Impact of flow on woodchip properties and subsidence in denitrifying bioreactors

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Schaefer, Abby
Werning, Kyle
Hoover, Natasha
Tschirner, Ulrike
Feyereisen, Gary
Moorman, Thomas
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Iowa Nutrient Research Center
The Iowa Nutrient Research Center was established to pursue science-based approaches to evaluating the performance of current and emerging nutrient management practices and providing recommendations on practice implementation and development. Publications in this digital repository are products of INRC-funded research. The INRC is headquartered at Iowa State University and operates in collaboration with the University of Iowa and the University of Northern Iowa. Additional project information is available at:
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Agricultural and Biosystems EngineeringIowa Nutrient Research Center

Woodchip bioreactors are edge-of-field practices that remove nutrients from agricultural drainage water, with an effective lifespan estimated between 10 and 30 yr. 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 three different hydraulic residence times (HRTs 2, 8, and 16 h) were excavated after 2 yr of operation, with wood samples collected from multiple depths and distances from the bioreactor inlet. Subsidence was observed in all six 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 ± SD = 64.9 ± 13.7) compared with the 8- 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.


This article is published as Schaefer, Abby, Kyle Werning, Natasha Hoover, Ulrike Tschirner, Gary Feyereisen, Thomas B. Moorman, Adina C. Howe, and Michelle L. Soupir. "Impact of flow on woodchip properties and subsidence in denitrifying bioreactors." Agrosystems, Geosciences & Environment 4, no. 1 (2021): e20149.