Demonstrating the nitrogen-removal effectiveness of denitrifying bioreactors for improved drainage water management in South Dakota

摘要

Nitrate-nitrogen exports from agricultural fields through subsurface (tile) drainage systems contribute to the eutrophication of natural aquatic ecosystems. Additionally, elevated concentrations of nitrate-nitrogen in drinking water area public healthconcern. The woodchip denitrifying bioreactor is one type of a cost effective, simple edge-of-field practice to reduce nitrate-nitrogen exports through agricultural subsurface drainage systems. The objective of this study is to evaluate the performanceof three bioreactors by determining the nitrate removal rate and cost per pound of nitrate removed by the bioreactors. During 2012, we installed two bioreactors near Baltic, SD and near Montrose, SD. In 2013, we installed another bioreactor near Arlington, SD. Water was sampled twice per week at the inlet and the outlet of the bioreactors. The water samples were analyzed for nitrate-nitrogen concentration using a spectrophotometer. The average concentration-based nitrate removal at the Baltic and Montrose bioreactors during the subsurface drain flow season (April-July) in 2012 were 81% and 51% respectively. We have not had any water flow through the Arlington bioreactor at this time and we experienced periodic monitoring equipment failure at the Balticsite. At the Montrose bioreactor, the maximum, minimum, and average removal rate of nitrate-nitrogen in unit volume of woodchips were found to be3.14, 0.03, and 0.98 g N /m3 volume of bioreactor per day respectively. The nitrate-nitrogen removal rate inunit active flow volume for the Montrose bioreactor was estimated as 12.58 g N/m3/d. The average annual cost for the bioreactor installation and maintenance, on a per hectare of effective drained area basis and assuming a 20 years lifetime of the woodchips, were $60, $70 and $130forthe Baltic, Montrose and Arlington bioreactors. The cost to remove one unit mass of nitrate-nitrogen was calculated for the Montrose bioreactor. It was approximately $8.68 per Kg N removed ($ 3.94 per lb N). These costs arecomparable with results from other bioreactor studies and are competitive compared to the costs of other nitrate-nitrogen reduction practices.