Hierarchical Optimization of Integrated Water Reuse Schemes
With up to 40% of the world’s population in over 80 countries and regions experiencing water stress, water reuse has emerged as a genuine and reliable alternative that can be used to supplement, and in some cases substitute, traditional water sources. The already large number of water reuse schemes, existing primarily in areas of fresh water short-age, is expected to increase in the future due to several factors. In industrialized countries, the main drivers are the lack of dependable water supplies and wastewater disposal sinks, while the need to provide economically feasible new water supplies and protect existing water sources from pollution are the key factors influencing water reuse in the developing world (Asano 1999).
An important factor that needs to be considered in the context of planning of future water reclamation undertakings is the scale of the projects. The water reclamation projects are driven by demand and lack of alternative sources, which lead to projects being considered, or even made feasible, by securing one or several large customers, which offers greater demand security than securing smaller customers. As the demand for reclaimed water rises in the future, the number of potential end-users will also increase, leading to larger and more complex pro-jects requiring adequate tools for their development. In the city of Chicago, for example, which currently reuses only 2% of effluents dis-charged from its seven wastewater treatment plants (WWTPs), a study of future water reuse opportunities identified over 800 potential users (Meng 2005).
Reclaimed water projects typically include construction of new, or upgrades to a municipality’s existing, treatment systems to treat wastewater to the required quality level, and construction of distribu-tion systems for reclaimed water. Their realization requires deciding on many relevant issues beyond the scope of this chapter, but the three broad questions that need be answered for every potential project are: (i) How much of the available wastewater should be treated? (ii) What level of treatment needs to be provided? and (iii) How is the reclaimed water going to be distributed? It is very likely that a water reuse system could have many possible design options: different sets of served end-users, with various water quality requirements; type and degree of treatment; number and location of treatment plants; number and loca-tion of pumps or pumping stations; number, size and location of storage tanks; layout and size of distribution pipe network. These elements are all linked, to give multiple interactions and a very large number of de-sign combinations, even for apparently small systems.
A simple trade-off example that highlights the interaction between system components can be made considering a hypothetical scenario with only two potential users. The first user is an industry located rela-tively close to the WWTP requiring a constant supply of high-quality reclaimed water for process use, and the second user, located further away from the reclamation plant, has the seasonal need for reclaimed water of lower quality for restricted irrigation. Providing reclaimed wa-ter to the first user would likely require a more expensive treatment option, but less expensive distribution system and no need for seasonal storage, whereas a more extensive distribution system and seasonal storage would be required to satisfy the second user’s demands, albeit with less treatment. Even if an assumption is made that the annual re-claimed water requirements for the two users are the same, there is an obvious need to perform a comprehensive analysis of design options prior to making the final decision.
The trade-offs involved in determining the optimal water reclama-tion system configuration illustrated above on a very simple case is amplified if different combinations of available treatment processes are contemplated and/or a greater number of potential end-users is considered. The complexity associated with planning of water reuse schemes is therefore very high due to a very large number of design combinations possible, and establishes the need for use of a decision support system (DSS) to aid in the planning process.
This chapter presents the development and results of application of a DSS for Water Treatment for Reuse and Network Distribution (WTRNet), developed as part of the AQUAREC project (AQUAREC 2006). A brief overview of the WTRNet components is first presented, followed by the description of the methodology developed to conduct optimal selection of reclaimed water customers, treatment processes and distribution components. The developed DSS was applied on a number of hypothetical case studies, and the results are provided in this chapter to illustrate some of the potential WTRNet applications.
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