PERFORMANCE EVALUATION OF AN URBAN WATER SUPPLY SYSTEM

Abstract

Next to air, clean and safe drinking water is essential element for the survival of any living being. Since water is a universal solvent, it helps human growth and affects public health. It also affects the various sustainable development goals having economic, environmental, and social dimensions. Urbanization is fast increasing in various parts of the world due to the availability of better living standards and job opportunities. However, such rapid urbanization has adversely affected the availability of various natural resources and imposed threats and challenges to basic existing urban infrastructures like drinking water, transportation, and energy. The existing urban drinking water systems (UDWS) are highly stressed and becoming unsustainable, particularly under changing hydroclimatic and socioeconomic conditions and growing populations with changing governance policies. Adequate sanitation and access to clean water are closely linked to improved public health and higher living standards in society. The lack of access to clean drinking water and poor sanitation facilities have led to millions of deaths worldwide, particularly in developing countries. Keeping these in mind, this study tries to carry out a performance evaluation of an urban drinking water supply system. It investigates the issues and complexities of UDWS in Sub-Saharan Africa, particularly Ethiopia, and suggests sustainable interventions. The first objective of the study is to identify the challenges of urban drinking water supply systems in Sub-Saharan Africa. The study highlights issues such as water supply and demand gap, water loss/non-revenue water, environmental, technical, institutional, and governance issues. Suggestions are extended for possible interventions to improve the situation on a sustainable basis. The methodology adopted for the purpose is a mix of methods involving primary (key informant interviews, field observations, and field measured data) and secondary (published articles, books, various reports, and design documents), as well as the data assessed through various computer-aided applications (mainly ArcGIS and WaterGEMS). The issues were deliberated through the UDWSs of four Ethiopian cities: Addis Ababa, Adama, Mekelle, and Dire Dawa. The main challenges faced by the Ethiopian cities' water utilities include water shortage, high and low pressure in the water distribution network (WDN), non-revenue water (NRW)/water loss, source pollution, ineffective policies and governance, and weak institutions. The study found that only 66% of the population in Addis Ababa has access to potable water, while in Mekelle, only half of the city population has access to potable water from the system. Furthermore, in Addis Ababa, Adama, Mekelle, and Dire Dawa, more than 35% of the freshwater produced is either NRW, unaccounted for, or lost which is significantly higher than the upper 25% limit suggested by the World Bank. Therefore, it is recommended to adopt certain sustainable interventions, such as integrated water resource management, installing appurtenances like pressure-reducing valves, check valves in the WDN, controlling and monitoring WDN through supervisory control and data acquisition and Internet of Things, effective and long-term planning and policy, etc. The second objective of this study is to focus on the hydraulic performance assessment of an urban water distribution network (WDN). A combination of ArcGIS and WaterGEMS are used for hydraulic modeling, and the performance is represented in terms of performance indices. ArcGIS is utilized for model building, whereas WaterGEMS is used for hydraulic simulation, and the network performance indices (pressure and velocity) are used to assess hydraulic performance. This objective assessment is carried out through a case study analysis of Dire Dawa city WDN (Zone 1). The study identifies the nodes with higher pressure (which may cause water leakage in the system) and the low-pressure nodes. Depending on the minimum, desirable, and maximum head at any node, the hydraulic performance is assessed for each node and the overall network performance. Further, velocity performance in the network is evaluated based on the minimum and maximum flow velocities in the various pipelines. The performance indices reveal problematic nodes and pipelines in the network that require improvement. The overall network performance for the pressure index was 0.62, which is above an acceptable level. Moreover, the network's overall performance for the velocity index is 0.41. To improve hydraulic performance, this study suggests installing pressure-regulating valves (PRVs) and changing the inefficient pipes in critical areas. The third important objective of this study is to propose a global reliability index (GRI) for an existing urban water distribution network. The proposed GRI integrates hydraulic and water quality reliability indices into a single metric to assess the WDN's performance under normal and