Graduation Year

2024

Document Type

Thesis

Degree

M.S.E.V.

Degree Name

MS in Environmental Engr. (M.S.E.V.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Sarina Ergas, Ph.D.

Co-Major Professor

Mahmood Nachabe, Ph.D.

Committee Member

Qiong Zhang, Ph.D.

Keywords

Controlled release fertilizer, Internal Water Storage Zone, Nitrogen, Non-point source, Phosphorus

Abstract

Excess inputs of nutrients, such as nitrogen and phosphorus, to surface water bodies can lead to the presence of harmful algal blooms (HABs). HABs can damage their surrounding environments and the economies dependent on them. Nutrient pollution can be properly controlled and monitored at point-sources, such as wastewater treatment plants, but is more difficult to control for non-point sources, such as fertilizer runoff. One form of fertilizer runoff that remains overlooked is runoff from plant nurseries. Controlled release fertilizers (CRFs) embedded in plant containers at nurseries can leach excess nutrients into the runoff. For nurseries in Florida, best management practices (BMPs) include preventative measures to prevent nutrients from leaching into the runoff; there are no BMPs that are designed for runoff treatment.

Bioretention systems are designed to control stormwater runoff by filtration and biodegradation. Modifications in a bioretention system’s design have the potential to promote nutrient removal from nursery runoff. An addition of an internal water storage zone (IWSZ), caused by changing the outlet configuration and an addition of woodchips, creates an anoxic environment that favors denitrification. Biochar is an inexpensive, carbon negative material, that exhibits chemical and physical properties that promote nutrient removal. Biochar’s high cation exchange capacity (CEC) promotes NH4+ adsorption and its high water holding capacity helps retain moisture between irrigation events. However, it is unknown whether these properties are useful in the IWSZ of a modified bioretention system. Lastly, vegetation can be planted on bioretention systems to promote rootzone aeration and microbial activities. Plants also provide an additional pathway for nutrient removal through plant uptake. There are two goals of this study: (1) to investigate the performance of biochar amended modified bioretention systems with varying designs and conditions, and (2) to investigate the significance biochar amendment in the IWSZ has on the nutrient removal of modified bioretention systems.

Two pilot-scale biochar amended modified bioretention systems were constructed at a commercial plant nursery in Lithia, FL. One system was completely biochar amended (CBA), and the other system was only amended with biochar above the IWSZ, or partially biochar amended (PBA). Each system was configured so that the outlet height could be adjusted to vary the hydraulic retention time (HRT) in the IWSZ. A study was performed on the nursery irrigation runoff to understand changes in flow rates and nutrient loads during an irrigation event. A tracer study was done on both units at each outlet height. The systems were monitored for three events each with a low, medium, and high hydraulic loading rate (HLR, 9 total events). The HLRs represented a bioretention system covering 5%, 2.5%, and 1% of the greenhouse area, respectively. After monitoring over varying HLRs, six additional events were monitored at the high HLR: three at an elevated outlet height, and three after Muhly Grass vegetation was planted on each unit. A one-way analysis of variance (ANOVA) test was performed at a 95% confidence interval to determine the statistical significance of these design factors on nutrient load reductions over each event.

Runoff pollutant loads increased over time as irrigation occurred possibly due to a greater dissolution of controlled release fertilizers. From the tracer studies, the microporosity of biochar prevented shortcutting and increased the mean retention time spent in the IWSZ. Average total load reductions improved for both total inorganic nitrogen (TIN) and total phosphorus (TP) as the HLR decreased, height of the IWSZ increased, and with biochar amendment in the IWSZ. The average total load reduction improved for TP yet declined for TIN after the addition of Muhly Grass vegetation. The increase of HLR from 0.11 cm/min to 0.55 cm/min, and biochar amendment in the IWSZ had a statistically significant impact on the TIN load reduction between each event. Statistical significance was not found for any other design factor or on TP load reduction due to the high variability of load reductions between events. The highest total load reduction was 85.6% for TIN and 89.2% for TP. These occurred from the CBA unit at the HLR representing 2.5% and 5% of the greenhouse area, respectively. This work suggests that installing properly designed biochar amended modified bioretention systems can effectively remove nutrients from commercial nursery runoff. Mathematical modeling should be used to optimize these systems under varying HLRs, IWSZ heights, media characteristics, and irrigation durations.

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