|CHRISTIANSON, LAURA - Freshwater Institute|
Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2014
Publication Date: 3/12/2015
Publication URL: https://handle.nal.usda.gov/10113/60613
Citation: Feyereisen, G.W., Christianson, L.E. 2015. Hydraulic flow characteristics of agricultural residues for denitrifying bioreactor media. Applied Engineering in Agriculture. 31(1):89-96.
Interpretive Summary: One means of reducing the load of nitrate-nitrogen lost through agricultural tile drainage is to route the water through a denitrifying bioreactor, which typically consists of a trench filled with wood chips alongside a field, or between a field and an open drainage ditch. Wood chips have worked successfully in this application, however, availability and performance can be issues with them. Previous studies have shown that agricultural residues have a higher nitrate-nitrogen removal rate than wood chips. Also, agricultural residues are readily available where bioreactors are needed. This study was conducted to evaluate the hydraulic properties of agricultural residues that are candidate bioreactor media - corn stover, barley straw, corn cobs, and wood chip/corn cob mixture - and compare them to wood chips. The materials were tested by packing them into a 12-inch diameter by 8-foot long PVC vessel and flowing water through them at 15 different flow rates. Wood chips exhibited the highest hydraulic conductivity, k, with a temperature corrected value of 4.47 cm/sec and barley straw was lowest with 2.81 cm/sec. The rank order of conductivities was wood chips > corn stover > wood chip/corn cob > corn cob > barley straw. There was actually not a statistical difference among the materials. This work showed some agricultural residues would provide similar initial hydraulic conductivities to woodchips. The information is valuable for other researchers and engineers who are investigating and designing denitrifying bioreactors.
Technical Abstract: Denitrifying bioreactors are a promising technology to mitigate agricultural subsurface drainage nitrate-nitrogen losses, a critical water quality goal for the Upper Mississippi River Basin. This study was conducted to evaluate the hydraulic properties of agricultural residues that are potential bioreactor media alternatives to the conventionally used wood chips, and to compare results with results previously published using log-log data transformation. Such residues’ higher inherent nitrogen removal rates may improve performance, especially under cool early-season drainage water temperatures critical in the upper Midwest. Hydraulic conductivities, k, of candidate media – corn stover, barley straw, corn cobs, wood chip–corn cob mixture – and wood chips were obtained with a 30-cm diameter by 2.5-m long permeameter. Statistically significant differences were observed between media types with the woodchips exhibiting the highest k followed by corn stover. However, when corrected for viscosity, significant differences between media types were eliminated (mean k’ = 2.81 to 4.47 cm/sec across media), although the more-labile agricultural resides consistently had lower conductivities than the woodchips. The log-log transformation proved to be critical for most accurately representing the data. When previously published data were corrected using this transform, there were no statistically significant differences between the older work and this study for both woodchips and woodchip-corn cob mixtures (p = 0.82 and 0.22, respectively). Despite focus on use of woody media for bioreactors, this work showed some agricultural residues would provide similar initial hydraulic conductivities to woodchips, further refining design criteria for solid carbon-source denitrifying bioreactors.