abnormal conditions. The performance of the WDN is evaluated when all pipelines are functional and when one of the major pipelines fails. Hydraulic simulation and residual chlorine assessment for normal and abnormal conditions are performed using the Pressure-Dependent Analysis (PDA) method by combining the ArcGIS and WaterGEMS models in the extended period simulation. To assess the water quality reliability, residual chlorine at various nodes is considered as the key indicator. The proposed methodology is illustrated through a case study of Dire Dawa city WDN (Zone 2) in Ethiopia. The study reveals that the network's overall performance in terms of GRI is low but satisfactory, with a value of 0.547. To improve reliability, additional (parallel) pipelines, booster pumps, and booster disinfectant stations can be provided.The fourth objective of this study aims to propose a fuzzy logic-based aggregated performance index (API) for assessing the overall performance of the water distribution network under all ranges of operation modes (normal and pipe failure state). Reliability, resilience, and vulnerability indices are integrated using fuzzy logic-based methods to achieve this objective. To assess the individual performance indices, this study recommends the use of PDA for hydraulic simulation. Furthermore, a fuzzy-rule base API is proposed to deliver the outcome in linguistic form and assist decision-makers in prioritizing WDN maintenance. The proposed method is illustrated using a real-time WDN for a part of Dire Dawa city (zone 2) in Ethiopia. The study found that the API for this network is 0.721 and 0.624, respectively, under normal and abnormal conditions, which is just satisfactory. It was observed that nodes J4, J44, J47, and J49 are critical based on their low API scores. Therefore, efforts should be made to improve these nodes' hydraulic and residual chlorine conditions to increase the API. The fifth and final objective of the study is to investigate the impacts of the pipes and isolation valve failure on the system performance and develop a prioritization model for the pipe and isolation valve repair/replacement. Pipe and isolation valve failures in a distribution network can cause service interruptions to the downstream users. To effectively manage such failures, it is crucial to identify and prioritize maintenance of the pipes and valves that cause the greatest impact. The impact of such failures is assessed in terms of the number of affected isolation elements, customers, and supply shortfall during the event. WaterGEMS software is used for segmentation and criticality analysis in the Dire Dawa city network (Zone 3). Four different isolation valve configurations and two states (normal and abnormal) are analyzed to determine the impact of pipe and valve failures. The proposed prioritization model methodology is based on the analytic hierarchy process (AHP) approach. The suggested model considers three major sub-indicators: system performance, socio-economic, and physical. These parameters are further divided into nine sub-parameters. The weights of these parameters are assessed through the AHP technique, which allows for prioritizing individual water pipes and isolation valves. This methodology is applied to the Dire Dawa city network (Zone 3). The results indicate that a higher density of isolation valves leads to a lower risk of failure. However, the effects of pipe and valve failures on the network's performance during abnormal conditions vary. The weight coefficients for the system performance, socio-economic and physical sub-indicators were assessed as 0.43, 0.14, and 0.43, respectively. It is noticed that the system supply shortfall is the most important parameter among the other system performance; the existence of important facility near failure, among the socio-economic factors; and the age of the pipe/valves among the physical factors are the most critical influencing sub-parameters. The proposed failure-impact analysis can aid in the development of efficient operation and maintenance strategies, improving the performance and reliability of an urban WDN. It is felt that the proposed model can help the decision-makers to ensure efficient and effective operation, maintenance, and rehabilitation in an urban WDN. Overall, this study focuses on the performance evaluation of an urban drinking water supply system (UDWS), especially in Ethiopia, highlighting challenges and suggesting some interventions for sustainable UDWS. The study proposed a methodology for hydraulic performance assessment and a global reliability index (GRI). A fuzzy-logic-based aggregated performance index (API) is also suggested to assess the overall WDN performance, considering reliability, resilience, and vulnerability to UDWS. Furthermore, a prioritization model for pipe and isolation valve repair/replacement is developed. It is felt that the study will help the decision-makers and operators of such utilities for the effective operation, maintenance, and rehabilitation of various components of UDWS.