Denitrifying bioreactor woodchip recharge: Media properties after nine years

Authors: CHRISTIANSON, LAURA - University Of Illinois; Feyereisen, Gary; HAY, CHRISTOPHER - Iowa Soybean Association; TSCHIRNER, ULRIKE - University Of Minnesota; KEEGAN, KULT - Agricultural Drainage Management Coalition; SOUPIR, MICHELLE - Iowa State University; HOOVER, NATASHA - Iowa State University

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2020
Publication Date: 2/11/2020
Citation: Christianson, L.E., Feyereisen, G.W., Hay, C.H., Tschirner, U.W., Keegan, K.J., Soupir, M.L., Hoover, N.L. 2020. Denitrifying bioreactor woodchip recharge: Media properties after nine years. Transactions of the ASABE. 63(2):407-416. https://doi.org/10.13031/trans.13709.
DOI: https://doi.org/10.13031/trans.13709

Interpretive Summary: Woodchip denitrifying bioreactors have begun to be deployed over approximately the last decade to remove nitrate-nitrogen from agricultural tile drainage. One of the first questions asked about the technology is “how long do they last?” This paper is the first report of a field woodchip bioreactor that has had to be “recharged” with new woodchips to renew its performance. The bioreactor was installed in August 2008 in Greene County, Iowa, and recharged in November 2017. Woodchips taken from the bioreactor were tested for particle size distribution, chemical properties, and capacity to support denitrification, which is the mechanism by which bioreactors convert nitrate into the gaseous form of nitrogen that is in the air. There were two types of wood media used in this bioreactor – chips and mixed shreds. The median particle size of the mixed shreds was significantly smaller than that of the woodchips, 7.7 and 12.1 mm, respectively, and likely reduced flow rate in the bioreactor compared to initial conditions. Sediment, likely entering the drainage system through a surface inlet, was also present in the media, which likely further restricted flow and may have led to development of short circuiting, thus reducing the effectiveness of the bioreactor bed. Lab batch testing showed that both media supported denitrification, although the carbon quality of the mixed shreds was reduced from that of the woodchips and nearing the condition at which they would not effectively support denitrification. The results generally support a rule-of-thumb design life of 10 y for tile drainage denitrifying bioreactors.

Technical Abstract: There is a lack of information on denitrifying bioreactors treating subsurface drainage water at the end of their initial design lives due to the relative newness of the technology and the estimated life. A denitrifying bioreactor (LWD: 15 x 7.6 x 1.1 m) installed in August 2008 in Greene County, Iowa, USA was recharged with new woodchips in November 2017 (age: 9.25 y) providing the novel opportunity to evaluate end of design-life wood media properties. The objective was to pair a battery of physical, chemical, and nitrate-N removal tests on the wood media harvested from the bioreactor with field observations to assess likely reasons denitrifying bioreactors treating tile drainage may need to be recharged. The two types of wood media harvested from the bioreactor had median particle sizes (D50) of 12.1 and 7.7 mm and saturated hydraulic conductivities of 4.2±3.0 and 3.1±1.0 cm/s (mean ± standard deviation) which were likely less than that of the initial wood media used to construct the bioreactor. The wood media carbon content and quality had degraded (e.g., lignocellulose indices of 0.63-0.74 nearing the range of decomposition stabilization), although batch studies suggested the robustness of wood as a carbon source to support nitrate removal. Woodchip degradation along with suspected sedimentation from the drainage system likely reduced conductivities over time. Possible flow restriction may have led to development of preferential flow paths through the bioreactor which were indicated by low bioreactor outflow rates (i.e., reduced permeability) and reduced hydraulic efficiency based on conservative tracer testing. These results generally support a rule of thumb design life of 10 y for subsurface drainage denitrifying woodchip bioreactors, with multiple causes and linked impacts of the changes in media properties.