Demonstrating the Nitrate-Nitrogen-Removal Effectiveness of Denitrifying Bioreactors in South Dakota for Improved Drainage Water Management.

A portion of excess nitrate-nitrogen residing in the soil has been found to be exported from agricultural fields through subsurface drainage (tile) systems. The nitrogen then ends up in natural water bodies such as streams and rivers where the excess nutrient can lead to hypoxic conditions. A number of strategies are being implemented to reduce the nitrogen exports through the tile systems in agriculture fields. Denitrifying woodchip bioreactors is one example of a practice removing nitrate-nitrogen from tile water at the field scale.

A woodchip bioreactor is a buried trench filled with carbonaceous material such as woodchips that promote denitrifying bacteria colonization that convert nitrate in drainage water to inert nitrogen gas through the multi-step process called dentrification. Research is on-going around the world to evaluate bioreactor nitrate removal efficiency and review designs, compare carbon substrates, impacts of tile systems and climate, and ways to reduce escape of unwanted gases during the denitrification process.

The goal of this project is to evaluate the effectiveness of four bioreactors by determining the nitrate removal rate and cost per pound of nitrate removed by the bioreactors at the different locations. During 2012, we installed two bioreactors at different locations: One near Baltic, SD and one near Montrose, SD. In 2013, another bioreactor was installed by Arlington, SD. We installed them by digging a trench about 5.5ft deep and 20ft wide. The length of the trench is dependent on the amount of water being treated which is dependent on the drained acreage and soil properties. Our design called for a trench 120ft long. Sensors connected to data loggers were installed near each bioreactor to measure the flow rate and electrical conductivity at the inlet and the outlet of the reactor, soil and air temperature and, precipitation. Water was sampled twice per week from both upstream and downstream control structure. The water was analyzed for nitrate concentration using a spectrophotometer. Substantial nitrate reduction was observed between the upstream and downstream ends of the reactors. One additional bioreactor is scheduled to be installed in late 2013. We will review the criteria for bioreactor design and share experiences from the installation of the bioreactors.