Agricultural runoff is a problem around the world, one that makes protecting and improving water quality a challenge.
In the United States Midwest, scientists have improved a type of bioreactor that may protect the Mississippi River watershed from nitrogen-heavy runoff.
What is a bioreactor? Put simply, it’s a container in which organisms undergo natural biochemical processes. They can range from the simple — for instance a yogurt maker — all the way to complex industrial facilities, like anaerobic digesters.
In the Midwest, farmers commonly keep their fields from becoming waterlogged by using a system of drainage pipes between 3 and 4 feet below the soil’s surface. These are known as tile-drained fields.
Runoff in these conduits tends to be filled with nitrogen leached from the soil by rain and irrigation. These systems drain into waterways that connect to the Mississippi River. When runoff containing extra nitrogen pours into streams and rivers, this creates problems throughout the watershed.
A simple bioreactor used as a buffer could provide a solution to this complex problem. It’s composed of a trench, typically 100 feet long, 20 feet wide, and 3 to 4 feet deep, filled with woodchips, corncobs, or other carbon-rich food for bacteria, and covered with a foot of topsoil.
A field’s tile pipes are rerouted through the bioreactor before the water reaches a stream. The bacteria naturally neutralize the nitrogen in the water, converting it into a gas through a process known as denitrification.
Laura Christianson, a University of Illinois researcher, said in a press release:
Good bacteria colonize the woodchips, and use them as food. […] We’re enhancing a natural process. There’s an elegance to it.
Her team has installed one of these bioreactors on a soybean farm in Illinois to further study the process.
This simple solution — if implemented — could have a huge, positive effect on local waterways, the Mississippi River, and ultimately, the Gulf of Mexico. The state of Illinois alone has roughly 10 million acres of tile-drained fields feeding into the watershed.
When fresh water is too rich with nutrients — called eutrophication — it becomes a breeding ground for algae. The nutrients include naturally occurring nitrates and phosphates.
Harmful Algal Blooms
Nutrient-rich agricultural runoff is a major source of nitrogen in surface water. An increasing number of municipal water utilities throughout North America are experiencing problems with harmful algal blooms. The Great Lakes region has been the site of numerous recent outbreaks, and they are a frequent occurrence at the mouth of the Mississippi River, creating oxygen-depleted “dead zones” in the Gulf of Mexico that can span more than 5,000 square miles.
Research into the use of denitrifying bioreactors has accelerated within the past 10 years and beyond use in agriculture, according to a literature review by Christianson. In the past five to eight years it has been investigated as a general water quality solution. Bioreactors could prove a useful, low-cost means for pathogen reduction in municipal wastewater, she said, as a form of tertiary treatment.
Christianson and other scientists are experimenting with different bioreactor designs to refine and improve them. Some researchers are using wood-based bioreactors in conjunction with other carbon-rich media such as biochar, for example. And others are using bioreactors in novel ways, such as within water treatment trains or with baffles. These innovations could eventually move the method into new areas and improve the performance of existing bioreactors, especially in cool conditions.
Balancing Watershed Issues and Greenhouse Gases
Among the challenges to be addressed, are how to remove unwanted by-products, such as hydrogen sulfide gas, from the bioreactor. More advanced designs are needed. Current bioreactors must have their food source replenished every 10 years to keep the bacteria population properly fed. More research is needed to balance the issues of watershed health and greenhouse gas emissions.
Then, too, there is the challenge of getting farmers interested in installing bioreactors. Christianson said some grants are available to offset costs, but because of the expense it remains a hard sell.
The literature study on bioreactor research — “Moving Denitrifying Bioreactors beyond Proof of Concept” — was published in the Journal of Environmental Quality.
Photo by Laura Christianson. Graphic provided by L. Christianson and Helmers/Iowa State Extension.