Food security

What are denitrifying woodchip bioreactors, and how do they help the environment?

Woodchip bioreactors are sophisticated, engineered water treatment systems. Yet, they are simple woodchip-filled trenches, too. If that sounds complicated, it’s really not. These bioreactors clean water with no negative crop production impact. My research at the University of Illinois-Champaign-Urbana is to study these structure.

pit in farm field with woodchips being installed
A bioreactor installation in Illinois. Blogger Hannah Dougherty stands next to the middle white pipe. Credit J. Chandrasoma/UIUC

Simply put, a denitrifying bioreactor is a relatively low-tech in-ground unit. Denitrifying bioreactors are excavated trenches filled with a solid carbon media, usually woodchips, as shown in the photo. Their most common application is for agricultural tile drainage water.1 They are also being researched for removing nitrate pollution from other types of waters and wastewaters.

Soil naturally contains bacteria and other microbes. In fact, soil is the earth’s best water filter (for more on that, read this Soils Matter blog). Bioreactors enhance the natural process of denitrification – the conversion of nitrate in water to harmless nitrogen gas – performed by bacteria already present in the surrounding soil. The woodchips provide an additional fuel source for the bacteria, in the form of carbon. Think of the woodchips as an extra cup of espresso in the morning! So, while the actual form of the bioreactor is simple, the chemistry and biology that happens inside them is complex.

Infographic of how a bioreactor works
Illustration of the location and size of bioreactors on farm field provided by L. Christianson/UIUC

Interest in this relatively simple edge-of-field practice has grown over the past decade. Both the seasonal hypoxic zone in the Gulf of Mexico and more local concerns about nitrate in drinking water are helped by bioreactors. Illinois and Iowa are generally the largest contributors of the nitrate polluting the Mississippi River. And the Mississippi drains into the Gulf of Mexico. Both states have a large amount of tile-drained acres, roughly 10 million tiled acres in each state.

In 2015, Illinois began the Illinois Nutrient Loss Reduction Strategy. This guides conservation practice implementation in the state. One goal is to install bioreactors on half of all the tiled acres in Illinois. Given the approximately 30 bioreactors in the state currently, there is a lot of opportunity to expand this practice. Based on a long term average, Midwestern bioreactors remove 25-45% of the annual drainage water nitrogen load.

Some of the most common questions about bioreactors are very practical. Building bioreactors on the edge of farmland does not take land out of crop production. Also, because they are on edges of fields, no field operations are affected. Special valves and other structures allow water to be routed correctly, and prevent water from backing up during storms. Bioreactors are designed to last at least 10 years and are typically designed for 30 to 80 acre tile-drained fields.

Other common questions have to do with the woodchips used as bioreactor fill. The tree species of the woodchip generally does not matter, with the exception of oak, cedar or high-tannin woods. The leaching of tannic acids from these types of woods is an area of current research. While the type of wood generally does not matter, the most important considerations are that the woodchips be roughly ½” to 2” in size and they be relatively free of fines or debris, so that water may pass through easily. Of course, woodchips are not the only source of organic carbon around, and other media have been used as bioreactor fill. Other researched carbon sources include corn cobs, corn stalks, cardboard, wheat straw, newspaper, and chipped municipal storm debris. Woodchips are the recommended fill material because they last longer and are generally more uniform than these other carbon sources. And, yes, the woodchips do lose their “power” over a decade, and need to be replaced.

Denitrifying bioreactors are a low-tech, yet highly sophisticated environmental solution. They have great potential for expansion. While they are not a silver bullet, they do offer important benefits in that they don’t negatively impact in-field crop production. Stay posted as researchers keep chippin’ away!

Answered by Hannah Dougherty, University of Illinois, Champaign-Urbana

  1. The Midwest typically has very wet springs. Planting crops was often delayed due to flooded farm fields. So, Midwestern growers began installing a set of pipes deep under their fields, which are called tile drains. The drains help fields dry out faster, so farmers can plant their crops for a longer growing season, and better yields.

Learn more about bioreactors here

To read more about bioreactors in action, visit this page about Laura Christianson’s research in the Midwest, or watch Laura’s video by clicking directly on the video below:

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

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