the productivity level that we are and not have some nitrate leak out of the system when there’s too much water.”
AURI-supported research with the USDA-Agricultural Research Services, the University of Minnesota (UMN) Water Resource Center and the UMN Southwest Research and Outreach Center (SWROC) is helping shed more light on one practice designed to reduce the level of nitrates leaving farm fields and escaping into surface water while potentially offering a value-added opportunity for agricultural biomass.
Denitrifying bioreactors are typically horizontal in-ground structures put on the edge of a field or on the bank of a drainage ditch so water is treated before it exits into the ditch. Drainage water flows through the bioreactor filled with a carbon source like wood or agricultural biomass. Microbes live on the biomass and convert nitrates in the water to harmless nitrogen gas. The nitrogen gas releases into the atmosphere and the treated drainage water flows on to ditches, creeks or nearby wetlands.
Still Refining
Bioreactors have proven to reduce nitrates in drainage water, but researchers are learning more about how to speed the bioreaction process. A shorter water residence time in the bioreactor could also reduce the bioreactor footprint and increase the economic attractiveness to farmers.
Nearly a decade ago, AURI scientists worked with Feyereisen to evaluate the potential of various agricultural residues and coproducts suitable for bioreactor media. Wood chips are typically used but they can be expensive and not readily available. Researchers tested an array of ag residues including barley straw, corn stover and corn cobs to compare how effective they were at hosting the microbes.
“Some previous AURI research looked at various ag products. We found that corn cobs performed as well or better than the wood chips that are typically used,” says Alan Doering, AURI senior scientist for coproducts.
Since the initial evaluation, Feyereisen has continued to test and refine the bioreaction process through benchtop research. Working with layers of wood chips and corn cobs, Feyereisen also began adding supplemental carbon to improve the reaction time.
“We started to add an additional dissolved carbon source to speed the flow. If we can speed up the flow through a given amount of space, the bioreactor footprint will be smaller and the cost will be lower,” Feyereisen says.
Feyereisen added acetate as an additional carbon source for the resident microbes to go along with the corn cobs and wood chips already in the bioreactor. One of the goals of the research was to see if a shorter hydraulic residence time was achievable within the bioreactors while still removing the nitrate without increasing greenhouse gases.
Most bioreactors allow drainage water to spend about eight hours in the system before it flows out. Feyereisen wanted to learn if that hydraulic residence time is reduceable to two hours or less through a combination of added carbon and a vertically oriented reaction chamber.
Results showed the nitrate removal rate for the treatment with additional carbon and a two-hour hydraulic residence time was more than 2.5 times that of the treatment without the added carbon, even after spending 12 hours in the baseline bioreactor.
Feyereisen found the positive treatment results encouraging, but he was also pleased to discover what did not happen.
“Once nitrogen is in nitrate form, a certain amount will go to nitrous oxide, which is a greenhouse gas,” Feyereisen explains. “Usually, when you shorten hydraulic residence time, the reaction becomes incomplete. The conversion from nitrous oxide to nitrogen gas, which is harmless and goes to the atmosphere is the last conversion. If there’s not enough time, a higher percentage of nitrate goes off as nitrous oxide. In these bioreactors, the conversion was quite complete and there was not a greenhouse gas increase.”
Feyereisen had hoped to test even shorter residence times using the added carbon, but a biofilm developed through the process which clogged the system and reduced effectiveness. Researchers are now working to address ways to prevent the biofilm development.
In the Field
At the University of Minnesota Southwest Research and Outreach Center (SWROC) in Lamberton, researchers use a small 300-acre watershed to evaluate various water and nutrient management practices. The site includes in-field, edge of field and beyond the field tactics including cover crops, controlled drainage, constructed wetlands and bioreactors.
“Bioreactors are one component, but our goal is to see how good we can make the water coming out using these combined systems,” says Jeff Strock, a soil scientist with the SWROC, whose research focuses on water and nutrient management. “There are distinct advantages and disadvantages to each system. It’s going to take multiple drainage water management systems on a landscape or farm in order to maintain ag productivity and profitability while trying to meet environmental goals.”
Strock partners with Feyereisen to take benchtop research to the field level.
“His [Feyereisen] work lays the foundation,” Strock explains, “we take it to see how it translates from the lab to the field edge.”
