Wood chip denitrification bioreactors can reduce nitrate in tile drainage

Widespread contamination of surface water with nitrate-nitrogen (NO3-N) has led to increasing regulatory pressure to minimize NO3-N release from agricultural operations. We evaluated the use of wood chip denitrification bioreactors to remove NO3-N from tile drain effluent on two vegetable farms in Monterey County. Across several years of operation, denitrification in the bioreactors reduced NO3-N concentration by an average of 8 to 10 milligrams per liter (mg L−1) per day during the summer and approximately 5 mg L−1 per day in winter. However, due to the high NO3-N concentration in the tile drainage (60 to 190 mg L−1), water discharged from the bioreactors still contained NO3-N far above the regulatory target of < 10 mg L−1. Carbon enrichment (applying soluble carbon to stimulate denitrifying bacteria) using methanol as the carbon source substantially increased denitrification, both in laboratory experiments and in the on-farm bioreactors. Using a carbon enrichment system in which methanol was proportionally injected based on tile drainage NO3-N concentration allowed nearly complete NO3-N removal with minimal adverse environmental effects.