Hierarchical Optimization of Integrated Water Reuse Schemes

As a result of decreased availability of fresh water supplies for built-up areas and the resulting water stress, water reuse has emerged as a verified alternative to meeting societal water demands. Reclaimed water projects typically include construction of new or upgrades to a municipality’s wastewater treatment systems to meet the required water quality level, and construction of distribution systems for reclaimed water. Their realization requires deciding on many relevant issues, but the three broad questions that need be answered for every potential project are: (1) who should be provided with the reclaimed water, (2) what treatment processes should be used to meet the requirements and (3) how is the reclaimed water going to be distributed to selected users. These issues are obviously interrelated, and need to be considered in an integrated approach as the decision on each component of a water supply scheme influences the choice of others. For simple projects, the questions outlined above can be answered at a planning stage by evaluating a small number of alternatives. However, the complexity associated with planning of larger water reuse schemes is 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 presentation will give an overview of a DSS for optimal integrated planning of water reuse schemes. The developed methodologies, provided in a software tool, take into account the interactions that exist between the individual schemes components (treatment trains, distribution system and end-users of reclaimed water) in evaluation and selection of most promising design alternatives. The approach adopted for determination of optimal water reuse schemes is hierarchical, in that it detaches the problem of the optimal sizing of distribution system from the optimization of the overall water reuse scheme. The inner problem is solved using a liner programming (LP) based approach, while the latter uses enumeration and different evolutionary algorithms appropriate for the size of the problem.

The DSS was used to investigate water reuse options for the City of Waterloo, Ontario, Canada, considering two options: the first considered provision of a dedicated water reclamation facility using raw wastewater as a source, and the second evaluated options for upgrading of the existing wastewater treatment plant to provide effluent of satisfactory quality. Results of the optimization conducted using the lifecycle scheme cost and percent demand satisfied as the two objectives to be minimized and maximized, respectively, will be presented. The impact of including other objectives/constraints in the optimization, such as qualitative scores for treatment trains, land and energy requirements, on the selection of unit processes and patterns in the selection of end-users was also conducted. The results of these analyses were used to derive several design principles for integrated planning of water reuse schemes, which will conclude the presentation.