Carbon dosing increases nitrate removal rates in denitrifying bioreactors at low-temperature high-flow conditions

Authors: ROSER, MARTA - Collaborator; Feyereisen, Gary; Spokas, Kurt; MULLA, DAVID - University Of Minnesota; STROCK, JEFFREY - University Of Minnesota; GUTKNECHT, JESSICA - University Of Minnesota

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2018
Publication Date: 7/1/2018
Citation: Roser, M., Feyereisen, G.W., Spokas, K.A., Mulla, D.J., Strock, J.S., Gutknecht, J. 2018. Carbon dosing increases nitrate removal rates in denitrifying bioreactors at low-temperature high-flow conditions. Journal of Environmental Quality. 47(4):856-864. https://doi.org/10.2134/jeq2018.02.0082.
DOI: https://doi.org/10.2134/jeq2018.02.0082

Interpretive Summary: One of the edge-of-field conservation practices promoted to reduce agricultural tile drainage nitrate losses from the Upper Mississippi River Basin is the woodchip bioreactor. In these bioreactors, bacteria convert nitrate in the water to the gaseous form of nitrogen that makes up most of the atmosphere. Since this is a microbial process, it proceeds at a slower rate during colder times of the year, such as in the springtime when snow melt and early spring rains cause tile drainage. Thus, bioreactors tend to be less efficient in these cold times during which a large percentage of annual drainage flow occurs. This research investigated materials other than woodchips to improve the cold performance of bioreactors. There were three groups of materials based on woodchips [plain woodchips; woodchips + biochar; woodchips + acetate (a carbon-rich compound)] and three groups based on corn cobs [plain corn cobs; corn cobs + biochar; corn cobs + biochar + chemically modified coconut coir]. The materials were tested in laboratory columns (6-inch diameter by 19-inch long PVC pipes) for 14 weeks at 41°F, 13 weeks at 59°F, and 7 weeks again at 59°F. The woodchip + acetate treatment removed 80 and 97% of the nitrate during the cold and second warm runs, respectively. For comparison, the plain corn cobs removed 30 and 74%, and plain woodchips 6 and 36% under the same conditions, respectively. The major finding was that adding acetate to woodchips greatly enhanced nitrate removal rates. A field trial and cost analysis of this material combination are logical next steps. The findings of this research will be of interest to researchers, agency personnel, producers/producer groups with interest in improving tile drainage water quality.

Technical Abstract: Nitrogen losses from croplands contribute to impairment of water bodies. This laboratory experiment evaluated various carbon sources for use in a denitrifying bioreactor, a conservation practice designed to reduce nitrogen losses. The nitrate-N removal efficiency of candidate treatments [corn cobs (CC), corn cobs with modified coconut coir (CC+MC), corn cobs with modified coconut coir and modified macadamia shell biochar (CC+MC+MBC), wood chips (WC), wood chips with hardwood biochar (WC+BC), and wood chips with continuous sodium acetate addition (WC+A)] were tested with up-flow direction. Effluent was sampled after hydraulic residence times (HRT) of 1.5, 8, 12, and 24 h. Column temperatures were 15°C for 14 weeks (warm), 5°C for 13 weeks (cold), and again 15°C for 7 weeks (rewarm). Cumulative nitrate-N load reduction was highest for WC+A, 80%, 80%, and 97%, during the warm, cold, and rewarms runs, respectively. Corn cob treatments (CC, CC+MC, and CC+MC+MBC) had the second highest cumulative load reductions for all three temperature experiments, and WC and WC+BC had the lowest performance under these conditions. The nitrate-N removal rate was optimum at the 1.5 h HRT for the WC+A treatment: 43, 30, and 121 g N m-3 d-1, for the warm, cold, and rewarms runs, respectively. Furthermore, acetate addition greatly improved wood chip performance and could be used to enhance nitrate-N removal under the cold and high flow rate conditions of springtime drainage for the North-Central USA.