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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Agroecosystems Management Research » Research » Publications at this Location » Publication #353208

Research Project: Agroecosystem Benefits from the Development and Application of New Management Technologies in Agricultural Watersheds

Location: Agroecosystems Management Research

Title: Impact of hydraulic residence time on nitrate removal in pilot-scale woodchip bioreactors

item MARTIN, EMILY - Iowa State University
item DAVIS, MORGAN - Iowa State University
item Moorman, Thomas - Tom
item ISENHART, THOMAS - Iowa State University
item SOUPIR, MICHELLE - Iowa State University

Submitted to: Journal of Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2019
Publication Date: 1/25/2019
Citation: Martin, E.A., Davis, M.P., Moorman, T.B., Isenhart, T.M., Soupir, M.L. 2019. Impact of hydraulic residence time on nitrate removal in pilot-scale woodchip bioreactors. Journal of Environmental Management. 237:424-432.

Interpretive Summary: The Midwestern United States is dominated by agricultural production on lands with artificial subsurface drainage which is an important source of nitrate pollution. Woodchip bioreactors are being implemented for the removal of nitrate in tile water drainage. One factor that affects performance of these bioreactors is the length of time that water stays in the bioreactor (hydraulic residence time, HRT). We used pilot-scale bioreactors (19 ft. long) with controlled water flows to investigate nitrate removal. Reductions in nitrate concentration were greatest in reactors with longer (HRT), but mass removal (grams of nitrate) were greatest at the shorter HRT. This is due to the shorter HRT bioreactors having much more water and nitrate entering the reactors. After considering emissions of the greenhouse gases nitrous oxide and methane, the 8-hour HRT was optimal. The results of this study will inform water quality specialists and engineers who work in the agricultural community on the design and installation of these bioreactors.

Technical Abstract: Nitrate (NO3-N) export from row crop agricultural systems with subsurface tile drainage continues to be a major water quality concern. Woodchip bioreactors are an effective edge-of-field practice designed to remove NO3-N from tile drainage. The NO3-N removal rate of woodchip bioreactors can be impacted by several factors, including hydraulic residence time (HRT). This study examined the impact of three HRTs, 2 h, 8 h, and 16 h, on NO3-N removal in a set of nine pilot-scale woodchip bioreactors in Central Iowa. NO3-N concentration reduction from the inlet to the outlet was significantly different for all HRTs (p < 0.05). The 16 h HRT removed the most NO3-N by concentration (7.5 mg L-1) and had the highest percent mass removal rate (PMRR) at 53.8%. The 8 h HRT removed an average of 5.5 mg L-1 NO3-N with a PMRR of 32.1%. The 2 h HRT removed an average of 1.3 mg L-1 NO3-N with a PMRR of 9.0%. The 2 h HRT had the highest NO3-N mass removal rate (MRR) at 9.0 g m-3 day-1, followed by the 8 h HRT at 8.5 g m-3 day-1, and the 16 h HRT at 7.4 g m-3 day-1, all of which were statistically different (p < 0.05). Significant explanatory variables for PMRR were HRT (p < 0.001) and influent NO3-N concentration (p < 0.001), (r = 0.80). The ideal HRT for the bioreactors was 8 hours to achieve maximum NO3-N removal while reducing the impact from greenhouse gas emissions.