Bioreactors are excavated pits filled with woodchips, with tile drainage water flowing through the woodchips. As water from the tile line passes into the bioreactor, denitrifying bacteria converts nitrate contained in the tile water into di-nitrogen gas. Bioreactors on a farm scale treat up to 100 acres of tile-drained land.
One study of four bioreactors in Iowa showed an average nitrate-N reduction of 43% for water going through the bioreactor. These systems can be designed with higher removal rates by maximizing retention time and minimizing by-pass flow.