Design Modification and Early Performance of an in-Ditch Denitrifying Bioreactor.

Denitrifying bioreactors have moved beyond the proof of concept for mitigating nitrate in subsurface tile drainage applications. However, the extensive use of agricultural drainage ditches, particularly in the Mid-Atlantic region’s Delmarva Peninsula, present a unique opportunity to modify existing bioreactor design procedures to adapt this technology to regions where surface drainage ditches are widespread. A woodchip bioreactor was designed for and constructed in a drainage ditch near Crisfield, MD in late 2015 to serve as a research and demonstration site for enhanced denitrification practices in areas that are heavily ditch drained. Approximately 92 m of the existing ditch was retrofitted to provide enhanced denitrification in three sections that each culminated with a short wooden berm at the downstream end (i.e., essentially three conjoined bioreactors of 27, 38, and 27 m length). The ditch bed was excavated and filled with woodchips topped by a gravel infiltration layer. Perforated flow collection manifolds were located directly upstream of each berm to route treated water into an underdrain. The bioreactor was not designed to treat high flow rates in the ditch, which would rather by-pass over the top of the berms thus allowing maintenance of surface drainage capacity at the site. Early nitrate removal results from this denitrifying bioreactor will be presented and discussed.