Graduation Year

2024

Document Type

Thesis

Degree

M.S.C.E.

Degree Name

MS in Civil Engineering (M.S.C.E.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Mahmood Nachabe, Ph.D.

Committee Member

Sarina Ergas, Ph.D.

Committee Member

Mauricio E. Arias, Ph.D.

Keywords

Bioinfiltration, Bioremediation, Bioretention, Nutrients, Rapid

Abstract

Nutrient pollution in stormwater drives the eutrophication of inland and costal waterbodies which leads to sea grass retreat and the proliferation of harmful algal blooms (HAB). These anthropogenic effects destabilize ecosystems, and some HABs can pose direct human health risk. Bioretention, or the storage and controlled discharge of stormwater run-off in an ecologically engineered setting, is a potential solution to this problem. However, it relies heavily on the settling of particles as a nutrient removal mechanism, and thus struggles with pollutants, such as dissolved nitrogen, which is a particular problem in Florida where the geological prominence of phosphorus leaves nitrogen as the limiting factor in HAB proliferation. In addition, many bioretention systems require large portions of their respective catchment areas and function at low Hydraulic Loading Rates (HLR) which make them unsuitable for urban applications.

To better understand potential remedies to both problems, bench scale, column, and pilot testing of high permeability gravel (HPG) amended with biochar were conducted to determine the characteristics of the new mixed media, to evaluate runoff elimination at a high HLR, representing an extreme storm event, and to evaluate the mass removal efficiency of dissolved nitrogen species at a low HLR representing a median storm event. At the bench scale, HPG was amended with biochar at different volumetric ratios and tested for porosity and moisture retention, as these properties should determine the air to water ratio within the soil and govern whether aerobic nitrification or anerobic denitrification is promoted. It was observed that porosity and moisture retention increased proportionally as the biochar fraction increased. vii Column tests were performed to determine saturated hydraulic conductivity, which was found to decrease slightly, demonstrating a decoupling of porosity and saturated hydraulic conductivity that is suspected to be a consequence of biochar’s nanoscale internal porosity but warrants further study.

Finally, pilot scale bioretention cells were constructed out of rain barrels with different drain configurations, one free draining from the bottom and the other featuring a raised drain, creating an Internal Water Storage Zone (IWSZ) from which water may drain. These systems were subjected to two different design storm events, a 10-year event using an HLR of 11.8 cm/min and a median storm event using a 2.4 cm/min HLR. Synthetic stormwater was created by pumping water from Lake Behnke, an urban stormwater pond on USF campus, and then spiking with chemicals to achieve dissolved Total Inorganic Nitrogen (TIN) concentrations consistent with urban stormwater runoff (0.8-1.0 mg NOx-N/L, 0.3-0.6 mg NH4+-N/L). Both systems were able to process the runoff of both storms without ponding more than 15 cm and were shown to have positive TIN mass removal efficiencies ranging from ~14-58%. The free drainage system favored nitrification and the raised drainage system favored denitrification, which is to be expected considering their relative oxygen distributions. However, TIN removal efficiency for both systems differed by about 2% within each design storm, suggesting similar denitrification occurred across systems. Both systems performed better under the lower HLR, with removal efficiency dropping by about 10% under higher loading. This level of passive nutrient treatment in stormwater is promising, similar systems would be suitable for urban settings where this level of removal is sufficient to bring dissolved N concentrations into regulatory compliance, or as the first line in a treatment train.

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