Bioreactors are an edge of field conservation drainage practice that is installed to improve water quality by reducing the amount of nitrates in drainage tile water before it is released to surface waters.
Results of research on pilot scale and field scale bioreactors show reduced nitrate levels in drainage water by 10% to greater than 90%, depending on the bioreactor, the drainage system, and the weather patterns for a given year. Some materials can be added to bioreactors to reduce phosphorous levels if present in the tile drainage water.
A bioreactor is a subsurface trench located along the edge of a field that is filled with a carbon source, most often wood chips. The trench is usually lined with a polymer material prior to the addition of the carbon source material to manage the water flows in the structure. A fabric material and topsoil cover the top of the bioreactor. The carbon source in the trench serves as a substrate for bacteria that breaks down the nitrate through denitrification processes converting it to nitrogen gas.
Water level control structures manage the flow of water into the bioreactor and allow excess flows to bypass the system so that the tile drainage system isn’t restricted. Usually two water control structures are used in the bioreactor design and each structure plays a different role. The inflow structure routes the water into the bioreactor, allowing excess water to bypass the bioreactor in the existing tile main during high-flow events. The outflow control structure manages the water within the bioreactor, ensuring that there is the proper time to remove nitrates from the water as it flows through and before leaving the structure. Depending on the topography of the site, new design methods may allow one water control structure to manage both inflows into and outflows from the bioreactor.
Bioreactors can be retrofitted to nearly any existing tile drainage systems and do not usually require any change in the drainage system. Good planning and careful siting can lead to little or no land being taken out of production. Soil type is not a criterion for locating these structures, so they can go in just about any location. The lifespan of a bioreactor is estimated to be 15 to 20 years. The area immediately above the bioreactor is not suitable for field traffic as the materials in the bioreactor are generally soft and can compress. Any method to exclude traffic from that area is advised.
The USDA-NRCS has financial assistance available in most states for the installation of bioreactors. Many nonprofits, Conservation Districts and some state agencies also have funding available to assist with the cost of implementing these structures.