|CHRISTIANSON, LAURA - Iowa State University|
|HELMERS, MATTHEW - Iowa State University|
|BHANDARI, ALOK - Kansas State University|
Submitted to: Ecological Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2012
Publication Date: 11/30/2012
Citation: Christianson, L., Helmers, M., Bhandari, A., Moorman, T.B. 2012. Internal hydraulics of an agricultural drainage denitrification bioreactor. Ecological Engineering. 298-307.
Interpretive Summary: Wood chip bioreactors are receiving increasing attention as a means of reducing nitrate in subsurface tile drainage systems. These reactors convert nitrate to nitrogen gas through a microbial process called denitrification. The internal flow dynamics and nitrate removal were investigated in a field-scale bioreactor in northeast Iowa. In spring, when drainage water flow and nitrate load into the reactor was greatest, approximately 10% of the nitrate was removed. Injections of bromide as a tracer revealed faster water flow than predicted indicating that the entire reactor volume was not utilized for nitrate removal. Thus, design modifications to better distribute flow through the entire reactor or management to increase residence time will result in more efficient nitrate removal. This information may be useful to scientists, water quality specialists, and conservationists.
Technical Abstract: Denitrification bioreactors to reduce the amount of nitrate-nitrogen in agricultural drainage are now being deployed across the U.S. Midwest. However, there are still many unknowns regarding internal hydraulic-driven processes in these "black box" engineered treatment systems. To improve this understanding, the internal flow dynamics and several environmental parameters of a denitrification bioreactor treating agricultural drainage in Northeastern Iowa, USA were investigated with two tracer tests and a network of bioreactor wells. The bioreactor had a trapezoidal cross section and received drainage from approximately 14.2 ha at the North East Research Farm near Nashua, Iowa. It was clear from the water surface elevations and the continuous pressure transducer data that flow was attenuated within the bioreactor. Over the sampling period from 17 May to 24 August 2011, flow conditions and internal parameters (temperature, dissolved oxygen, oxidation reduction potential) varied widely resulting in early samplings that showed little nitrate removal ranging to complete nitrate removal and sulfate reduction at the final sampling event. The bioreactor's non-ideal flow regime due to ineffective volume utilization was a major detriment to nitrate removal at higher flow rates. Regression analysis between mass nitrogen reduction and theoretical retention time suggested minimum design retention times should be increased, though caution was also issued about this as increased design retention times and corresponding larger bioreactors may exacerbate detrimental byproducts under low flow conditions. Operationally, outlet structure level management could also be utilized to improve performance and minimize detrimental by-products.