Interest in bioreactors is growing in Iowa if the field day at the Roger and Louise Van Ersvelde farm is any indication.
The late August field day brought approximately 45 neighbors, fellow farmers and landowners, and Grinnell College students to the Brooklyn, Iowa farm to learn about the benefits of this conservation structure.
“I’m looking for cleaner water and removing nitrogen from my farm,” Van Ersvelde said. “Bioreactors are a good practice to install if you want to improve water quality for those downstream and to make an impact.”
A bioreactor consists of a large trench dug alongside a crop field, filled with woodchips and covered with topsoil and perennial grasses. Water from an intercepted tile line is diverted into the woodchip-filled chamber. The microorganisms on the woodchips convert the nitrates in the tile water to nitrogen gas, releasing it into atmosphere. A tile outlet at the other end of the chamber moves the denitrified water downstream.
Van Ersvelde, an Iowa Soybean Association member, shared the details of last fall’s bioreactor installation with attendees. The 16-by-64-foot structure sits slightly lower than most at 6.5 feet deep, where his existing tile line ran. But it contains the same volume of woodchips as most bioreactors, at 4.5 feet deep, then covered with 2 feet of topsoil. Most bioreactors are set 4 to 5 feet deep with lesser depths of topsoil covering the chips. He used cost-share funding for the bulk of the installation cost and paid for a fourth from his pocket.
“We’re seeing an average of 46 percent nitrate reduction in Roger’s bioreactor. We are also seeing higher dissolved organic carbon concentrations at the bioreactor outlet when there were higher nitrate reductions,” said Mindy Sieck, watershed coordinator for the Little Bear Creek Watershed Improvement Project.
These nitrogen removal amounts are in line with the Iowa Nutrient Reduction Strategy’s estimates for bioreactor nitrate-N removal. It recommends a suite of practices that contribute to a 45 percent total reduction in nitrogen and phosphorus from ag runoff and subsurface drainage water. According to the strategy, bioreactors reduce nitrogen by an average of 43 percent.
Next generation education
Sieck is working with Grinnell College chemistry professor Andrew Graham to monitor the bioreactor performance. He guided two students this spring and summer to run the lab tests for the water samples.
“I have students who are doing research with me on nutrient cycling in a small watershed in Jasper County,” said Graham. “We’ve been looking at the efficiency of managed ponds and wetlands in removing nitrogen and phosphorus. We’re doing the bioreactor testing in parallel with the work in Jasper County.”
Sieck collects the water samples from the bioreactor and delivers them to Grinnell for the students for testing. She said they’ve run 12 sets of samples between March and July. The sets include water samples at the inlet to the bioreactor and at the outflow. They are testing for nitrate reduction as well as dissolved organic carbon.
Graham said they are monitoring for nitrate and nitrite and total dissolved nitrogen. During low flows they’ve seen close to 100 percent removal of nitrate and strong evidence for denitrification.
“It’s not just removal of the nitrate, but we’re seeing total nitrogen removal,” Graham said. “If you were converting that nitrate to ammonia and releasing that downstream, that wouldn’t be much of a solution.”
The bioreactor monitoring as well as the students’ work with ponds and wetlands serve as learning tools as they connect the laboratory with the environment and agriculture.
“The students have a greater appreciation for the complexity of [environmental] problems. They are learning that there are tradeoffs between any solution that you implement, for instance a tradeoff between performance and costs,” remarked Graham. “I think having that kind of appreciation for complexity of problems serves them well in whatever they decide to go on and do.”