Graduation Year

2025

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Sarina Ergas, Ph.D.

Co-Major Professor

Mahmood Nachabe, Ph.D.

Committee Member

E. Christian Wells, Ph.D.

Committee Member

Aydin Sunol, Ph.D.

Committee Member

Jeffrey Johnson, Ph.D.

Keywords

Ammonium, Elevated Drainage, Free Drainage, Nitrate, Stormwater Runoff

Abstract

Urban stormwater runoff can transport high nutrient loads to receiving waters, leading toeutrophication, harmful algal blooms, and the formation of hypoxic zones. Bioretention systems are a form of green infrastructure designed to mitigate these impacts, but their application in dense, ultra-urban environments is often limited by large land area requirements. This has led to the adoption of high permeability media (HPM) to accommodate higher hydraulic loading rates (HLRs) and reduce the system footprint. However, conventional HPM systems often show inconsistent removal of dissolved nitrogen species due to short hydraulic retention times (HRTs), a lack of organic carbon, and insufficient anoxic conditions suitable for denitrification.

The goal of this dissertation was to investigate and optimize strategies for enhancing dissolved nitrogen removal in HPM bioretention systems. This research provides a comprehensive understanding of the mechanisms driving nitrogen transformations by systematically evaluating the combined effects of two key factors: media amendments using biochar and wood chips, and hydraulic designs comparing free drainage with elevated drainage configurations. The research was guided by a multi-scale approach, progressing from abiotic batch experiments and laboratory-column studies to unvegetated and vegetated pilot-scale systems.

In Objective 1, abiotic nitrogen removal mechanisms of biochar were characterized by quantifying its adsorption capacity and kinetics for ammonium (NH4+-N) and oxidized nitrogen (NOx-N = NO2--N + NO3--N) in batch studies with concentrations typical of urban stormwater. Results showed that biochar had a significantly higher adsorptive capacity for NH4+-N than for NOx-N, which is attributed to its cation exchange capacity. The Langmuir isotherm model provided the best fit for the experimental data, with a maximum adsorption capacity (qmax) for NH4+-N of 0.13 mg/g, indicating monolayer adsorption. Kinetic studies revealed that the adsorption process was best described by the pseudo-second-order model, suggesting that the rate-limiting step is chemisorption. This process involves direct chemical interactions, such as the sharing or exchange of electrons, between the ammonium ions and functional groups on the biochar's surface.

In Objective 2, the effects of biochar amendment on the hydraulic characteristics of HPM and the resulting impact on nitrogen removal were evaluated in column-scale systems. HPM columns that were not amended with biochar demonstrated limited nitrogen removal performance; the free-drainage (FD) configuration was a net source of oxidized nitrogen (median export = -3.12% NOx-N). The addition of biochar, however, caused a large and statistically significant improvement in nitrogen removal in both FD and ED configurations (p < 0.05). The addition of biochar also improved performance by altering the media's physical properties; it increased the saturated hydraulic conductivity, porosity, and overall moisture retention. Tracer studies confirmed that biochar created a more tortuous flow path that more than doubled the mean solute retention time (MRT) in the FD configuration from 5.7 to 13.7 minutes.

In Objective 3, the effects of wood chips and drainage configuration on nitrogen removal performance were evaluated in laboratory-scale column studies. A statistical analysis of aggregated data showed no statistically significant difference in TIN removal between the HPMB FD and ED columns (p = 0.356). The addition of wood chips shifted the HPM FD column from exporting NOx-N to a net remover; however, it did not provide a statistically significant improvement in TIN removal for the already high-performing HPMB columns (p > 0.18).

In Objective 4, the dominant nitrogen removal pathways and performance trade-offs were investigated in pilot-scale systems that included a wood chip mulch layer and vegetation (Muhlenbergia capillaris). Pilot-scale studies confirmed that drainage design dictates the primary nitrogen transformation pathway. During the initial phases (I & II), the FD system showed higher NH4+-N removal (Median = 37.7%) via nitrification, while the ED system was more effective at removing NOx-N (Median = 44.2%) (Table C5). The aggregated statistical analysis (Table C5) revealed that adding a wood chip mulch layer (Phases III-IV) created a significant trade-off. It significantly increased NH4+-N removal in both systems (p < 0.001) but also significantly decreased NOx-N removal (p < 0.05). These competing effects canceled each other, resulting in no statistically significant change in overall TIN removal (p > 0.5). Finally, introducing plants (Phases V-VI) created another performance trade-off: while NH4+-N removal was significantly enhanced in the ED system (p = 0.005), driven by a combination of direct plant uptake and enhanced microbial activity in the rhizosphere, overall TIN removal was significantly hindered in the FD system (p = 0.022), most likely because plant root activity disrupted the anoxic conditions required for denitrification.

This dissertation provides a comprehensive, multi-scale analysis of nitrogen removal mechanisms in modified bioretention systems. The findings highlight the trade-offs between competing nitrogen transformation pathways and demonstrate that amending HPM with biochar and using a simple FD configuration is an effective and practical strategy for managing nitrogen in urban stormwater.

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