Bioreactors: Science, not magic
The rollout of state-based nutrient reduction strategies across the Midwest means farmers, agribusiness professionals and agency staff alike are looking for cost-effective methods to clean up tile drainage. Woodchip bioreactors have risen to the top in these strategies as a low-maintenance option to remove nitrate from tile drainage water.
These trenches filled with woodchips have gained traction over the past few years due to their efficiency in removing nitrate. Our ambitious goal to reduce nitrogen loads in the Mississippi River by 45% will require many more conversations about a variety of in-field and edge-of-field practices, including bioreactors.
One confusion about woodchip bioreactors is that the name “bioreactor” conjures visions of a mystical, mad scientist hovering over steaming beakers as Frankenstein comes to life in the background. Actually, the name “bioreactor” refers to the workhorses behind how a bioreactor works: the good bacteria that live on the woodchips and convert the nitrate in the drainage water to harmless nitrogen gas.
Because nitrate is removed from drainage water through this biological reaction using these good bacteria, it is called a bioreactor.
Placement of bioreactors
Bioreactors are applicable on most tile drainage fields. While they most commonly fit in grassy areas at the edge of the field near a tile outlet, they can also be installed “in-line” between two fields. If placed smartly, no land is generally removed from production, and a bypass line that is a part of every bioreactor design means no major reduction in drainage capacity.
Most bioreactors to date have been sized to receive drainage water from field-sized areas (30 to 80 acres, for example). Some researchers are pushing this envelope to provide more centralized treatment of drainage water from much larger areas.
In the Iowa Nutrient Reduction Strategy, woodchip bioreactors can remove an average 43% of the nitrate leaving a field in a tile drain.
The Iowa Strategy also assessed bioreactors at a cost efficiency of 92 cents per pound of nitrate that is kept from moving downstream. This assessment makes bioreactors one of the most cost-effective practices when measured in terms of dollars per pound of nitrate.
They are cost-effective
Bioreactors are so cost-effective because they are solely focused on taking nitrate out of drainage water. In other words, a bioreactor is designed to create an ideal work environment, specifically for good, nitrate-removing bacteria (think: lots of free coffee, snacks and comfy chairs for these bacteria inside the bioreactor).
Bioreactors’ specific focus on nitrate in drainage water means this practice does not typically provide other benefits like some other practices. For example, wetlands and cover crops can help keep nitrate out of drainage water, and wetlands additionally provide wildlife habitat and flood retention, while cover crops can also help improve soil health.
Woodchip bioreactors are a proven practice to reduce the amount of nitrate in drainage water, but we “mad scientists” still have many research questions to continue to improve how well bioreactors work. In the end, bioreactors are just one of a number of practices to help keep our water clean, and we’ll keep chippin’ away to move the needle in the right direction.
Christianson is an assistant professor of water quality at the University of Illinois. She graduated from Iowa State University in 2011 with a Ph.D. in Ag Engineering and Sustainable Agriculture.
This article published in the June, 2016 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2016.
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