Corn Cobs as an Alternative Carbon Source to Enhance Bioreactor Performance for Improved Water Quality
The Iowa Nutrient Reduction Strategy identified woodchip bioreactors as a promising strategy for removing NO3-N from drainage water. While reported reductions are promising, bioreactor performance varies greatly and is influenced by temperature, carbon (C) availability, influent nitrate concentration, and residence. Besides temperature, bioavailability of C governs the N removal rate. Many lab scale studies have demonstrated increased N removal when exploring sustainable alternative C sources, especially agricultural residues, including corn cobs.
The overall goal is to compare nitrate and phosphorus fate in bioreactors using a combination of corn cobs and woodchips, plus just woodchip bioreactors, at the pilot scale. Objectives are to evaluate performance of bioreactors with three different ratios of corn cobs and wood chips by monitoring bioreactor influent and effluent for nitrate and phosphorus; conduct a tracer study to assess hydraulic properties; and conduct a technoeconomic analysis to determine the cost of N removed and identify opportunities to make bioreactor installation more cost effective.
Nine pilot scale woodchip bioreactors already in place will be used for this project. The woodchip fill material in six of the bioreactors will be modified in fall 2018. Corn cobs will fill one-third of three of the bioreactors, and two-thirds in another three. The remaining three will remain all woodchips. Experiments will begin in spring 2019 when flow is available, and run for about six months. Water samples will be collected weekly.
Woodchip bioreactors are a promising technology for nitrogen (N) removal, but there are challenges, including limited woodchip supplies in the Midwest and poor performances in low-temperature conditions. This project aimed to investigate the potential of using corncobs as an alternative carbon source for bioreactors through the following objectives: evaluate and compare i) hydraulic efficiencies, ii) N removal rates (NRR), and iii) N removal costs in nine 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 was operated at 2-, 8-, and 16-hr HRTs, respectively.
The hydraulic efficiency was highest in CC75, followed by WC100 and CC25. At the respective HRTs, the highest nutrient removal rates were by CC75, then CC25 and WC100.
When comparing N removal costs, CC75 ($20.83, $22.51, and $34.35 kg-1 N at 2-, 8-, and 16-hr HRT, respectively) had the highest cost-efficiency; then WC100 ($51.61, $31.85, and $44.05 kg-1 N); and CC25 ($52.47, $53.01, and $48.06 kg-1 N). The N removal costs of PBRs were higher than typical full-scale bioreactors, but the relative cost comparisons using PBRs showed that N removal costs were 22-60% lower in CC75 than in WC100. However, the N removal costs were 2-66% higher in CC25 than in WC100.
Overall, this two-year monitoring project demonstrated improved hydraulic efficiency, nutrient removal rates and cost-efficiency in CC75 than in WC100. We recommend future work to investigate the long-term performance of CC75.
Hoover, N.L., M.L. Soupir, R.D. VanDePol, T.R. Goode, and J.Y. Law. 2017. Technical note: Pilot scale denitrification bioreactors for replicated field research. Applied Engineering in Agriculture. 33(1): 83-90. DOI 10.13031/aea.11736 DOI: 10.13031/aea.11736
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
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
Schaefer, A., K. Werning, N.L. Hoover, U. Tschirner, T.B. Moorman, G. Feyereisen, A.C. Howe, M.L. Soupir. (in press) Impact of flow on woodchip properties and subsidence in denitrifying bioreactors. Agrosystems, Geosciences & Environment
Other activities and accomplishments:
Curriculum for highschool or middle school Environmental Science/Biology/Engineering courses,”From Here to There with with Woodchip Bioreactors.” Developed by Eric Hall and the Soupir Lab, Iowa State University. Three modules, available at: https://hallscience.us/curriculum and https://www.oercommons.org/courseware/lesson/71904