A new approach in optimal sensor placement for smart hydraulic monitoring in intermittent water supply (IWS) systems

Abstract

This thesis proposes a novel design approach for a monitoring system that can detect hidden leaks in intermittent water supply (IWS) systems. Cities with IWS conditions in their drinking water network, such as Nairobi and Harare, often have a high percentage of non-revenue water (NRW) in their system. Estimations of the amount of NRW in these cities range from 40% to 50%, of which a large part is due to a leaky infrastructure. Intermittency of water supply is usually caused by a shortage of available supply, making it extra poignant to notice that these areas lose significant volumes of water. The leaks are also important locations for contaminant intrusion, which deteriorate the quality of drinking water. Additionally, intermittency of supply results in people using storage to fulfill themselves with their weekly water demand, which provides new challenges when constructing hydraulic models. Hidden leaks, which are leaks that do not appear at the surface, can be noticed in continuously supplied areas through reports of pressure deficiencies or the absence of supply. As these are regular circumstances in IWS areas, these hidden leaks are seldom noticed. Therefore, methods that are applicable in IWS systems need to be developed to detect these hidden leaks. This thesis proposes a new approach to detect hidden leaks in IWS areas with a smart hydraulic monitoring system. The approach optimizes the design of such a system in a district metered area (DMA) with IWS conditions in sub-Saharan Africa, by balancing information density and investment costs. By using as little equipment as possible, this optimization study aims to be not only scientifically and practically relevant, but also cost-effective. The methodology that was used to design the monitoring system makes use of a similar concept as the Dynamical Bandwidth Monitor (DBM), which is a smart hydraulic monitoring system that has been applied regularly in networks with continuous supply. The monitoring system consists of sensors that continuously measure flow or pressure and it compares these measurements to a range of expected values, attributing deviations from these expected values to a potential leak. A case study of Ashdown Park, a DMA with IWS conditions in Harare, was used to assess the performance of the design. The flow into this DMA and the pressure at its inlet had been monitored for one year. Two designs of the monitoring system were made, one which mainly consisted of flow sensors and one with mostly pressure sensors, to showcase which type of sensor could best be used in Ashdown Park. A hydraulic model was constructed for the DMA using pressure dependent outflow modelling. Daily demand patterns were constructed from analyzing the inflow measurements and used to calibrate the hydraulic model. The proposed calibration method assumes linear relationships between the demands and inlet pressure on one side and the pressure at a specific node and flow at a specific pipe on the other side. The range of expected flows and pressures within the DMA was calculated by Monte Carlo analyses, during which demand realizations were modelled by using a novel method which made use of a random weighted choice of demand, based on the outflow from a single tap. The ability of the monitoring system to detect leaks during different demand realizations was stored in a three-dimensional Boolean matrix, which was then used to determine the optimal sensor placement. A social and financial analysis, summarized in a business model canvas, shows more practical challenges and opportunities that could arise from implementing the monitoring system. The lessons learnt from this thesis were used to showcase whether the monitoring system could be applicable for IWS systems around the globe. Several conclusions can be drawn from the results of this thesis. The daily demand patterns in Ashdown Park showed a different pattern than in continuously supplied systems, showing less strong peaks. This could be due to a constant water demand for filling storage, leaks in the system or different consumer behaviour. The calibration method made it possible to model flows and pressures at the DMA inlet which were comparable to the measurements. The novel method to model demand realizations with a random weighted choice and a single tap capacity, showed promising results since the spread of the modelled inflow was well comparable to the spread of the inflow measurements. This standard tap capacity is especially suitable for IWS areas, since most people in IWS areas usually only have one tap directly connected to the water supply system and water end-use devices are not directly connected to the network. Furthermore, it was found that the water use behaviour of inhabitants of Ashdown Park had been more constant than the supply behaviour of the water utility. This irregular supply behaviour of the utility increased the difficulty of designing a pressure monitoring system. Using a flow monitoring system to detect leaks showed a better performance (leaks could be found on a daily basis in 25% of the pipes in the DMA) than using a monitoring system with pressure sensors (leaks could be found in 1% of the pipes). Making the monitoring system with pressure sensors dependent on the inlet pressure increased its performance (from 1% to 8.3%). Branched parts of the system were more favorable locations to place sensors and sensors at the DMA inlet were crucial for calibrating the hydraulic model. Practical barriers that were identified during this thesis were irregular operational schemes, unknown demand patterns and incomplete GIS data. Furthermore, costs can be saved as soon as leaks are detected, making the financial profitability very dependent on the performance of the system and the occurrence of leaks. The applicability of the monitoring system in IWS areas around the globe is determined by the priorities of a local water utility, its network characteristics and the ability of the local utility to overcome implementation barriers. The main limitations in this research are due to making some simplified assumptions, such as assuming a constant flow-rate from the tap in all households in Ashdown Park, and due to a lack of understanding of the local situation, since this research was performed in the Netherlands. To validate assumptions and get better understanding of the local situation, it is advised to conduct follow-up research at the location of interest. Especially a pilot project of the proposed monitoring system would likely find more practical barriers and limitations than could be thought off in this thesis and therefore bring more valuable information for the implementation of a smart hydraulic monitoring system. If prioritized, properly installed and operated, the proposed smart hydraulic monitoring system could generate substantial water savings and provide many social benefits, such as an increased access to clean drinking water and employment opportunities. Above all, it can assist a local utility with fulfilling their responsibility: supplying people with the basic need of drinking water.