Evaluation of corncob-amended woodchip bioreactors

Title: Continued assessment of corncobs as an alternative carbon source to enhance bioreactor performance for improved water quality

Location: Boone, Iowa

Time Period: 2021 - 2023

Research Team: Michelle Soupir, Ji Yeow Law, Thomas Isenhart, Natasha Hoover, Gary Feyereisen, Morgan Davis

Project Description: Woodchip bioreactors are a cost-efficient practice to remove nitrates (N) from tile drainage, but the limited regional woodchip supply poses challenges for broad-scale bioreactor implementation efforts. Additionally, poor N removals were observed in woodchip bioreactors when the temperature was low, and N export was high in the spring. These challenges motivated researchers to seek alternative bioreactor carbon substrates that are low-cost, locally available, and can provide improved N removals. Among the carbon substrates evaluated in lab studies, corncobs have shown promising N removals rates and costs. This project aims to investigate the long-term performance of corncob-amended woodchip bioreactors under field conditions. The project objectives are to evaluate and compare i) hydraulic efficiencies, ii) N removal rates, and iii) N removal costs in nine replicated pilot-scale bioreactors (PBRs) using three different woodchip-corncob mixture ratios at three hydraulic retention times (HRTs). Three PBRs were filled with woodchips only (WC100); three PBRs were filled with 75% (by vol.) woodchips and 25% corncobs (CC25); and three PBRs were filled with 25% woodchips and 75% corncobs (CC75). The PBRs from each carbon treatment were operated at 2-, 8-, and 16-hr HRTs, respectively. Our early results showed that hydraulic efficiency was highest in WC100 (0.92±0.14), followed by CC25 (0.86±0.16) and CC75 (0.58±0.17). CC75 exhibited the highest N removal rates (11.6-21.1 g N m-3 d-1), then followed by CC25 (5.7-9.7 g N m-3 d-1) and WC100 (2.0-8.0 g N m-3 d-1). When compared to WC100, the N removal costs of CC75 and CC25 were up to 8.3 and 2.1 times lower, respectively. Overall, the early data suggested that corncob-amended woodchip bioreactors had better nitrate removal rates and cost-efficiencies but lower hydraulic efficiencies. Nonetheless, continued assessment of these PBRs is essential to understand the long-term performance that could be affected by different carbon substrate degradation rates.

Publications:

Schaefer, A., K. Werning, N.L. Hoover, U. Tschirner, T.B. Moorman, G. Feyereisen, A.C. Howe, M.L. Soupir. 2021. Impact of flow on woodchip properties and subsidence in denitrifying bioreactors. Agrosystems, Geosciences & Environment DOI: http://dx.doi.org/10.1002/agg2.20149

Martin, E.A., M.P. Davis, T.B. Moorman, T.M. Isenhart, M.L. Soupir. 2019.  Impact of hydraulic residence time on nitrate removal in pilot-scale woodchip bioreactors. Journal of Environmental Management. 237:424-432. DOI: 10.1016/j.jenvman.2019.01.025

Davis, M.P., E.A. Martin, T.B. Moorman, T.M. Isenhart, M.L. Soupir. 2019. Nitrous oxide and methane production from denitrifying woodchip bioreactor at three hydraulic residence times. Journal of Environmental Management. 242: 290-297. DOI: 10.1016/j.jenvman.2019.04.055

Funders: Iowa Nutrient Research Center

Disclaimer: This is an active research site, please contact Michelle Soupir (msoupir@iastate.edu) prior to planning any site visits.