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Demonstrating the Effectiveness of Nitrate-nitrogen Removal of Denitrifying Bioreactors in South Dakota for Improved Drainage Water Management
There is a critical need to develop additional practices for reducing nitrate nitrogen load deposition through tile drainage systems in the Mississippi River Basin and ultimately achieving the target of reducing the hypoxic zone in the Gulf of Mexico. Denitrifying woodchip bioreactors are one example of a method to remove nitrate nitrogen from the drainage water. The goal of this project were to evaluate the performance of three bioreactors by determining the nitrate-nitrogen removal rate and cost per pound of nitrate-nitrogen removed by the bioreactors. We installed two field-scale bioreactors in 2012 and one in 2013 and equipped each reactor with flow monitoring equipment. Water was sampled twice per week immediately upstream and downstream of the bioreactors and analyzed for nitrate-nitrogen concentration. The average concentration-based nitrate removal at both the Baltic and Montrose bioreactors were 81% and 51%, respectively during 2013. At the Montrose bioreactor, the average removal rate of nitrate-nitrogen in calculated for the entire volume of woodchips (wetted and unwetted) was 0.98 g N /m3 per day. The nitrate-nitrogen removal rate calculated for the volume of wetted woodchips for the Montrose bioreactor was estimated as 12.58 g N/m3/d. A bench top experiment was conducted to develop the following flow equation for the V-notch weir used in the study to estimate flow rates: Q=1.7406 H 1.9531, where Q is flow rate (l/min), and H is thickness in cm of the nappe flowing over the V notch. An economic analysis showed that the annualized installation costs for the three bioreactors were between $60 and $130 per hectare of effective drained area. The cost to remove one unit mass of nitrate-nitrogen was calculated for one of the bioreactors only due to monitoring equipment failure or lack of flow at the two others. It was approximately $8.68 per Kg N removal.