Development of a framework to evaluate the hybrid water supply systems

Centralized water services in many major cities are increasingly being considered to be inadequate in achieving important goals for the urban water sector. It has been argued that new approaches need to be considered in order to cater for additional demand due to increasing population, changing climate, limited resource availability and a desire to protect ecosystems and build more livable cities. It has therefore been argued that providing additional capacity by using only conventional centralized systems is not economically or environmentally the best option. The use of decentralized water supply options such as rainwater tanks; storm water harvesting and reuse; and localized wastewater treatment and reuse in combination with centralized systems can help provide a sustainable solution to address these challenges by reducing the load on fresh water and decreasing the amount of wastewater to receiving environment. It is currently unknown how such technologies impact on the operational performance of the downstream infrastructure and existing treatment processes. For instance, reuse of greywater reduces the wastewater flow and hence there will be more change of solid deposition in the sewerage system. A comprehensive literature review has identified several significant research gaps related to interactions between centralized and decentralized water supply services. One of the prominent gaps is the effects of such hybrid water supply systems (combination of centralized and decentralized systems) on changes in the quantity and quality of wastewater and storm water of the existing system. Therefore, research is necessary to assess the feasibility of their implementation in conjunction with existing centralized systems. Prior to implementing these hybrid water supply systems (WSS), the interaction of these systems with the local environment needs to be understood. The interactions between centralized and decentralized systems are highly complex. Current practices do not consider the impacts of these systems on the existing infrastructure. Furthermore, implementation of these systems does not consider the external impacts on the rest of the water cycle. This paper proposes a comprehensive framework that focuses on the interactions between decentralized and centralized water supply systems while planning a well integrated hybrid water supply system. Such a system is expected to enhance the performance of water supply in terms of meeting increased water demand with less impact on other urban water cycle components including sewerage and drainage. In addition, it makes it possible to understand, predict and manage the various impacts on the urban water cycle components. However there is a paucity of research in the area of hybrid water supply systems. In order to fulfill this major research gap, this study presents a framework integrated with a number of analytical tools and modeling approaches to evaluate the hybrid water supply systems. The proposed framework would evaluate the impacts from the implementation of hybrid water supply systems on the quantity and quality of wastewater and storm water in the existing centralized system. This generalized framework coupled with associated models and tools (i. e., water balance modeling, contaminant balance modeling, multi-criteria decision analysis (MCDA), and uncertainty analysis) considers the varying nature of urban areas and is sufficiently generic to analyze the impacts of hybrid water supply systems in any type of urban developments. Hybrid water supply systems can be assessed based on volume and peak flow rates of wastewater and storm water; wastewater and storm water quality; and water supply reliability.