Location: Kandiyohi County, Minnesota
Client: Middle Fork Crow River Watershed District
Background
In 2012 the Middle Fork Crow River Watershed District (MFCRWD) received a grant from the Minnesota Board of Water and Soil Resources (BWSR) to construct a woodchip bioreactor. A woodchip bioreactor is an emerging best management practice (BMP) that can address problem drainage water from agricultural production. Runoff is routed through a buried trench filled with woodchips. The woodchips are a carbon source that serve as a substrate for bacteria that breaks down the nitrate through denitrification. This reduces the nitrates in the runoff before it continues on to area lakes.
The Project
HEI designed an effective woodchip bioreactor solution capable of treating approximately 50 acres of agricultural property within the District.
Our team faced several design challenges while developing the BMP, as woodchip bioreactors have not been widely used in Minnesota. HEI addressed issues like unknown woodchip properties, groundwater tables, unknown conditions of existing tile lines, and optimization of the hydraulic residence time to provide the maximum treatment without producing undesirable reactions (as the result of using all the nitrate). We worked cooperatively with the client to find solutions for these concerns.
The final design features a bioreactor that is connected to the existing tile line. Agricultural runoff enters the woodchips through a perforated pipe. Within the bioreactor, bacteria from the runoff “eats” the carbon in the woodchips and “breathes” nitrate during the respiration process.
HEI also provided construction observation services to ensure that the BMP was built according to design. After its construction in 2013, water samples were collected bi-weekly from both upstream and downstream of the bioreactor throughout the summer of 2014. These samples measured the project’s effectiveness, and have shown a significant decrease in nitrate.