Hydraulic Flow Characteristics of Agricultural Residues for Denitrifying Bioreactor Media

Abstract

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 them with previously published results 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, and wood chip – corn cob mixture – and wood chips were obtained with a 30-cm diameter A— 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/s across media), although the more-labile agricultural residues 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.