The relationship between consumer demand and pressure in a selected pressure managed zone in Cape Town

Master Thesis


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Water is a basic need and a limited resource across the world. Climate change, pollution, population growth, irrigation and urban development, among others, contribute to the issues faced with respect to availability of quality of water resources and security of water supply for consumption. Pressure management, is the most common and feasible demand management initiative implemented by the City of Cape Town Metro. The main focus of these initiatives is to reduce water losses within the water distribution system. Influence of pressure on water consumption is also observed, but has not been as well investigated as with leakage-pressure relationships. This study aims to assess the impact of change in system pressure on consumer water demand. To do this a pressure managed DMA and Control DMA was identified. The billed consumption data was analysed for 11 months before and 11 months after the pressure management period. A control DMA served to verify that the consumption reduction was as a result of pressure management and not any other intervention. Furthermore, this study involved the collection and analyses of the logged system flow data prior to and post commissioning of pressure management. Pressure is not fixed and varies overtime. The Average Zone Pressure was not available from logged data and was calculated by simulating the hydraulic model to reflect the system conditions prior and post commissioning of the pressure managed DMA. Following that, an investigation into how the leakage responds to pressure was performed. Since the latter affects the demand response to pressure. It was then decided to separate the leakage from consumption. In order to do this, various leakage parameters were calculated and randomly distributed across the system. To analyse the leakage before and after pressure management, two types of models were used, namely 1) Epanet Model (based on the Orifice Equation) and 2) the Epaleaks Model (based on the Modified Orifice Equation). N3 is the coefficient of elasticity. This coefficient represents the relationship between pressure and flow rate. Normal N3 analysis was performed on the available data. N3 was calculated for the system consumption, based on the logged data and a sample of the billed consumption records. The power regression model suggests an N3 of approximately 0.05 to 0.06 for the system based on a sample of filtered billed consumption data. However, in the case of the entire system's end use consumption the N3 value is approximately 0.4. Overall, the N3 values compared reasonably well with other studies in the range of 0.04 to 0.29 and in some cases ≈0.5.