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ARS Home » Midwest Area » St. Paul, Minnesota » Soil and Water Management Research » Research » Publications at this Location » Publication #379338

Research Project: Developing Agricultural Practices to Protect Water Quality and Conserve Water and Soil Resources in the Upper Midwest United States

Location: Soil and Water Management Research

Title: Impact of flow on woodchip properties and subsidence in denitrifying bioreactors

item SCHAEFER, A - Iowa State University
item WERNING, K - Iowa State University
item HOOVER, N - Iowa State University
item TSCHIRNER, U - University Of Minnesota
item Feyereisen, Gary
item Moorman, Thomas
item HOWE, A - Iowa State University
item SOUPIR, M - Iowa State University

Submitted to: Agrosystems, Geosciences & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2021
Publication Date: 3/15/2021
Citation: Schaefer, A., Werning, K., Hoover, N., Tschirner, U., Feyereisen, G.W., Moorman, T.B., Howe, A.C., Soupir, M.L. 2021. Impact of flow on woodchip properties and subsidence in denitrifying bioreactors. Agrosystems, Geosciences & Environment. 4(1). Article e20149.

Interpretive Summary: Woodchip bioreactors are an edge-of-field conservation practice that removes nitrate-nitrogen (N) from agricultural drainage water. Anticipated lifespan of these devices is a decade or more, but settling of the woodchip bed has been observed after a few years. This research reports on the condition of woodchips removed from 6 bioreactor beds that were operated under 3 different flow rates, or hydraulic residence times (HRT): 2, 8, and 16 hours. The settling in these beds was more pronounced near the inlet end. Particle size distribution remained the same over the two years. Hydraulic tracer testing indicated that some short circuiting had developed, but the changes were similar for all 3 flow rates. A batch denitrification test showed that woodchips from all 3 flow rates supported denitrification, even though the carbon-to-nitrogen ratio of the woodchips from the 2-h HRT bioreactors was degraded. The conclusion is that to offset wood aging, bioreactor fill material nearest the inlet could be replenished without excavation of the entire bioreactor. The findings of this research are pertinent to water and conservation managers, agency personnel, practitioners, researchers, and academics with interest in using denitrifying bioreactors to remove nitrate-N from water.

Technical Abstract: Woodchip bioreactors are edge-of-field practices that remove nutrients from agricultural drainage water, with an effective lifespan estimated between 10 and 30 years. Subsidence, or bioreactor settling and subsequent depression formation, is a concern of producers and stakeholders and little is known regarding its effect on bioreactor performance. Six woodchip bioreactors set at 3 different hydraulic residence times (HRTs 2, 8, 16 h) were excavated after 2 years of operation, with wood samples collected from multiple depths and distances from the bioreactor inlet. Subsidence was observed in all 6 bioreactors and was greater near the inlet. Particle size distribution did not change over the study period, indicating that smaller woodchips were not degrading preferentially or washing out of the bioreactor while the macropore space was simultaneously decreasing. Flow path analysis showed an increase in Morrill Dispersion Indices and short-circuiting as well as decreases in drainable porosity and hydraulic efficiency; these changes were uniform across all three HRTs, suggesting that the decline in hydraulic properties was independent of flow. Further, despite increased woodchip decomposition as measured by C:N ratio in the 2-h HRT bioreactors (mean ± standard deviation = 64.9 ± 13.7) compared to the 8-h and 16-h HRT systems (90.3 ± 19.0, 95.6 ± 27.2, respectively), denitrification was still supported at all HRTs based on the results from a batch denitrification test. To offset wood aging, bioreactor fill material nearest the inlet could be replenished without excavation of the entire bioreactor.