Graduation Year

2023

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

Qiong Zhang, Ph.D.

Keywords

Aggregate Stability, Biochar, Dimensionless Numbers, Legacy Phosphorus, Specific Energy

Abstract

The Cargill ditch is located on the property of Se7en Wetlands, a 1600-ac treatment wetland system in Lakeland, Polk County, Florida. The Se7en Wetlands property was previously utilized for phosphate mining prior to the construction of the system. Nonpoint nutrient sources derived from remnants of abandoned phosphate mines – known as “legacy phosphorus” – become mobilized by stormwater runoff and impair surface water bodies by promoting harmful algal blooms (HABs). Thus, the Cargill ditch likely conveys nutrient rich flow resulting from legacy phosphorus and is thought to be one contributing factor in the occurrence of HABs within the treatment wetland cells. The objective is to implement a PRB utilizing an adsorbent media at the Cargill ditch in Lakeland, Florida. The media removes phosphorus from runoff through filtration, sedimentation, and adsorption processes. This study evaluates four types of media for use in a PRB at the Cargill ditch: three types of unmodified biochar materials (Sunshine Organics biochar, Biochar now, and Biochar Supreme) and a mineral-based adsorptive media called Phosflow.

Permeability, porosity, aggregate stability, column/adsorptive studies, and flume-scale tests were conducted. Two dimensionless parameters may be used to scale laboratory tests to field conditions. The Froude number can be used to make comparisons between laboratory flume experiments and onsite conditions at a selected cross-section of the Cargill ditch. The Stanton number has been used to evaluate the applicability of scaling between laboratory column experiments to understand how media thickness and flow rate impact the adsorptive performance of the media.

Laboratory experiments were conducted to evaluate the media characteristics and hydraulic behavior in a laboratory-scale barrier. A constant-head permeability test and porosity testing were completed on all four materials to assess the ease with which water flows through particles. Of the four materials tested, Phosflow yielded the largest value of permeability at 1.95 cm/s. Inter-aggregate (flow between particles) and intra-aggregate porosities (flow within the pore space of particles) also control implementation of a PRB in terms of build-up of flow upstream and adsorptive capacity. As phosphorus comes into contact with media it adsorbs to the media surface, eventually diffusing into the inner pore space or chemically precipitating out of the solution. Phosflow in particular was found to have high intra-aggregate porosity relative to the biochar materials. Wet and dry sieve analyses were performed on Phosflow and Sunshine Organics biochar to assess the stability or resistance to fragmentation, in which a stability index of 0.98 and 0.94 were determined. However, more fragmentation of particles was observed for the biochar. Both Phosflow and the Sunshine Organics biochar were placed in a flume-scale barrier to observe the relationship between flow rate and energy (loss) gradient across the barrier. It was found that at higher flow rates, there was a slight departure from the linear relationship between flow and head loss described by Darcy’s law, indicating transition to more turbulent flow patterns. Through the Froude numbers, implementing a PRB would also cause a transition from subcritical flow, compatible with existing conditions at the Cargill ditch, to supercritical flow. Finally, column experiments were conducted on Phosflow alone, considering its high stability, uniform particle size distribution, and expectations for adsorptive performance. For two columns of varying thicknesses and flow rates, adsorptive performances below 1 mg {\mathrm{PO}}_\mathrm{4}^{\mathrm{3-}}-P/g Phosflow were obtained, and a model utilizing the Stanton number as an input was fit to column removal data. Overall, these results show that dimensionless numbers, such as the Froude number and Stanton number, can be used to make inferences about the flow conditions and the rate of mass transfer observed for laboratory tests.

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