System dynamics modelling as policy decision support for retaining and recycling water within the urban water system to address water scarcity – the case of the City of Cape Town

Van Zyl, Amore (2019-12)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Water scarcity is a worldwide phenomenon, identified as the number one global risk, that if neglected, can result in social and economic collapse. The City of Cape Town is one The results found that rainwater harvesting and decentralised wastewater reuse did not significantly decrease the water supply stress in the City. However, the effective reuse of greywater lowered the water stress experienced, while combining several intervention actions simultaneously had the most success in reducing water supply stress in the City. Furthermore, the model presented expected oscillatory behaviour due to the system’s reaction to constraints on water allocation. Finally, as time progress the impact of interventions declined, indicating that continuous evaluation and intervention actions are required to balance the growing demand. such urban region experiencing ongoing water shortages in conjunction with several other urban regions in South Africa. Subsequently, the need for innovative and sustainable solutions, to address the growing water deficit, has become essential to assure sustainable water resources for the future. The City is predominantly dependent on traditional water resources from the Western Cape Water Supply System that are vulnerable to climate change and rising water demands. Therefore, alternative sources of water serve to diversify the water resources for City consumption. Using an integrated approach to alleviate water stress in the City, value can be extracted from within the water system. Retaining and reusing water in the system and reducing water consumption are interventions that align with circular economy principles to zero waste for value generation. However, legislation aimed at extracting value from the urban water system need to be tested and evaluated to ensure sustainability in the City’s economic, social and environmental sectors. The urban water system is a complex system with interconnecting elements. To avoid unexpected outcomes as a result of implemented changes, a holistic approach to water management is required. Literature found system thinking to be a suitable approach for evaluating policy changes in complex water systems, providing insight and understanding to decision-makers. To develop a decision support model that represent the real-world system behaviour, technical and census data as well as case study research was used to link causal behaviour by constructing equations and stock-and-flow diagrams. The constructed system dynamics model, once validated, allowed for the development of policy scenarios. These scenarios tested the impact of utilising rainwater harvesting, greywater reuse and decentralised wastewater treatment plants together with consumption restrictions to alleviate water supply stress in the City over the period 2001-2040.

AFRIKAANSE OPSOMMING: Watertekort is ‘n wêreldwye verskynsel. Dit sluit ook die Stad Kaapstad in wat ernstige watertekorte in die gesig staar as gevolg van verminderende reënval in die area. Die stad is ’n digbevolkte verstedelikte metropool wat oorwegend op oppervlakwater staatmaak vanuit die Wes-Kaapse Watertoevoerstelsel. Oppervlak- en grondwater is tradisionele waterbronne, maar is kwesbaar vir klimaatsverandering en stygende wateraanvraag. Om die dreigende waternood wat in die streek ondervind word aan te spreek, sal alternatiewe waterbronne dien om diversifisering van waterbronne vir die stad se behoeftes, vry te stel. Om die waternood in die stad te verlig, kan waarde uit die huidige watersisteem verhaal word deur retensie en hergebruik van water binne die sisteem sowel as deur gebruiksvermindering. Die tipe intervensie is in lyn met sirkulêre ekonomiese beginsels wat lei tot nul vermorsing om waarde te genereer. Wetgewing wat daarop gemik is om waarde uit die stedelike watersisteem te genereer moet getoets en geëvalueer word om die volhoubaarheid van die stad se ekonomiese, sosiale en omgewingsektore te verseker. Dit moet daarop gelet word dat die stedelike watersisteem ‘n baie ingewikkelde sisteem is wat baie interverbindings bevat. Om onverwagte gevolge wat spruit uit veranderinge wat aangegaan word te vermy, is ‘n holistiese benadering ten opsigte van waterbestuur uiters noodsaaklik. Bestaande literatuur wys daarop dat sistemiese denkwyse die geskikste manier is om beleidsveranderinge ten opsigte van komplekse watersisteme te evalueer en ook om insig en begrip aan besluitnemers te verskaf. Om ‘n besluitsondersteuningsmodel, wat werklike sisteemsgedrag weergee, te ontwikkel is tegniese- en sensusdata sowel as gevallestudie navorsing gebruik om veroorsakende gedrag te koppel deur die gebruik van vergelykings en dinamiese vloei diagramme. Die opgestelde stelsel dinamika model, sodra gevalideer, laat vir die ontwikkeling van beleidscenarios toe. Die scenarios toets die impak van die gebruik van reënwater, gryswater hergebruik gedesentraliseerde afvalwatersuiwering asook gebruiksverminderingmeganismes op water spanning verligting en die watervoorraad vir die stad oor die tydperk 2001-2040. Die resultate het gevind dat die gebruik van reënwater en hergebruik van gedesentraliseerde afvalwater nie ’n beduidende impak op watervoorraad spanning in die Stad het nie. Egter, die effektiewe hergebruik van gryswater verlig die watervoorraad spanning oor die tydperk, maar daar word die meeste sukses behaal as verskillende intervensies gelyktydig aangepak word. Verder het die model verwagte ossilerende gedrag openbaar wat gekoppel is aan reaksies teenoor beperkings op waterallokasies. Daar moet ook op gelet word dat soos die tyd verloop het, het die effek van intervensies afgeneem wat daarop wys dat deurlopende evaluering en ingryping aksies nodig is om die groeiende vraag na water te balanseer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107026
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