by Sally Colby
Savvy farmers in early civilizations managed cropland drainage for higher yields. Over the years, farmers experimented with a variety of drainage methods, including clay pipes that were widely used until the 1970s. But no matter what the year or design, the goal was the same: to keep excess water away from roots.
The terms “tile drain” and “tiling” are still widely used, but instead of cylindrical clay pipes, drainage systems are comprised of poly tubing. Although tiling improves yields on poorly drained acreage, the practice comes with a cost in the form of excess nitrate levels in water exiting the drainage system. Increased concern about water quality in sensitive watershed areas has led to the development of systems to reduce the amount of nitrate leaving farm fields via drainage systems.
One edge-of-field solution used around the world is a wood chip bioreactor. According to NRCS, wood chip bioreactors positioned close to the end of a tile drain system at the edge of a field should treat peak flow from a 10-year, 24-hour drain event, and treat at least 15% of the peak flow from the drainage system, using locally proven criteria that will result in the treatment of at least 60% of the long-term average annual flow from the drainage system.
Dr. Laura Christianson, assistant professor, Department of Crop Sciences at the University of Illinois, has been studying bioreactors for the past 12 years. Christianson explained the relatively simple system as a pit filled with wood chips that cleans nitrate from tile drainage systems. The process of denitrification occurs as microorganisms, fueled by carbon in the chips, colonize the wood chips and convert nitrates in the water to gas.
“There are two parts to get right with bioreactors,” said Christianson. “First there’s the microbiology, then the hydraulics, or flow routing. The wood chips are really just the fuel … The good, naturally present denitrifying bacteria are the workhorses of this nitrate cleaning process. They use the organic carbon supplied to them in the wood chips, then convert nitrate in the water to harmless nitrogen gas. It’s the denitrifying bacteria that put the ‘bio’ in bioreactor.”
The second critical aspect of a functional bioreactor is flow routing. Bioreactors typically use two separate control structures to manage water flow through the wood chips. The inflow control structure diverts drainage water into the bioreactor and allows excessive water to bypass the bioreactor during high flow events. “We don’t want to back drainage water up into the field,” said Christianson. “That doesn’t work for farmers.”
The outflow control structure helps retain water in the bioreactor long enough to allow denitrifying bacteria sufficient time to convert the nitrate in the water to gas, which is the process that cleans the drainage water. “It’s a biological water cleaning process,” said Christianson, “and bacteria don’t work instantaneously, so we need to provide the right hydraulic retention time.”
Denitrifying wood chip bioreactors are practical technology that can be designed to meet local needs, goals and designs, and use local materials. The bottom, sides and top of the media chamber holding the wood chips is lined with geotextile or plastic to prevent soil from entering the bioreactor. In certain areas where cedar is a waste product, cedar chips might be part of the wood chip content. However, because cedar is rot-resistant, small-scale studies are needed to determine whether cedar will decompose sufficiently over time for denitrification.
The key to an in-ditch bioreactor that works efficiently is sizing it correctly for the drainage. Christianson said ditches are an important part of the flow conveyance. They should to be large enough for hydraulic retention and long enough for anoxic conditions to develop for the denitrification process. Although nitrate removal rate varies among locations, the average volumetric removal rate ranges from 0.5 to 5.0 grams of nitrogen removed per cubic meter bioreactor per day.
Christianson said there is enough carbon in a given amount of wood chips to last 100 years or more, but that isn’t a realistic expectation. After a seven- to 12-year lifespan, the old wood chips are excavated, the site undergoes any necessary repairs and new chips are added. At this time, there’s no research showing any risk in land-applying spent chips in an agronomic field.
Ongoing bioreactor project studies in both the U.S. and in other countries will help researchers learn more about the practice. Christianson said in Denmark, about 50% of ag land is tile drained. As is the case in many parts of the U.S., about 45% to 60% of nitrogen losses in that country are attributed to tile drained areas. “What’s different, and interesting,” she said, “is that in Denmark, they attribute about 33% of phosphorus losses to tile-drained areas.”
In one Denmark study, a bioreactor situated in a wetland area was constructed with several drainage discharge designs: vertical downward flow, vertical upward flow and horizontal flow. The horizontal flow treatment system, which is how most systems in the U.S. are designed, performed the best and removed the most nitrogen.
Christianson noted that global partners who have done work with bioreactors have discovered which woods don’t work, and report that wood species with low tannin content are best. When possible, avoid rot-resistant wood such as cedar or redwood. Wood chips with significant amounts of sawdust, leaf litter or bark should not be used.
Winter housing for cattle often involves a concrete pad or sacrifice area to prevent pasture damage. Christianson cited a project in West Virginia in which cattle are being held through winter on a wood chip pad rather than on a typical concrete pad. Wood chips are easier on cows’ hooves than concrete, and manure settles through the chips, providing a cleaner surface for animals and less nitrate in runoff water.
A new multi-state project funded by an NRCS Conservation Innovation Grant is aimed at determining the performance of established bioreactors. Other studies, including one in New Zealand and one at South Dakota State University, are evaluating the microbiology of bioreactors. Those studies will also look for antibiotic-resistant bacteria in discharged water.
“In-field nitrogen management practices are absolutely essential for farmers to consider,” said Christianson. “We always need to be managing nitrogen to the best of our ability in the field. Getting nitrogen management every year in every field is difficult because you can’t predict when nitrogen is going to mineralize until after the fact. While managing nitrogen fertilizer smartly and efficiently in the field to balance agronomic and environmental goals is always going to be important, there will be years when it’s harder to get that right. We’re going to have nitrate loss because we have tile drains, and that’s where edge of field practices come in.”