A bioreactor is an edge-of-field practice that reduces excessive nitrate concentration in water leaving fields through tile drainage. Drainge water is diverted through a buried trench filled with woodchips. Microorganisms convert the nitrate in the drainage water to harmless nitrogen gas (denitrification). Woodchip bioreactors also are known as denitrification bioreactors. Typically, the bioreactor is installed at the end of a tile system just before reaching an outlet to a stream or drainage ditch. Most installations in Iowa to date have been approximately 100 to 120 feet long and 10 to 25 feet wide. Usually, no land is taken out of production for a bioreactor. Research has estimated bioreactor lifespans of 15 to 20 years, after which the woodchips would be replaced if treatment was to be continued. Bioreactors can reduce nitrate levels by 15-30% on 30-80 acres of tile-drained fields.