Doctoral Degrees (Civil Engineering)

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Browsing Doctoral Degrees (Civil Engineering) by Subject "Biofiltration"

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    Plant biofiltration for urban stormwater runoff purification in South Africa
    (Stellenbosch : Stellenbosch University, 2022-04) Jacklin, Dylan Michael; Brink, Isobel; Jacobs, Shayne Martin; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: A major consequence of urbanisation is the large-scale conversion of pervious to impervious surfaces, which significantly alters the natural hydrological cycle in terms of both hydrology and quality. Together with climate change, urbanisation and the associated stormwater runoff are regarded as major threats to water resource security worldwide, due to the variety of pollutants generated and transported. Conventional urban stormwater management rapidly collects and transports runoff for discharge into the nearest watercourse, triggering a plethora of public and ecological health concerns. In response, the Water Sensitive Urban Design (WSUD) concept, of which plant biofiltration is a component, is increasingly preferred as a sustainable alternative to conventional systems as it considers stormwater as a resource to protect rather than a substance of which to dispose. Plant biofiltration promotes a spatial network of passive, ecologically sound treatment solutions to the diffused nature of urban stormwater pollution, for discharge into existing drainage systems or watercourses. South Africa, one of the most rapidly urbanising countries in Africa, experiences some of the worst environmental deterioration globally. Stormwater runoff discharge, together with aging and defective conventional treatment systems, threaten the country’s already limited freshwater resources. Therefore, the South African Water Research Commission seeks to promote the adoption of WSUD, thus representing a significant shift from the linear drainage strategy currently adopted at local level to a holistic management approach of the urban water cycle and its integration into urban design. Although the value of ecosystem services is increasingly recognised, WSUD and particularly plant biofiltration as one of its components, is under-utilised, as the current framework in South Africa only provides broad philosophical guidance lacking scientific premise for practical design considerations. Variable performances have been reported in standard and modified plant biofilters, stagnating treatment optimisation knowledge. The difficulty of plant biofilter optimisation stems from the complex pollutant removal processes, which vary between physical designs and operational conditions. Therefore, appropriate design demands that both engineering hydrology and the scientific functioning of natural elements be considered; however, the latter is not currently included in the training of the civil engineer who can be the professional responsible for plant biofilter planning and design. Furthermore, current plant biofilter models insufficiently account for design modification and its associated removal processes. Limited local research and design specifics are currently available, which has resulted in injudicious plant selection and erroneous plant biofilter design, inhibiting treatment performance and threatening the recipient site’s natural biodiversity. Thus, the main aim of this research is to advance knowledge in stormwater plant biofiltration for improved urban water management in South Africa. This research initially presents potential phytoremediators, plants for the in situ treatment of pollutants, which are indigenous to the Western Cape, South Africa, as an aid to the practicing engineer for use in local plant biofiltration initiatives. Although chosen plant species were from the local Western Cape area for logistic reasons (the University is situated in the Western Cape), the techniques presented to identify species are transferable to other biogeographic areas. Informed by this undertaking, a phyto-guide is developed for identifying novel phytoremediators, adept at adjusting to the recipient habitat’s dynamic conditions and further incentivised by South Africa’s extensive biodiversity. The initial approach to plant biofilter optimisation investigated four engineered materials as potential growth media amendments, promoting attapulgite combinations for use in small-scale stormwater biofilters in the spatially constrained urban area. Progressing with plant biofilter optimisation, nine indigenous South African biofilters were investigated as effective yet sustainable alternatives to exotic phytoremediators, and Prionium serratum, among others, was found to exhibit enhanced removal capabilities. Physically modifying the plant biofilter as the final component to optimisation, following growth media and plant species, showed that combining standard biofiltration techniques with upflow filtration, plenum aeration and anaerobic zone saturation is the most efficient solution; removing on average 96% of synthetic stormwater loads. The novel sequential modifications between designs highlighted pollutant-specific removal processes and proffered plant biofilter designs for optimised treatment performance. Empirical findings based on the data captured by the preceding investigations contribute statistical output for future local in-depth modeling endeavours. Additionally, in developing the conceptual deterministic plant biofilter model for stormwater treatment, possible applications of existing models for the various nutrient, and potentially heavy metal, removal processes are summarised. In conclusion, this research contributes physical design specifics and both experimental and mathematical models to urban stormwater treatment researchers and practitioners, constantly improving the understanding of plant biofilter complexity.