The Performance of Woodchip Bioreactors in South Dakota.

Subsurface (tile) drainage on agricultural land with poor natural drainage allows for timelier field operation access and contributes to improved crop yields. While properly designed and installed subsurface drainage typically reduces sediment and total phosphorus losses, subsurface drainage may enhance the movement of nitrate-nitrogen to surface waters. Excess nitrogen leads to eutrophication and hypoxic conditions in aquatic ecosystems, particularly in estuaries, and high intake levels pose a health hazard, especially for infants and pregnant women. Denitrification woodchip bioreactors are one of several practices that can be installed to reduce the nitrate-nitrogen concentrations in the drainage water. Bioreactors utilize biological denitrification to reduce nitrate-nitrogen to inert dinitrogen gas (N₂). This requires suitable bacteria, a carbon source, and anoxic conditions. Bioreactors consist of an excavated trench filled with a carbon source, typically woodchips. The drainage water from the tile system is routed through the reactor and subsequently discharged into the normal receiving surface water body. In the design phase, it is critical to size bioreactors appropriately to create hydraulic retention times sufficient to maximize nitrate removal but short enough to minimize potential negative side effects. Inlet and outlet control structures are used to adjust the hydraulic retention time. We have installed three woodchip bioreactors in eastern South Dakota on drainage systems each draining between 25 and 40 acres. Following the installations, we have collected water samples and monitored discharge rates during periods of flow. The concentration-based nitrate removal rate from the bioreactors ranged from 10% to 100 %. The nitrate-nitrogen removal rates ranged from 0.03 – 3.14 g N/m³ of woodchips, and the total load removal rate was estimated as 8 Kg N/ha.