Strock says bioreactor work at the SWROC spanned many years and included testing various media and bioreactor designs. Experiments even include the introduction of heat to the bioreaction chamber through solar panels to keep microbes active during colder months.
Strock’s work confirmed Feyereisen’s research that a vertical bioreactor supplemented with additional carbon can reduce the time water spends in the bioreactor while still achieving substantial nitrate removal. While largely proven, the concept still has plenty of hurdles that need to be cleared.
“One of the things we discovered was the water quality aspect works because of the hydraulic residence time,” Strock says. “Horizontal bioreactor designs take 12 to 24 hours for the water to leave. With the vertical flow, residence times are between 2 and 4 hours. We’re reducing a lot of nutrients really fast. That translates to the field as well.”
“If we can cut the time to two hours to treat the same amount of water, a bioreactor could be one-quarter the size,” Feyereisen says. “That’s one difference that this research could make. It’s not that we can just make smaller bioreactors and turn up the flow, we have to have more available carbon, which comes from the corn cobs and the added carbon.”
The Next Phase
Feyereisen says dozens of denitrifying bioreactors function in the Upper Midwest. Some are used for research while others are fully functioning.
“They’re out of the incubator, so to speak,” Feyereisen says.
Scientists know the concept works, but they are focused on refining the process to make bioreactors more efficient and economical. Adding the carbon to the corn cobs and wood chips is still experimental for nitrate removal but results so far are encouraging. Researchers also want to evaluate if bioreactors are usable for the removal of other nutrients besides nitrates from drainage water.
“As long as water is passing through some treatment device, we’d also like to remove the phosphorous which can be difficult to capture and hold,” Feyereisen explains. “Phosphorous tends to build up in bioreactors or wetlands. With a big storm or flush, it can be washed out. It’s really hard to come up with a phosphorous sink that removes phosphorous from the system.”
Feyereisen says the next phase of bioreactor research will include discovering how much flow is realistically treatable. Because drainage water often comes in big storms and the flow is so fast, some water bypasses the treatment system. Researchers are still working out how much can be treated and evaluating the cost of that treatment.
System Economics
Every management practice has an economic component. Engineering and installing a bioreactor takes money. During challenging economic times, installing management systems to improve water quality without any compensation is a difficult sell to most farmers. Understanding how much environmental benefit is actualized through a bioreactor can help set a value.
Feyereisen says monitoring equipment will be placed on a bioreactor system already operating near Blue Earth to measure nitrate levels flowing into the system and how much is coming out.
“When it comes to paying people for conservation for things like cover crops, for example, you don’t really know how much improvement you’re getting in that specific case. But when you’re measuring the concentration and load going into a device and out, you know what was removed,” Feyereisen says.
Value-Added Opportunity
Improved drainage water quality may be the primary goal of denitrifying bioreactors, but there is also a value-added component. While most current functioning bioreactors are wood chip-based, some have been modified to include corn cobs in the base media.
Bioreactors require a recharge. The carbon source hosting the microbes only lasts so long and depends upon water flow. Bioreactors must be cleaned out as they can fill with sediment. This process may only be necessary about every 10 years, but it provides the optimal time to introduce corn cobs to the process.
“I’m excited to see how well corn cobs have performed in the bioreactors,” Doering says. “Farmers produce corn cobs and the technology is there to collect cobs. When bioreactors have to be cleaned, remaining nutrients can be land applied as fertilizer. So not only can the cobs help improve water quality, they can also help replenish the soil. All while utilizing a farmer’s product.”
Next Steps
Feyereisen and Strock know the bioreaction process works for converting nitrates to nitrogen gas, but it needs to make sense economically, so research continues.
“Our goal is to take one more step,” Strock says. “We’re building on what we’ve learned.”
“AURI is funding research that helps advance the technology so that it’s more economical for farmers,” adds Becky Philipp, AURI project manager. “The research also continues to be applied and built upon stretching the value of the funding further yet.”
It will likely take some time and refinement before producers rely heavily upon bioreactors for nutrient removal. However, the potential is too good to ignore.
“Water quality issues are not going away,” Doering says. “If we can use agricultural byproducts and residues to help solve another agricultural issue, that would be a huge win.”
The published peer-reviewed journal article is available at Agricultural & Environmental Letters via the following link: http://dx.doi.org/10.1002/ael2.20024
