Leopold Center for Sustainable Agriculture

Bioreactors show promise for improving water quality

Back to Leopold Letter Summer 2011

By MELISSA LAMBERTON, Communications research assistant

Matthew Helmers and Laura Christianson face a daunting question: How can Midwest farmers protect waterways from nitrate, a pollutant that is a local and national problem? The answer: harness microbes to do the hard work. The initial results of an ongoing research project suggest that denitrifying bioreactors can remove nitrate from tile-drained fields while remaining compatible with farming practices.

Christianson, a Ph.D. candidate in Agricultural and Biosystems Engineering at Iowa State University, discussed the benefits and challenges of bioreactors in a webinar hosted by Iowa Learning Farms on May 18. The project, currently led by Matthew Helmers, associate professor in ISU’s Department of Agriculture and Biosystems Engineering, has been funded by the Leopold Center since 2009. Former ISU associate professor Alok Bhandari, now at Kansas State University, initiated the research.

Subsurface drainage makes Iowa’s productive fields possible, but also impacts Iowa’s water quality, in particular by carrying nitrate into streams and rivers. The Environmental Protection Agency (EPA) lists the Cedar River watershed in northern Iowa as impaired because of high nitrate levels, which raise concerns about exceeding the drinking water standard.

On a national scale, the Midwest contributes heavily to the growing hypoxic zone in the Gulf of Mexico. Nitrogen from fertilizers moves into the Mississippi River system and eventually arrives in the Gulf, where it causes explosive algae growth. This depletes oxygen and creates a “dead zone” that significantly impacts the region’s ecology and fishing-based economy. The EPA has called for a 45 percent reduction in the nitrogen delivered annually to the Gulf by 2015.

Helmers and Christianson are examining how denitrifying bioreactors might be used to reach that goal. Bioreactors capture nitrogen-laden water from underground tile drainage and deliver much cleaner water to Iowa’s streams and rivers. Bioreactors are deep trenches filled with a carbon source, such as wood chips, and installed along the edges of fields. Control structures direct water from tile drainage through the bioreactor, and a bypass line ensures that water won’t back up during high flow events.

Christianson said the idea is to create “a vacation resort” for beneficial soil microbes. If provided with carbon food and denied oxygen, the microbes will breathe by transforming nitrate into dinitrogen gas (N2), a harmless atmospheric gas.

“Denitrifying bacteria are doing this natural process, which we can harness for water quality purposes,” Christianson said. “They are the workhorses of this whole system.”

The investigators constructed three small-scale pilot bioreactors for controlled experiments at ISU’s Agricultural Engineering/Agronomy Research Farm near Boone. They also partnered with the Iowa Soybean Association to analyze data from two field-scale bioreactors, and worked with Coldwater Palmer Watershed Group to design, install and monitor a third device. Initial results from the pilot bioreactors suggest that beneficial bacteria can reduce nitrates by 30 to 70 percent in water that spends four to eight hours flowing through the wood chips. Longer retention times result in higher levels of nitrate removal.

One challenge to this new technology stems from its success. When nitrate-breathing bacteria remove too much nitrate from the system, they are quickly outcompeted by sulfate-breathing bacteria. These unwanted bacteria utilize the wood chips for food and produce small levels of hydrogen sulfide. According to University of Illinois researchers, their activity corresponds with increases in the methylated form of mercury, a toxin that accumulates in fish and mammal tissue.

Christianson expects that these negative side effects can be minimized by managing the bioreactor with quicker flow rates, so that not all the nitrate is removed. Scientists can determine how to adapt the flow by periodically collecting grab samples to analyze. In the field, Christianson knows right away if too much nitrate has been removed, because the outlet control structure smells like hydrogen sulfide.

The investigators stress that the technology merits further research. They plan to continue analyzing data from field-scale bioreactors, which organizations like the Iowa Soybean Association are beginning to install around Iowa as demonstration sites. They also will conduct an acceptability survey among farmers.

“This is an exciting chance for me as an engineer to get out of my comfort zone and explore the social science side of water quality,” Christianson said.

A major benefit of bioreactors is their compatibility with farming practices. They can be installed at the edges of fields and fitted into existing grass buffers, so no land needs to be taken out of production. Christianson estimates that a bioreactor can last 10 to 20 years with little maintenance and no energy inputs beyond the initial installation. They work best at a small scale, draining 40 to 80 acres. The cost of installing a bioreactor for that acreage averages $7,000 to $8,000. In Iowa, the EQIP program offers a one-time payment that covers roughly half the cost.

“Bioreactors provide a very focused treatment for nitrate, and that can help us improve water quality locally and also nationally,” Christianson said. “It provides one more tool in the toolbox.”

Back to Leopold Letter Summer 2011