AMES, Iowa – Woodchip bioreactors, installed at the edge of agricultural fields, can remove 15 to 60 percent of the nitrate in tile-drained water annually. This innovative approach for protecting the water quality in Midwest streams and rivers is described in a new fact sheet available from Iowa State University Extension and Outreach.
Iowa State University graduate student Laura Christianson and ISU ag and biosystems engineer Matthew Helmers authored the publication. They have researched bioreactors in a project funded by a competitive grant from the Leopold Center for Sustainable Agriculture since 2009.
Bioreactors — buried trenches filled with wood chips — capture and treat water from tile drainage before it reaches streams and rivers. Inside the bioreactor, beneficial microbes breathe by transforming nitrate into harmless dinitrogen gas. The landowner can adjust the amount of water flowing through the bioreactor with control structures at either end, and a bypass line ensures that water won’t back up during high flow events.
Because bioreactors fit well in buffer strips or grassy areas at the edges of fields, typically no land is taken out of production. The Iowa Soybean Association has installed six bioreactors to date, and plan to double that number this fall. Several farmer-led watershed improvement associations are partnering with ISU Extension and Outreach to install bioreactors as well. The Environmental Quality Incentive Program (EQIP) offers a one-time payment of $3,999 for installation, which covers roughly half the cost.
Christianson notes that this new technology merits further research. The research team continues to analyze data from working bioreactors in the field, and plans to conduct an acceptability survey with farmers. They estimate that bioreactors will last 15 to 20 years before the woodchips need to be replaced, although no working bioreactor has reached the end of its lifespan yet. Most bioreactors remove 15 to 60 percent of the nitrate in drainage from 30 to 80 acres.
Management practices can help reduce the negative side effects of bioreactors. One concern is that if nitrate-breathing bacteria use up their supply of nitrate too quickly, they will be replaced by sulfate-breathing bacteria. These unwanted bacteria transform any mercury present in the bioreactor to a toxic form called methyl mercury. Managing the control structures with quicker flow rates, so that not all the nitrate is removed, can minimize this concern.
Answers to frequently asked questions about how bioreactors work, how to install and manage bioreactors, and where to find technical and financial help can be found in the new fact sheet, “Woodchip Bioreactors for Nitrate in Agricultural Drainage.” Download the publication, or order print copies at no charge, from the ISU Extension Store at http://store.extension.iastate.edu (search for PMR 1008).