Evaluating Woodchip Bioreactors for Nitrate Removal from Tile Drainage Water.

Tile drainage is the predominant conveyor of nitrate from corn and soybean fields to surface waters in Illinois. Woodchip bioreactors have been proposed as an end-of-pipe remediation technique to remove nitrate from tile water via the process of denitrifiction. We have installed three bioreactors in central Illinois in various aspects of the landscape and on different size drainage systems. We found bioreactor performance was excellent (80% removal of tile nitrate) in a dry year; however, performance was limited in a wet year (10% removal of tile nitrate). The major limitation during the wet year was excessive bypass flow (approximately 50% of the tile flow did not pass through the woodchips and went untreated). In our third year of this study we modified the stoplog configuration in the Agri Drain structure to limit bypass flow and force more of the tile water through the woodchips. This paper will discuss what we have learned during the past three years about bioreactor performance especially in regard to the balance between bypass flow and reduced residence time within the woodchips.