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
Earlier this year, analysis of the previous season’s tests (tracer tests and woodchip analysis) were completed. The second year of sample collection at the pilot-scale bioreactors began in March, with samples initially collected weekly for nutrient and dissolved gas analysis, then increased to twice per week. Repairs and maintenance to pumps and meters have been necessary. The second-year sample collection and analysis of the data will continue into the next reporting period. A proposal has been submitted to INRC for a third year of project funding.
Three project-related presentations are planned for July. Natasha Hoover will present an overview of the pilot-scale bioreactor work at the International Soil and Water Conservation Society conference in July, and two master’s students will give presentations at the ASABE Annual International Meeting in July.
This was the first year of operation with the corn cob modifications to six of the nine pilot-scale bioreactors. Sample collection was scheduled twice per week during operation for dissolved carbon, ammonia and nitrate/nitrite. Dissolved gasses were sampled, and dissolved oxygen levels were measured once per week. The water quality lab acquired a new analyzer and has developed a new methodology to analyze nitrite separately from nitrate. The samples have been analyzed in the lab, and data analysis will be completed for the next report.
Either ground corn cobs or whole corn cobs broken into 2-3 pieces were placed inside mesh aquarium bags and placed in the sample wells of the corn cob modified bioreactors. Two bags of each were placed in each well. Moisture content was analyzed on the corn cobs that were placed in the wells so that measurements can be compared in 2021 when the bags are removed for analysis. Tracer tests were conducted on each of the pilot-scale bioreactors to evaluate the flow characteristics with an estimated 4-hr hydraulic residence time (HRT) for each bioreactor. The samples have been analyzed to provide valuable information about the differences in flow characteristics with bioreactors filled with woodchips and those with the ¼ and ¾ corn cob replacement. Drainable porosity, the volume of water that could be drained from the saturated bioreactors, was measured at the end of the 2019 experimental season. Flow was stopped to the bioreactors, and the depth of water in each bioreactor was measured. The bioreactors were then drained and the total flow volume was measured at the outlet plumbing of each bioreactor. The drainable porosity data will be useful for accurately estimating the HRT, which is dependent on the flow rate and saturated pore volume of each bioreactor. Ultrasonic meters that were purchased and installed at the beginning of this sampling season were effective at improving inlet flow rate measurements and maintaining more precise flow rates.
Outreach this period included 4 field days and 3 presentations.
The pilot-scale bioreactors were modified with a change in fill material in the fall of 2018. This was the first reporting period with experimentation and data collection at the corn cob filled pilot-scale bioreactors. General maintenance was conducted this spring to improve flow control and measurement and to repair and/or replace damaged plumbing and equipment.
Mesh bags were filled with ground corn cobs, similar to the woodchip-filled bags that were deployed during the installation of the bioreactors in 2014. These bags will be secured to a nylon rope and added to the sampling wells in the corn cob-filled bioreactors for future analysis of microbial populations on the fill materials. An incoming graduate student with the Water Quality Research Lab (WQRL) has been trained to sample and maintain the pilot-scale bioreactors during the summer and has been conducting sample and data collection weekly with Morgan Davis’ research group. The student is planning her graduate research component related to the bioreactors. Water samples collected to date are preserved and in line for analysis.
Potassium bromide tracer tests at a target 4-hour hydraulic residence time were conducted in June and early July at each of the bioreactors to evaluate flow characteristics for comparison among the various fill compositions (woodchips, 3/4 woodchip/ 1/4 corn cobs, 1/4 woodchip / 3/4 corn cobs). Analysis of the test results will be completed and included in the next semi-annual report.
Outreach this reporting period included a visit to the pilot-scale bioreactors on June 12 as part of the Leopold Center Board Meeting.
Converting the bioreactors from all woodchip bioreactors to a combination of woodchip and corncob fill materials in different proportions was completed during this quarter. The initial plan for excavating all bioreactors and mixing the existing woodchips for refill was reevaluated, and it was decided that the most practical (due to equipment capabilities) and cost-effective excavation plan would be to leave three bioreactors intact (not removing the existing woodchips), and partially excavating the other six bioreactors to fill with one-fourth or three-fourths corn cobs at the inlet ends. The sampling wells in the excavated portion of each bioreactor were replaced. During excavation, multiple samples of woodchips from the bioreactors and corn cobs from the stockpile were collected for particle size analysis and additional evaluation. The completed bioreactors were saturated with drainage or well water to keep rodents from burrowing into the bioreactors over winter. Supplemental work with collected corn cobs has been conducted in the laboratory, and analysis is ongoing.
Preparations have been made for the excavation and removal of woodchips from the pilot-scale bioreactors. A meeting was held in September to begin planning for this phase of the pilot-scale bioreactor study. It was determined that all bioreactors will be excavated and the woodchips will be mixed to ensure even distribution of chip sizes in the bioreactors. Internal plumbing repairs and modifications will be made while the bioreactors are empty, including anchoring the sampling wells to the flanges with screws, instead of relying on glue, to ensure the wells remain in place, and installing a diffusion pipe at the outlet end of each bioreactor. A contractor will be hired to complete the woodchip removal this fall.
Additional plumbing modifications discussed in September include reducing the inlet pipe and valve sizes to improve flow rate control and shortening the float control length in the supply tank to reduce the pressure variation impacting achieved flow rates.
Corn cobs were ordered from Green Products and delivered to the bioreactor site September 25th.