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Denitrifying bioreactors: Moving beyond ‘proof of concept’ to optimization, implementation

Nitrogen: one of the many elements with which humans seem to have a love–hate relationship. We need it in our agricultural soils—it’s literally what makes our crops grow—but when it percolates into our waterways, nitrogen becomes hazardous to wildlife and public health.


The risk of too much nitrogen in the form of nitrates entering our water system is not a new one. Scientists and farmers have long known about the risks associated with agricultural nitrogen runoff: high nitrate levels in water contribute to large algae blooms that negatively impact aquatic wildlife; too much nitrate can also pose health risks to humans.Installation of a denitrifying bioreactor


The problem has gotten so serious in some areas that it’s pitting urban communities against their agricultural neighbors. The Des Moines Water Works—the public utility responsible for providing safe drinking water to hundreds of thousands of Iowans—has filed a lawsuit against three counties in northwest Iowa alleging that nitrates from agricultural runoff in their jurisdictions threatens downstream Des Moines’ water supply. It’s set to go to trial in August of 2016, but no matter how the case turns out, it’s already brought the issue of nitrogen leaching to the forefront of political conversations in one of the nation’s largest farming states.


What many are not talking about is that there is already a proven concept for effectively removing nitrates from agricultural runoff before it reaches our watersheds.


Enter the bioreactor.

A best practice for reducing nitrate runoff

Don’t be fooled by the high-tech name; bioreactors are rather humble, low-tech instruments. Essentially, they are buried trenches filled with woodchips. That’s it. The secret to their denitrifying power lies in bacteria that sustain themselves on the woodchips and respire nitrogen. In so doing, these bacteria are able to convert nitrates into nitrogen gas, effectively removing the nitrates from agricultural runoff when the bioreactors are connected to agricultural drainage systems.


The idea behind denitrifying bioreactors isn’t new, but they haven’t reached agricultural fields until relatively recently. They’re already proving themselves as a best practice for reducing nitrate runoff, along with planting grass cover crops like cereal rye and oats.


“In 2016, we know that denitrifying bioreactors work. There is no question,” says Laura Christianson, University of Illinois assistant professor of water quality and guest editor of a special collection of articles (see http://bit.ly/1TLFykv) on bioreactors in the May–June 2016 issue of the Journal of Environmental Quality. Progress from when scientists first floated the idea to finally proving of concept was slow at first, with researchers proposing bioreactors first in the mid-1990s.


“Research was kind of slow and trickling in, in the early 2000s,” Christianson says. Then around 2008, bioreactor research took off and spread from universities to agricultural groups, environmental nonprofits, and government agencies like NRCS.

Optimizing spring performance

Today bioreactors are an established practice, though scientists are still figuring out how to optimize them, especially during the early spring. It’s a critical period for nitrogen loss as frost and snow melt, washing away nutrients from soils before farmers have a chance to sow their fields and fix those important nutrients in their crops. Instead, free nitrates can be washed out of fields through drainage systems in melt water and early-season rains. This is when denitrifying bioreactors could be most useful.


But Christianson says that as they currently work, bioreactors often become overloaded during the spring thaw. There is simply too much water flowing through them, and it’s too cold for the microbes that convert the nitrates to work very efficiently. While bioreactors still remove some nitrate from the drainage water under these conditions, they could work much better. If researchers can solve the problem, Christianson says bioreactors could potentially solve most of our nitrate issues.


“If you could take all the nitrate out of that early spring drainage, you would be addressing most of the problem for the whole year,” Christianson says. “So the number one thing for improving nitrate removal at this point is to figure out a way to treat more water and treat it better (when) the water is cool.”


One potential solution is to increase the amount of time the water sits in a bioreactor. The simplest way to achieve that is to build larger bioreactors, but Christianson says that’s not going to be an attractive—or affordable—solution for most farmers.


“It gets expensive,” she says. Farmers would have to build much larger trenches for their bioreactors and would have to fill them with more woodchips, all to capture the extra nitrates flowing out of the system during a relatively brief time in early spring. “You’re wasting money most of the year because you don’t need that capacity.”


Researchers are still working on the issue, testing bioreactor designs in an effort to determine the optimal size for all conditions.


Meanwhile, Midwestern farmers are beginning to pick up the practice. Christianson estimates that there are currently between 50 and 100 bioreactors on working farms in Illinois, Iowa, and Minnesota.


“They are definitely the early adopters,” she says, noting that agricultural practices with no yield benefits tend to be adopted at a slower rate.

Easy to manage, no annual investment


Still, the benefits are proven, and denitrifying bioreactors are easy to manage and do not require an annual investment like cover crops. After an upfront investment of around $10,000, denitrifying bioreactors are designed to last 8 to15 years and require little maintenance.


Christianson hopes more farmers consider installing bioreactors soon.


“One of the biggest hurdles with any sort of water quality effort is that we’re asking producers to voluntarily provide the benefit of clean water to the public using practices that more often than not don’t provide a private benefit such as a yield bump. Right now it’s voluntary. The voluntary part means that we get to choose how we’re going to meet our water quality goal, not that we get to choose if we’re going to meet our water quality goal.”


The USEPA expects the agricultural economy to meet its water quality goals one way or another, and Christianson says a voluntary approach is likely to be more preferable than new regulations. And if the Des Moines Water Works case is any indication of the nitrate spats to come, farmers would be well served by getting ahead of the issue on their own terms.


View this special section of papers in the Journal of Environmental Quality