Performance of a Woodchip Bioreactor Receiving Agricultural Tile Drainage.

A woodchip bioreactor is an edge-of-field remediation technique that employs denitrification to remove nitrate from tile drainage waters and can fit into the landscape without impacting row crop production. Bioreactor performance depends on a variety of factors including nitrate loading rates, water temperature, and residence time, as well as the potential for undesirable processes (nitrous oxide flux, sulfate reduction, and mercury methylation). Secondary factors such as the degradation of the woodchip media (decreased particle size) and the potential for biofilm formation may affect the hydraulic conductivity of the woodchips and lead to greater by-pass flow and reduced removal rates. This study evaluated bioreactor performance by quantifying input/output balances for carbon (dissolved organic carbon and carbon dioxide flux); nitrogen (nitrate, ammonium, total N, and nitrous oxide flux); and phosphorus (dissolved reactive phosphorus and total P). Under extreme reducing conditions during low flow and high temperatures, we evaluated the reduction of sulfate and the production of methyl mercury. At the outlet of a patterned tile system draining 20 ha of land in a corn/soybean rotation in east-central Illinois, we constructed a bioreactor with a 6 x 15 m footprint, about 1.3 m deep. Based on design parameters, the bioreactor was sized to remove approximately 50% of the tile nitrate load. In addition, we tested a new generation of control structure that contained both ports that carry flow into and out of the woodchip chamber. We found that removal percentages of nitrate varied greatly with temperature and flow rates, with little removal during January and February and during high flows in April of 2013. Overall, we hope to better understand design constraints and performance limitations to ensure that this technique can be one of the solutions to the surface water nitrate problem that exists throughout the intensively tile drained regions of the Corn Belt.