PhD Oral Exam - Antônio Cavalcante Pereira, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Surface water remediation has been a subject of extensive debate worldwide, as nutrient pollution, particularly phosphorus (P), is projected to increase in the coming years. Anthropogenic activities such as deforestation near water bodies, overfertilization of soils, improper livestock practices, unplanned land-use changes, insufficient nutrient removal in wastewater, and the impacts of climate extremes and colonialism are major factors contributing to the upsurge presence of this limiting nutrient in aquatic systems. These nutrient streams, concomitant with the former external ones (i.e., legacy pollution discharge), accumulate P on suspended solids and sediment of the waterbodies, thereby becoming internal nutrient sources. Because no specific regulations are in place to mitigate this rising P nutrient pollution, these water bodies are increasingly threatened by eutrophication and its associated concerns, and the problem is being passed on to future generations. Effective methods for remediating eutrophic waters and P-rich sediments include sediment dredging, sediment capping, hypolimnetic aeration/oxygenation, algaecide addition, controlled management of macrophyte or fish populations, and various combinations of these. Although P remediation methods are established, they remain inadequately sustainable, inconsistently implemented, and underreported worldwide. This is due to business-as-usual environmental practices, climate colonialism and its persistent perspectives, and the absence of effective circular-economy management strategies for waste produced. Consequently, this thesis aims to investigate and elucidate a feasible P attenuation method that involves the entire lake system, and to propose a new, practical, easily reproducible remediation technique for preventing or attenuating eutrophic scenarios related to P-enriched particles management in the sediment and suspended solids using geotextiles. For this, the method investigated mesoeutrophic water filtration through a non-woven geotextile tube to remove polluted suspended solids and, thereby, particulate P, and air-induced sediment resuspension, followed by geotextile bag filtration to attenuate P-enriched sediment particles. The water geotextile filtration experiment was performed in situ in a silt-curtain-enclosed area at Lake Caron (a shallow mesoeutrophic lake in Quebec, Canada), and sediment resuspension was conducted in the laboratory using sediment samples from the same study lake. After employing both practices, a circular-economy approach for reusing the highest-volume waste generated (i.e., captured suspended solids and sampled/captured sediment) was studied. Results show that attenuation of suspended solids in eutrophic water reduced particulate P availability in the water column, thereby suppressing algal growth. Studied lake water P, initially in a mesoeutrophic trophic state, was brought to a mesotrophic level boundary using the proposed filter media and filtration method in the enclosed area in situ. Furthermore, most of the P in the lake's cohesive sediment, which acts as a fluid-like slurry, resides in particles smaller than 45 µm, is redox-sensitive, and is closely linked to organic matter. A 3-minute sediment resuspension did not cause significant leaching of heavy metals, soluble reactive phosphorus, or dissolved organic carbon into the system. The proposed non-woven geotextile filter combinations statistically significantly attenuated concentrations of suspended sediment particles, total organic carbon, and total phosphorus in the filtrate in both cone and bag configurations, demonstrating their scalability. Additionally, P-mass in the sampled sediment decreased by 23% using the proposed method. Reusing remediation waste materials and filtration by-products showed that P-enriched sediment and suspended solids have some agronomic benefits, but only when incorporated into balanced substrates rather than heavily sediment-loaded ones. Lake Caron sediment has the potential to support direct plant growth at low- to intermediate-sediment levels on the following wt:wt ratios: 95% substrate:5% sediment and 75% substrate:25% sediment. Assessments also confirmed minimal ecotoxicological risks associated with this sediment. Subsequently, the pot experiments conducted using Lepidium sativum, throughout all watering regimes and mixture treatments, showed that seed germination characteristics depended more on the substrate employed than on the watering method. Biomass production increased on sediment-rich substrate, and plant nutrient uptake was clearly demonstrated by the results, with no greater heavy-metal bioaccumulation compared with the 100% substrate (normal soil local condition). Also, the results suggested that intermittent application of captured suspended solids for plant watering was beneficial to the studied plant. Therefore, restoring eutrophic aquatic ecosystems for current and future generations requires a comprehensive, region-specific strategy that combines internal P reduction with sustainable regulation of external inputs. This work demonstrates that integrating in situ geotextile filtration and sediment resuspension remediation, followed by geotextile filtration, coupled with circular reuse pathways, can transform P lost to water bodies into a manageable resource, providing a scalable, reproducible and sustainable route to curb eutrophication and any associated event.