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

Date: 
Aug 2021

Issue

Preliminary bioreactor work has shown that corncobs were a promising carbon source for nitrate removal from from agricultural water and showed improved NO3-N removal for all flows studied. However, better understanding of the corncob lifespan is needed to better estimate N removal costs for corncob reactors, relative to woodchip reactors.

Objective

The overall goal of this study is to compare nitrogen fate in corncob and woodchip bioreactors at the pilot scale.

Approach

Evaluation of the longer-term performance of bioreactors will be done with three different carbon sources 1)woodchip, 2) corncob-25 (25% corncob by length or volume) , and 3) corncob-5 (75% corncob by length or volume) . This will be done by monitoring total organic carbon, nitrate, ammonium, nitrous oxide, and methane (dissolved GHG) at the bioreactor inlet, within the bioreactor, and at the bioreactor outlet. Additionally, the research team will refine the existing technoeconomic analysis (TEA) to determine $/kg N removed using longer-term performance data and identify long-term supply sources of corncobs.

Project Updates

Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.

December 2022

Due to the lack of inflow, researchers shut down the bioreactor on July 25th, which has not been able to be restarted due to persisting drought condition. During the downtime between August and September, the team inspected and repaired worn bioreactor parts, including pumps and other plumbing components. Early freezing conditions led to winterizing and storing the smaller parts and equipment in the Agricultural Engineering shed in mid-October. Research Farm staff winterized the larger influent tanks. All nitrate and ammonia samples collected in 2022 were analyzed.

Observations included:

- Nitrate removal efficiencies of 11-15%, 17-25%, and 26-38%, respectively, in the WC100 (100% woodchips), CC25 (25% corncobs + 75% woodchips), and CC75 (75% corncobs + 25% woodchips) bioreactors.

- Relatively higher nitrate removal efficiency in corncob-amended woodchip bioreactors than in woodchip-only bioreactors at lower temperatures.

- The production and removal of ammonia in all bioreactors.

- No consistent trend was detected between the carbon treatments.

The 2022 data will be aggregated with previous years’ data to perform additional analysis. The analysis of TOC samples collected in 2022 was delayed because the previous collaborator capable of analyzing these samples has retired. A new collaboration with a lab in the Agronomy Department allowed us to analyze 120 of the 390 collected samples in December. A preliminary data analysis indicated no difference in TOC leaching between the carbon treatments. The remaining samples will be analyzed in early 2023.

Bromide tracer tests were conducted on all bioreactors between May 11th and June 14th. These samples were stored at 4ºC until analysis. As mentioned, the previous collaborator capable of analyzing these samples has retired, thus delaying the analysis. As a new chromatography analyzer became available in the Department of Civil Engineering, we analyzed bromide samples from five of the nine bioreactors between November and December. Samples will continue to be analyzed as the instrument becomes vacant in early 2023.

Other activities included two field days/tours and work on a related publication submitted to the Journal of Environmental Management.  

June 2022

research bioreactor equipment at field siteThe instrument installation at the pilot bioreactor site began on March 25th, but the bioreactors were not operated due to the prolonged overnight freezing conditions in the spring. The bioreactors began operation on May 4 after replacing several old, damaged plumbing parts. Weekly samples were collected beginning on May 6.

The samples are analyzed for nitrate+nitrite, total organic carbon, dissolved nitrous oxide and dissolved methane. Flow, temperature and dissolved oxygen data were also collected during each sampling date. Bromide tracer tests were conducted on all bioreactors between May 11-June 14. The bromide samples are stored at 4ºC until analysis in Summer 2022. The data will be used to calculate the hydraulic efficiency, short-circuiting potential and Morill Dispersion Index of each bioreactor. These parameters will be compared to the initial parameters when the corncob bioreactors were installed in 2019. Additionally, comparisons between the bioreactors will be made.

Water quality and dissolved greenhouse gas samples will continue to be collected weekly through October when the flow is present.

A field tour was conducted at the pilot-scale bioreactor demonstration site on March 8 as part of a workshop led by Eric Hall (ISU Biotechnology Outreach Education Center Program Coordinator). The attendees included 12 Keokuk High School AP biology students and two teachers. During the field trip, Ji Yeow Law provided an overview of the bioreactor site and its purpose for field research. He also shared the research need, current research, future research and applications of bioreactors.  

On June 29, a field tour was conducted at the pilot-bioreactor demonstration site as part of the Research Experience for Teacher (RET) workshop led by Hall. Attendees included nine high school teachers from Iowa and other surrounding states. During the field trip, Ji Yeow Law provided an overview of the bioreactor site and its purpose for field research. He used nitrate test strips to visually present the nitrate concentrations measured at the bioreactor inlet and outlet and discussed some preliminary results from these corncob and woodchip bioreactors.

January 2022

The lack of precipitation in 2021 resulted in insufficient flow to keep all nine pilot-scale bioreactors (PBR) flowing. Therefore, there were no water samples collected during this reporting period. A tracer study initially planned to be conducted in fall 2021 was also postponed until spring 2022.

In November, all PBRs were partially excavated near the inlets to extract woodchips and corncobs samples. Subsidences were measured during the excavation process. The extracted woodchips and corncobs samples were analyzed for internal porosity, particle density and particle size distribution. The samples were also processed and sent to the USDA collaborator in Minnesota for analysis of carbon quality (e.g., carbon, nitrogen, lignin and cellulose contents).

A permeameter was constructed in the lab and will be used to determine the hydraulic conductivity of carbon media in bioreactors.

Other activities and outreach included a virtual field trip in July, which was part of a high-school teacher workshop led by Eric Hall (ISU Biotechnology Outreach Education Center Program Coordinator), conducted at the bioreactor site. In collaboration with INRC, several educational/outreach video shoot sessions were conducted in the lab and at the PBR site in November. The video content includes current issues, research approaches and future research needs.

The project was also featured in an oral session at the 2021 ASA, CSSA, SSSA International Annual Meeting, which covered the comparisons of nitrogen removal rate and cost between standard woodchip bioreactors and corncob-woodchip bioreactors A manuscript was developed to disseminate the early findings of this project. The manuscript discusses and compares nitrogen removal efficiency, rate, and cost between standard woodchip bioreactors and corncob-woodchip bioreactors. The manuscript is currently under revision and will be submitted in early 2022.

In addition, several upcoming presentations are planned for an Innovative Practices Workshop: Woodchip Bioreactors, funded by a REAP CEP grant.

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