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Corn Cobs Can Improve Performance, Reduce Costs of Water Quality Bioreactors - If Supply is Available

Corn cobs may perform better than woodchips to reduce nitrates in water quality bioreactors -- and for a lower cost -- say Iowa State University researchers. The challenge is getting an adequate supply of corn cobs.

A bioreactor is an edge-of-field water-treatment technology used to treat nitrate-nitrogen in tile drainage coming from crop fields. Basically, a bioreactor is a below-ground trough, where tile water interacts with a high-carbon material like woodchips in a low-oxygen environment. The carbon source nourishes microbes that transform the nitrate primarily to harmless nitrogen gas before the water exits the trough and flows into waterways or ditches.

Iowa State’s Michelle Soupir, a professor in agricultural and biosystems engineering, has been a leader in developing bioreactor technology, working with partners on campus and with the USDA Agricultural Research Service. They have shown that woodchip bioreactors – predicted to have a life span of about a decade before they need to be recharged with a new carbon supply – provide average nitrate reductions of 15-60%, at estimated costs of $10,000 to $15,000 (around $10.23 per treated acre).

To explore alternative carbon sources for bioreactors that might be just as effective and cheaper, Soupir began a project in 2018 to study the potential of corn cobs.

“We had lab studies that showed corn cobs are more biologically active and could increase nitrogen removal over woodchips,” she said. “We wanted to test this at a larger scale to answer a number of questions, including how long corn cobs would last compared to woodchips, their hydraulic properties and their costs.”

To look for answers, Soupir’s research team set up an experiment at Iowa State’s pilot-scale mini-bioreactor research site at the Agricultural Engineering and Agronomy Research Farm near Ames. They evaluated nitrate removal rates, efficiencies of three different woodchip-corn cob mixtures at three different hydraulic (water) residence times and costs. The bioreactor cells were filled with 75% woodchips and 25% corn cobs by volume; 25% woodchips and 75% corn cobs; and 100% woodchips. Each set of the three carbon treatments had a bioreactor operated at 2-, 8- and 16-hour hydraulic residence times.

After monitoring the systems for four years, they found:  

  • The bioreactors with corn cobs had higher nitrate removal rates than 100% woodchips, with the 75% corn cob mix providing the best performance.
  • The corn cob bioreactors had hydraulic properties similar to woodchips, with 75% corn cob systems having the best hydraulic efficiencies.
  • The relative costs per amount of nitrogen removed were 22-60% lower in the 75% corn cob bioreactors than in 100% woodchips.

“It was great to see that the systems with higher levels of corn cobs showed such positive results for these parameters,” Soupir said. “Also, we were surprised to find that the corn cobs, which tend to be larger than the woodchips, clumped less and degraded more slowly than the woodchips. This suggests they are likely to have a longer life span than we originally estimated, which would further reduce costs and maintenance requirements.”

The catch, the researchers say: It takes about 60 – 180 cubic yards of material, about two to three semi-truck loads, to fill a field-scale bioreactor.

“That’s a lot,” said research partner Ji Yeow Law, an agricultural and biosystems engineering research engineer who conducted the economic evaluation for the project. “We think of them spread across Iowa fields, but it is actually tough to find them available in the amounts needed. There is already a pretty good market for them for bioproducts and other purposes. But we are looking into possible sources, such as seed corn companies.”

An article in the Journal of Environmental Management summarized preliminary findings from the research. Support for the first three years of the project came from the Iowa Nutrient Research Center at Iowa State. Green Products of Conrad, Iowa, contributed the corn cobs, and the wood chips came from Golden Valley Hardscapes in Story City, Iowa.

In addition to Soupir and Law, the research team included Tom Isenhart, professor, natural resource ecology and management; Morgan Davis, former graduate student, now an assistant professor, School of Natural Resources, University of Missouri; and USDA ARS scientists Tom Moorman (now retired) and Gary Feyereisen.

This year, the team will study a bioreactor that uses 100% corn cobs as a carbon source, with funding from a Conservation Innovation Grant through the Natural Resources Conservation Service. While that project only provides funding to research corn cob treatments for one more year, Soupir said the team hopes to be able to continue to study the corn cobs’ effectiveness.

They are also looking at other carbon sources, including different types of woodchips, and other ways to increase bioreactors’ effectiveness, such as pumping water from fields to increase flow, and thus, the total amount of nitrate that can be removed.  

Bioreactors started gaining interest as a promising edge-of-field practice after the Iowa Nutrient Reduction Strategy’s 2012 science assessment suggested that the state needed from 76,000 to 133,000 bioreactors in combination with other practices to reach the strategy goals. More than a decade later, the state is far from those numbers, though momentum for bioreactor installation continues to increase.

“Bioreactors can be an important tool to improve water quality in our agricultural state,” Soupir said. “This research represents continuing work to make the next generation of the technology even more effective, practical and cost-effective.”

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