Features from the Farm: Impact of bioreactor use for filtering nitrogen

In today’s agriculture, field tiling is a common practice to increase crop production by draining excess water out of the plant root profile which creates a more favorable growing environment. Of major concern is the high nitrate-nitrogen concentrations contained in tile water that leaves the site and eventually makes its way into the Mississippi river, emptying out into the Gulf of Mexico, creating a hypoxic zone.

A hypoxic zone is created by high levels of nutrients, such as nitrate (NO3) nitrogen, which trigger large blooms of algae to grow. When the algae bloom dies, it falls to the bottom of the Gulf and bacteria decomposes the algae. The decomposition process consumes most of the oxygen in a significant area of Gulf water near the shoreline. Without oxygen aquatic life in the area flees or dies. The hypoxic zone, or dead zone, in the Gulf has averaged about 5,000 square miles over the last five years.

Staff at the West Central Research and Outreach Center (WCROC) in Morris constructed a bioreactor, which is a system for filtering out nitrogen from water, in an effort to reduce the nitrate flow into the Mississippi River. A woodchip-based bioreactor system was constructed at the discharge point of a 14 acre pattern-tiled field in the summer of 2012.

The bioreactor is approximately 60 feet long, 20 feet wide and five feet deep. The bioreactor is filled with three feet of woodchips, covered with woven fabric, and two feet of topsoil overhead. Tile water flows into the bioreactor and through the bed of woodchips. Woodchips, a natural carbon source, provide the material for certain denitrifying bacteria to colonize. Depending on conditions, these bacteria can transform nitrate into dissolved dinitrogen gas as the water passes slowly through the bioreactor. The bacteria basically steals the oxygen from the nitrates present in the water, converting it into a dinitrogen gas. The dinitrogen gas, which is harmless, eventually escapes to the atmosphere.

The denitrification potential of bioreactors is influenced by the rate of water flow through the bioreactor, as well as temperature, pH, dissolved oxygen, and oxidation/reduction potential. Sizing of the bioreactor in relation to the area drained has not been established yet. Bioreactors may be the best fit for fields receiving manure application where nutrient availability is variable, more so than in agricultural drainage systems where nitrogen best management practices that reduce nitrate-nitrogen concentrations in tile water are already in place. Bioreactors could be one of many methods to reduce environmental damage from nutrient discharge.

The goal is to have the amount of nitrate exiting be less than the amount of nitrate entering, indicating a reduction of nitrates in run-off water. Results from 2013 and 2014 indicate that a wood chip based bioreactor of this size will substantially reduce the nitrate concentration of tile water when less than 10,000 gallons of water flow through on a daily basis.


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