Plant biofiltration for urban stormwater runoff purification in South Africa

Thesis (PhD)--Stellenbosch University, 2022.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.AFRIKAANSE OPSOMMING: Groot nagevolge van verstedeliking is die grootskaalse omskakeling van deurlaatbare na ondeurdringbare oppervlaktes, wat die natuurlike hidrologiese siklus aansienlik verander in terme van beide hidrologie en kwaliteit. Tesame met klimaatsverandering word verstedeliking en die gepaardgaande stormwaterafloop beskou as primêre bedreigings vir waterhulpbronsekuriteit wêreldwyd, as gevolg van die verskeidenheid besoedelstowwe wat gegenereer en vervoer word. Konvensionele stedelike stormwaterbestuur versamel en vervoer vinnig afloop vir afvoer in die naaste waterloop, wat 'n oorvloed van openbare en ekologiese gesondheidsprobleme veroorsaak. In reaksie hierop word die “Water Sensitive Urban Design” (WSUD)-konsep, waarvan biofiltrasie 'n komponent is, toenemend verkies as 'n volhoubare alternatief vir konvensionele stelsels, aangesien dit stormwater beskou as 'n hulpbron om te beskerm eerder as 'n stof waarvan ontslae geraak moet word. Plantbiofiltrasie bevorder 'n ruimtelike netwerk van passiewe ekologies gesonde behandelingsoplossings vir die verspreide aard van stedelike stormwaterbesoedeling, vir afvoer in bestaande dreineringstelsels of waterlope. Suid-Afrika, een van die mees verstedelikte lande in Afrika, is besig om vinnig te verstedelik en ervaar van die ergste omgewingsagteruitgang wêreldwyd. Stormwaterafloopafvoer tesame met veroudering en gebrekkige konvensionele behandelingstelsels bedreig die land se reeds beperkte varswaterbronne. Daarom poog die Suid-Afrikaanse waternavorsingskommissie om WSUD te bevorder, wat dus 'n beduidende verskuiwing verteenwoordig van die huidige lineêre dreineringstrategie na 'n holistiese bestuursbenadering van die stedelike watersiklus en die integrasie daarvan in stedelike ontwerp. Alhoewel die waarde van ekosisteemdienste toenemend erken word, word WSUD onderbenut, aangesien die huidige raamwerk in Suid Afrika slegs breë filosofiese leiding verskaf sonder wetenskaplike uitgangspunte. Wisselende optredes is egter in standaard en gemodifiseerde plantbiofilters gerapporteer, wat gevolglik lei tot die stagnering van behandelingsoptimalisering. Die probleem van plantbiofilteroptimalisering spruit uit die komplekse besoedelingsverwyderingsprosesse, wat wissel tussen fisiese ontwerpe en operasionele toestande. Daarom vereis toepaslike ontwerp dat beide ingenieurshidrologie en die wetenskaplike funksionering van natuurlike elemente oorweeg moet word, met laasgenoemde tans nie ingesluit in die opleiding van die siviele ingenieur wat dikwels verantwoordelik is vir plantbiofilterbeplanning en -ontwerp nie. Verder slaag huidige plantbiofiltermodelle nie daarin om ontwerpmodifikasie en die gepaardgaande verwyderingsprosesse tot ‘n voldoende mate in te reken nie. Beperkte plaaslike navorsing en ontwerp spesifikasies is tans beskikbaar, wat gelei het tot onoordeelkundige plantseleksie en foutiewe plantbiofilterontwerp, wat behandelingsprestasie inhibeer en die ontvangerterrein se natuurlike biodiversiteit bedreig. Die hoofdoel van hierdie navorsing is dus om stormwaterplantbiofiltrasie te bevorder vir verbeterde stedelike waterbestuur in Suid-Afrika. Hierdie navorsing se aanvaklike doel was om potensiële fitoremediators vir oorsprong kontaminant behandeling wat inheems is aan die Wes-Kaap, Suid-Afrika, te versaf as 'n hulpmiddel vir die praktiserende ingenieur vir gebruik in plaaslike plantbiofiltrasie-inisiatiewe. Alhoewel gekose plantspesies om logistieke redes uit die plaaslike Wes-Kaap area kom (weens die Universiteit wat in die Wes-Kaap geleë is), is die voorgestelde plantspesies seleksie tegnieke oordraagbaar na ander biogeografiese gebiede. Op grond van hierdie onderneming word 'n fitogids ontwikkel vir die identifisering van nuwe fitoremediators, wat in staat daarvan is om aan te pas by die ontvangerhabitat se dinamiese toestande en verder aangespoor word deur Suid-Afrika se uitgebreide biodiversiteit. Die aanvanklike benadering tot plantbiofilteroptimering het vier gemanipuleerde materiale as potensiële groeimedia aanpassings ondersoek, wat attapulgietkombinasies vir gebruik in kleinskaalse stormwaterbiofilters in die ruimtelik beperkte stedelike gebied bevorder het. Met die optimering van biofilterontwerp is nege inheemse Suid-Afrikaanse plantbiofilters ondersoek as effektiewe tog volhoubare alternatiewe vir eksotiese fitoremediators en het Prionium serratum, onder andere, geïdentifiseer om verbeterde verwyderingsvermoëns te toon. Die fisiese wysiging van die plantbiofilter as die finale komponent tot optimalisering, opvolgende groeimedia en plantspesies, binne die ontwerpingenieur se beheer, het gekombineerde standaard biofiltrasietegnieke met opvloeifiltrasie, plenumbelugting en anaërobiese soneversadiging as die mees doeltreffende geïdentifiseer; met verwydering van gemiddeld 96% van sintetiese stormwaterladings. Die nuwe opeenvolgende wysigings tussen ontwerpe het besoedeling spesifieke verwyderingsprosesse uitgelig, sowel as plantbiofilterontwerpe vir geoptimaliseerde behandelingsprestasie.Doctora