Internal hydraulics of an agricultural drainage denitrification bioreactor

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 hydraulicdriven processes in these 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 (i.e., reduction in peak flow as the hydrograph moved down gradient). 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 (7-100% mass reduction; 0.38-1.06 g N removed per m 3 bioreactor per day) 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 (7.5-79 h) 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 by-products under low flow conditions. Operationally, outlet structure level management could also be utilized to improve performance and minimize detrimental by-products. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. b s t r a c tDenitrification 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 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 (i.e., reduction in peak flow as the hydrograph moved down gradient). Over the sampling period from 17 May to 24 August 2011, flow conditions and internal parameters (tempe...

Temperature and Substrate Control Woodchip Bioreactor Performance in Reducing Tile Nitrate Loads in East-Central Illinois

“…They found that temperature was not a good predictor of removal rates, with measurements from May through August and water temperatures varying from 8.9 to 17.1°C (Christianson et al, 2013b). Our results and those of Christianson et al (2013b) suggest that bioreactors may have to be much larger to effectively remove nitrate from a typical tile line because the retention time would have to be substantially longer. On the same farm as Bioreactor 1, we monitored two constructed wetlands intercepting tile lines and found they were much more effective at reducing nitrate N loads during 2012 and 2013 (Groh et al, 2015).…”

“…Previous studies on wood chip bioreactors have found that N 2 O emissions are small, but these reports are limited (Moorman et al, 2010;Woli et al, 2010;Warneke et al, 2011a;Christianson et al, 2013b).…”

“…However, our average monthly rates were only 1 to 3 g N m −3 d −1 when water temperature was colder in February and March. Christianson et al (2013b) reported rates of 0.38 to 1.06 g N m −3 d −1 for a bioreactor in Iowa and determined that internal hydraulics limited the removal rates. They found that temperature was not a good predictor of removal rates, with measurements from May through August and water temperatures varying from 8.9 to 17.1°C (Christianson et al, 2013b).…”

“…Christianson et al (2013b) reported rates of 0.38 to 1.06 g N m −3 d −1 for a bioreactor in Iowa and determined that internal hydraulics limited the removal rates. They found that temperature was not a good predictor of removal rates, with measurements from May through August and water temperatures varying from 8.9 to 17.1°C (Christianson et al, 2013b). Our results and those of Christianson et al (2013b) suggest that bioreactors may have to be much larger to effectively remove nitrate from a typical tile line because the retention time would have to be substantially longer.…”

Reactive ditches: A simple approach to implement denitrifying wood chip bioreactors to reduce nitrate exports into aquatic ecosystems?

“…Thus, the flow time through the filter media is of particular importance for high filter effects (e.g. Schipper and McGill 2008;Christianson et al 2012Christianson et al , 2013. A simple opportunity to extend flow times through the filter might be barriers within the filter, which allow longer times of contact.…”