Woodchips to the rescue!
How denitrifying bioreactors can help purify our water
By: Austin Yantes
Edited by: Katherine Hill and Sienna Schaeffer
Let’s talk about nitrogen. This tiny element, number 7 on the periodic table, is perhaps the most important element on Earth. 78% of the air we breathe is nitrogen. The fertilizer that allows us to grow enough food to feed the world is made of nitrogen. To make it even more personal, nitrogen is woven into the very fabric of our DNA. And not only human DNA — every single living cell on the planet contains nitrogen. Without nitrogen, the world as we know it would not exist.
Although nitrogen is a critical component of life, it is a classic case of “too much of a good thing.” In balance, nitrogen gives rise to the diversity of life around us, but in excess, it can have disastrous effects. Our Earth has maintained this essential equilibrium for over 2 billion years, cycling nitrogen seamlessly from land to water to air. It was a seemingly ceaseless, unbreakable cycle, that is, until recently.
Humans are great at a lot of things, including producing and using massive amounts of synthetic nitrogen. In the early 1900s two bright scientists figured out a way to pull usable nitrogen out of thin air — literally. This process, called Haber-Bosch, allows us to take nitrogen gas (which plants cannot use) and ultimately turn it into ammonia (which plants, including crops, are totally into). Since then, our application of ammonia-based fertilizer has increased exponentially and is expected to surpass 200 million tons in 2018 (FAO 2015). In fact, humans produce, or fix, more nitrogen today than all other natural sources combined (Harrison 2003). Unfortunately, this sudden shift in the balance has caused the planet’s nitrogen cycle to spiral out of control.
Nitrogen-based fertilizer is not the problem in and of itself. The larger issue is that an extraordinary quantity of nitrogen is being lost to the environment. A shocking 60% of the nitrogen applied to crops as fertilizer is not even used by the plants, it’s just… lost.
Where does it go, you may ask? Nitrogen leaving agricultural fields is typically in a form called nitrate, which moves very easily with water. A large proportion of the farming operations in the Midwest drain water from their fields with long lines of plastic tubing called drainage tiles. These tubes take excess water with high concentrations of nitrate and dump it into the nearest waterway, such as a stream, which may eventually supply our drinking water. Nitrate also enters our water supplies via runoff from farm fields during snowmelt or rainfall. High nitrate levels in drinking water can cause adverse health effects such as blue baby syndrome, a condition which decreases the oxygen in infants’ blood, leading to coma or death (Knobeloch et al. 2000).
Excess nitrogen in the environment can lead to all sorts of other issues; it causes huge blooms of algae, destroys habitats, and reduces biodiversity. These repercussions are not limited to agricultural areas. Nitrogen applied as fertilizer in the Midwest can make its way down the Mississippi River, leading to massive fish kills in the Gulf of Mexico. The Gulf’s famous “Dead Zone” is estimated to cost tourism and fishing industries over $82 million each year (NOAA). Fortunately, many innovators are looking for ways to solve our perplexing nitrogen problem.
One such solution is denitrifying bioreactors. The name of this practice may sound a bit foreign — so let’s break it down. The word “denitrifying” refers to the process of denitrification. In this process, bacteria take nitrate and convert it to nitrogen gas, thereby moving nitrogen from land (where it’s a contaminant) to the air (where it’s harmless). This is a natural process that can happen anywhere with the proper conditions, but is particularly useful to us when applied to agricultural systems. By installing some sort of structure that encourages denitrification at the end of tile lines, we can turn the nitrate in agricultural drainage water back into nitrogen gas. The word “bioreactor”, then, refers to the structure itself.
To put it all together, “denitrifying bioreactor” is another way of saying “structure with the right conditions for denitrification”. These conditions include the absence of oxygen and a good source of carbon (e.g. straw, sawdust, or most commonly, woodchips). Defined most simply, a denitrifying bioreactor is a large pit filled with woodchips. Some added plumbing connects the main tile line to a perforated pipe that distributes the drainage water throughout the bioreactor, allowing the bacteria to do what they do best, cleaning up our water in the process.
Bioreactors are being constructed and studied across the Midwest. Data from Iowa, Minnesota, and Illinois shows that bioreactors can reduce the load of nitrogen passing through them by anywhere from 11 to 65% (Christianson et al. 2013). Another research group in Quebec, Canada found that the water coming out of bioreactors contained up to 99% less nitrates than the water going in (Husk et al. 2017). This study also found that bioreactors maintained their nitrate-removal capacity through the long Canadian winters, even when the soil froze. In the future, additional research may show that woodchip bioreactors can also remove other pollutants such as phosphorus, fecal bacteria, and pesticides.
The installation of bioreactors, or any nitrogen reduction strategy for that matter, requires the willingness of farmers and other landowners. As you and I both know, convincing people to reform their ways is usually easier said than done. Encouragingly, the Quebec study revealed that farmers were able to cultivate crops on top of two of the four bioreactors with no notable difference in crop yield, which may just be convincing enough to elicit change.
While our nitrogen problem is vast and complicated, it is not insurmountable. With the right technologies and proper cooperation from landowners, there is hope that the natural nitrogen cycle can one day be restored. And who knows? It just might be woodchips that save the day.
References
- Food and Agriculture Organization of the United Nations. (2015, February 16). Fertilizer Use to Surpass 200 Million Tonnes in 2018. http://www.fao.org/news/story/en/item/277488/icode/http://www.fao.org/news/story/en/item/277488/icode/
- John Arthur Harrison, Ph.D. “The Nitrogen Cycle” Visionlearning Vol. EAS-2 (4), 2003.
- Knobeloch L., Salna B., Hogan A., Postle J., Anderson H. (2000) Blue babies and nitrate-contaminated well water. Environ Health Perspect 108:675–678
- Christianson, L., Tyndall, J., & Helmers, M. (2013). Financial comparison of seven nitrate reduction strategies for Midwestern agricultural drainage. Water Resources and Economics, 2, 30–56.
- Husk, B. R., Anderson, B. C., Whalen, J. K., & Sanchez, J. S. (2017). Reducing nitrogen contamination from agricultural subsurface drainage with denitrification bioreactors and controlled drainage. Biosystems Engineering, 153, 52–62.
