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Title: Non-linear hydraulic properties of woodchips necessary to design denitrification beds

item GHANE, EHSAN - University Of Minnesota
item Feyereisen, Gary
item ROSEN, CARL - University Of Minnesota

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/2016
Publication Date: 9/13/2016
Citation: Ghane, E., Feyereisen, G.W., Rosen, C.J. 2016. Non-linear hydraulic properties of woodchips necessary to design denitrification beds. Journal of Hydrology. 542:463-473. doi:10.1016/j.jhydrol.2016.09.021.

Interpretive Summary: Denitrifying bioreactors, consisting of a trench filled with wood chips through which tile drainage water is routed, are being used in agricultural producing areas to reduce nitrate-nitrogen losses to surface water bodies. The common design assumption is that water flow through the wood chip bed increases linearly with increasing pressure, or head, on the inlet end. However, recently, the flow through fresh wood chips has been shown to be nonlinear; that is, as the inlet pressure or head increases, the flow rate increases proportionately less. This research reports that for wood chips that had been installed in a field bioreactor for four years, the flow is also nonlinear. The finding is important for the design of bioreactors because if linear flow assumptions are used, the flow through the bioreactor will be less than anticipated. The findings of this research will be of interest to NRCS who has developed a denitrifying bioreactor standard that will likely be directly affected by this work, engineers who design bioreactors, researchers, and producers/producer groups with interest in reducing tile drainage nitrate losses.

Technical Abstract: Denitrification beds are being used to reduce the transport of water-soluble nitrate via subsurface drainage systems to surface water. Only recently has the non-linearity of water flow through woodchips been ascertained. To successfully design and model denitrification beds for optimum nitrate removal, a better understanding of flow in beds is needed. The main objective of this study was to characterize and compare the hydraulic properties of old, degraded woodchips using Forchheimer’s and Izbash’s equations. To achieve this goal, we conducted constant-head column experiments using old woodchips that were excavated from a four-year old denitrification bed near Willmar, Minnesota, USA. For Izbash’s law, the non-Darcy exponent (n) ranged from 0.76 to 0.87 that indicates post-linear regime, and the permeability coefficient (M_10) at 10°C ranged from 0.9 to 2.6 cm s^-1. For Forchheimer’s equation, the intrinsic permeability of 5.6 ' 10^-5 cm^2 and ' constant of 0.40 (drainable porosity of 0.41) closely resembled the in-situ properties found in a previous study. Based on the Forchheimer number, the error of ignoring the non-Darcy flow in old woodchips was on average at least 7% at the average hydraulic gradient of 0.006 ± 0.003 with a greater error occurring at higher hydraulic gradients (i.e., at bed peak flows). Forchheimer’s coefficients provided stronger correlations with drainable porosity and slightly superior fit to data than that of Izbash. In conclusion, Forchheimer’s equation was better than that of Izbash’s for describing water flow through old woodchips, and using Darcy’s law can lead to overestimation of peak flow in denitrification beds.