Bioreactor: Redirects tile water to an underground bed of wood chips where nitrate is removed naturally by microorganisms. Vegetation on top of the bioreactor can provide other benefits such as wildlife habitat.
A bioreactor is an edge-of-field treatment process that allows the producer to reduce the amount of nitrate leaving the field from a tile line and therefore improving water quality of the receiving stream. It consists of a buried pit filled with a carbon source such as wood chips, through which tile water is diverted. The carbon provides a food source for microorganisms that use the nitrate to metabolize the carbon, converting the nitrate to harmless atmospheric nitrogen (N2) gas. Bioreactors can reduce nitrate by an average of 43 percent.
Two control structures are used to divert tile water into the bioreactor, control the depth of water and to control how long each gallon of water stays in the bioreactor. The control structure at the upper end of the bioreactor determines the amount of water diverted. The structure intercepts or “T’s” into the existing field tile. When tile flow exceeds the bioreactor’s capacity, excess water bypasses the system and flows down the existing tile line, preventing backup into the field. The lower structure determines the depth of water within the bioreactor and the retention time.
Bioreactors are generally used where the drainage area is about 40 to 100 acres. The footprint is small, typically covering less than 0.05 acres. Most bioreactors installed in Iowa to date have been 100 to 120 feet long and 10 to 25 feet wide. They work well in existing filter strips. The wood chips may need to be replaced every 10 to 15 years to maintain high levels of nitrate removal. The Iowa Soybean Association Environmental Programs and Services team has installed about 40 bioreactors. It is estimated Iowa needs about 180,000 bioreactors to reach the goals set out in the Iowa Nutrient Reduction Strategy.
For more information on bioreactors:
• Denitrifying Woodchip Bioreactors
• Woodchip Bioreactors for Nitrate in Agricultural Drainage