Improving performance and expanding application of denitrifying bioreactors for nutrient management in agricultural landscapes

Subsurface tile drainage is widely implemented across agricultural landscapes in the U.S. Midwest, creating direct hydrologic pathways that transport excess nutrients from fields to surface waters. As a result, nitrate (NO₃) can bypass natural soil filtration and attenuation processes, contributing to elevated nutrient loads in downstream ecosystems and increasing risks to environmental and human health. To address these challenges, denitrifying bioreactors have been developed as an edge-of-field conservation practice to intercept tile drainage and promote microbial denitrification within a carbon-rich, anoxic environment. Optimizing the design and performance of these systems is critical for achieving regional nutrient reduction goals and mitigating downstream impacts. The first study (Chapter 3) evaluated the performance of corncob (CC) versus woodchip (WC) bioreactor media across a range of flow conditions and identified key drivers of performance. Results demonstrated that CC consistently outperformed WC, achieving greater nitrate removal efficiency (NRE) and NO3 load removal. Total organic carbon (TOC) and hydraulic residence time (HRT) were identified as primary drivers of performance, while targeted nitrogen-cycling gene abundances (nirS, nirK, nosZI, nosZII) were generally unreliable predictors of NRE in both media types. The second study (Chapter 4) used a four-year field-scale dataset to evaluate saturated water depth and test common hydraulic assumptions on bioreactor performance for NO3 removal. A saturated depth coefficient was developed using a grid search and then applied to inlet and outlet water depths to generate HRT estimates comparable to reference values derived from internal well measurements. The internal water table at this study site was found to be fundamentally non-linear, as typically assumed. Linear assumptions resulted in an overestimation of HRT and underestimation of NO3 load removal. The third study (Chapter 5) evaluated the potential to expand bioreactor application by determining if an existing system could effectively remove E. coli. A field-scale bioreactor was retrofitted with a pump to deliver surface water to the system, allowing comparison of pathogen removal between tile drainage and surface water sources. The system achieved significant E. coli removal, with influent E. coli concentration and water temperature significant to removal.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

dissertation

Discipline thesis:degree_discipline

Environmental science

Grantor

Iowa State University - Thesis & Dissertation

Year dc:date.issued

2026

Author dc:creator

• Vroman, Taylor

Advisor dc:contributor.advisor

• Soupir, Michelle, L

• Agricultural and biosystems engineering

Language dc:language.iso

en_US

Repository record dc:identifier.uri

https://dr.lib.iastate.edu/handle/20.500.12876/106779

OAI identifier oai:identifier

oai:dr.lib.iastate.edu:20.500.12876/106779