January 18, 2022

Chipping away at water quality issues

There are several in-field and edge-of-field agricultural conservation practices that are a part of the suite of practices approved by the Illinois Nutrient Loss Reduction Strategy to reduce nutrients leaving our state. Edge-of-field practices, as the name implies, are placed at the edge of a field and typically do not take any land out of production. One edge-of-field practice that is gaining popularity is the woodchip bioreactor.

A woodchip bioreactor typically consists of a rectangular-shaped pit filled with woodchips at the end of a field tile line. Woodchip bioreactors can come in many shapes and sizes depending on the size of the drainage area and the proposed location of the bioreactor. The woodchip bioreactor works by channeling tile drainage water into the bioreactor through a drainage water management control structure. As water moves through the woodchips and fills the pit, forming saturated conditions, denitrifying bacteria use the woodchips as a food source and convert the nitrate to nitrogen gas through the denitrification process. The water leaves the other end of the bioreactor via another control structure to continue to the outlet. By converting nitrate to nitrogen gas in the woodchip bioreactor, the bioreactor is removing nitrate from the water before it goes downstream.

With more frequent high-flow rain events, there is a concern that water might not flow quickly enough through the tile drain and bioreactor to properly drain the field. To eliminate this concern, a bypass at the intake control device prevents the water from backing up in the field during high flow events and allows the tile drainage to do its job. The downside to the bypass is all the water that isn’t treated within the woodchip bioreactor will enter the nearest waterway still containing the mobile nitrates.

Feedstocks other than woodchips, such as corn cobs, have been studied in bioreactors. Woodchips are typically used due to the longevity of the woodchips, but research on alternative materials is ongoing.

When selecting a location for a woodchip bioreactor, there are some key things that need to be taken into consideration. One is the size of the drainage area, which will determine the size of the bioreactor. Another is that woodchip bioreactors are best suited in areas with slopes ranging from 0 – 1%. Some caution should be taken while planning the location of the bioreactor to ensure it is in a location where it will not be driven over by machinery.

There are many positives to installing a woodchip bioreactor along with primary reason of removing nitrates from tile water. The woodchip bioreactor is placed at the edge of a field, so no farmland is taken out of production. Though there is a large up-front cost to installing a woodchip bioreactor, there are minimal maintenance costs and operation over the life of the woodchip bioreactor. The life of the woodchip bioreactor is also relatively long. Current research indicates that the woodchips will last for at least 10 years before they will need to be replaced, but continuing research is being conducted to determine the longevity of the woodchips. The life of the woodchips is also dependent on the type of wood used. Hardwoods, which are often recommended, have a longer lifespan compared to soft woods.

If you are interested in putting in a woodchip bioreactor on your farm, contact your local Soil and Water Conservation District and/or USDA-NRCS office for more information on cost-share opportunities available to help offset high initial installation costs. For more information regarding ongoing research on woodchip bioreactors and other drainage research, check out the Illinois Drainage Research and Outreach Program page at draindrop.cropsci.illinois.edu. You can also listen to Episode 41 of the Illinois Nutrient Loss Reduction Podcast on woodchip bioreactors at https://go.illinois.edu/NLRS_episode41. On this episode, you will hear about a recently installed bioreactor in Rock Island County from Bryan Maxwell, bioreactor designer and post-doc at University of Illinois, and Drew Hohenboken, the farmer on whose property the bioreactor was installed.

Rachel Curry is a University of Illinois Extension watershed outreach associate.