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Corn in the USA

A potential alternative carbon source for bioreactors.

May 21, 2024  By Bree Rody


LEFT: A research team from Iowa State University examines a site of mini-bioreactors near Ames, where different woodchip-corncob mixes are being studied. All images courtesy of Michelle Soupir/Iowa State University

Bioreactors are seen as an increasingly important tool in agricultural water management. A new Iowa-based research project hopes to provide landowners with more options, flexibility and resources to implement those bioreactors.

As subsurface drainage – in particular, nutrient transport from subsurface drainage – is subject to increasing scrutiny for its role in downstream water quality issues, farmers have been in a tough spot. For all its potential links to adverse water quality, subsurface drainage also has an undeniably positive relationship with crop yield and quality. Fortunately, scientific research has been on the cutting edge to shed light on, and further improve, edge-of-field practices that ensure that waters leaving the fields are not taking with them excess nutrients such as nitrogen and phosphorus.

Edge-of-field practices come in a number of different forms (for more on various edge-of-field practices and how close they are to widespread adoption, see Jack Kazmierski’s feature on page 20). One of those practices is denitrifying bioreactors. In the simplest of terms, bioreactors are underground water troughs where tile water interacts with a carbon source in a low-oxygen environment. The carbon source nourishes microbes, transforming nitrates in the water primarily into harmless nitrogen gas, which then flows out of the reactor and into waterways.

That high-carbon material used in bioreactors is, most commonly, woodchips. But the Iowa project looks at a potential alternative to woodchips. And, in the Midwest, what better material than corn?

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Well, corn cobs, to be exact.

Michelle Soupir, professor of agricultural and biosystems engineering at Iowa State University, explained that the primary purpose of this project was to explore corn cobs as an alternative to woodchips, for a few different reasons.

According to Soupir, lab studies showed that corn cobs are more biologically active than woodchips, which could increase nitrogen removal. But corn cobs could also be a viable alternative because of the cost and demand for bioreactor woodchips.

A highly specialized carbon source
Soupir explained to Drainage Contractor that the woodchips found in bioreactors must fit very particular specifications. These aren’t just any woodchips – they’re highly specialized.

“The woodchips need to be at least one inch, and most chippers do not produce a chip size that meets that requirement,” she explains. “There’s [only] two facilities that produce woodchips that are the appropriate size to meet NRCS specifications.”

These chippers will chip up the logs, and the chips then enter into a separate screening process with a screen that is between three-quarters and one inch, in order to screen out the too-small pieces. This is not the most efficient process, says Soupir. “Because they have to screen out all of the smaller pieces, by our estimate, [they lose] about 40 percent – almost half of the wood product.”

Then, she says, there’s the logistics piece: adding on the cost of transportation, and the timing of getting woodchips where they are needed, when they are needed. “It does get complicated, because there are so few places where the woodchips can be sourced from in the state.”

In short, even though these woodchips literally grow on trees, they don’t exactly “grow on trees” when it comes to immediate and easy availability.

“You would think… if there’s a clearing, we might say, ‘Oh, here’s all this wood, let’s just put it into some bioreactors!’ But the equipment required to get the chips to the size that we need, is not easily available. So that’s a big barrier, to get us from where the wood is available to where it’s needed.”

As such, the Iowa project began in 2018.

The study
With the base knowledge that corn cobs are valuable because of their biological activity, as well as their similar size properties to the woodchips, Soupir’s research team set up an experiment at Iowa State’s pilot-scale, mini-bioreactor research site near Ames. The bioreactor cells were fitted with: 75 percent woodchips and 25 percent corn cobs by volume; 25 percent woodchips and 75 percent corn cobs; and 100 percent woodchips. Each set of the three carbon treatments had a bioreactor operated at two-, eight- and 16-hour hydraulic residence times.

Monitoring the systems, which took place over four years, revealed that the bioreactors with corn cobs present had higher nitrate removal rates than the bioreactors whose carbon sources were made of entirely of woodchips. The 75 percent corn cob/25 percent woodchip mix performed best. That mix also showed the best hydraulic efficiencies, and clogging has not been a concern so far.

On the topic of efficiencies, the 75 percent corn cob mix also showed relative costs per amount of nitrogen was lower than the 100 percent woodchip mix, with costs ranging from 22 to 60 percent lower. Soupir says it’s better to look at costs in terms of a “per removal basis.” “It could cost the same to build two bioreactors, but you may get a better investment for your money if you remove more nitrates.”

For lifespan, woodchip bioreactors are predicted to have a lifespan of about 10 years before they require a new carbon supply. Initially, the team thought that because corn cobs are a different kind of carbon source, they would break down more quickly than the woodchips. Now in their sixth year, Soupir says they did not see the drop-off in the ability to remove nitrates.

Bigger picture: Bioreactor adoption
The importance of bioreactors in the broader picture of water quality can’t be overstated, with their ability to reduce nitrates at an average of 15 to 60 percent. However, bioreactors aren’t free. They have an estimated cost of $10,000 to $15,000 – around $10.23 per treated acre.

The goal of the corn cob study, says Soupir, is to provide landowners with more options to reduce nitrates in their water, ultimately resulting in the adoption of more bioreactors. 

But it’s not as though corn cobs are regarded as a waste product; Soupir says that getting the corn cobs gathered and available for bioreactors has been “harder than expected” and that feed corn facilities have been the more common source of corn cobs. “There’s not a ton of corn cobs just sitting around; we had to work rather hard to source the corn cobs, almost as hard as gathering the woodchips.”

But Soupir is hopeful that because of the biological activity of corn cobs and their many benefits in the broader agricultural sphere, they can not only become more readily available but also help provide producers with another revenue source.

As for further adoption of bioreactors, Soupir acknowledges that efforts from various agencies such as the Iowa Department of Agriculture and Land Stewardship (IDALS) and the Department of Natural Resources have helped increase not only adoption, but also awareness.

“There have been a lot of efforts to get more practices on the ground, and to streamline some of the processes so that they can be a little more efficient for the contractors, for the engineers and for the farmers.” She credit’s the state’s batch-and-build approach to getting more bioreactors built and in the ground.

“It’s allowed contractors to do the ‘batch’ part, where all the things that are needed for a bioreactor can be delivered; [contractors have built] multiple bioreactors within a relatively close area, and so that makes installation go much more quickly, which makes a more profitable operation.”

In 2012, an assessment as part of Iowa’s Nutrient Reduction Strategy suggested that the state needed anywhere from 76,000 to 133,000 bioreactors, in combination with other practices, to meet the nutrient reduction goals. The state is still far from those numbers, but more research and incentives can play a part in adoption.

As for the Iowa research team, this year, the team will study a bioreactor comprised of 100 percent corn cobs as a carbon source, with funding from a Conservation Innovation Grant through the NRCS. They are also looking at other carbon sources, including different types of woodchips, and other ways to increase bioreactors’ effectiveness. DC


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