Research is currently being conducted as part of the AQUAREC project, with the objective being the development and validation of design principles for water reuse systems. To achieve the project objective, simulation and optimisation software for integrated water reuse systems need to be developed. A water reuse system is likely to have many possible design options: type and degree of treatment, number and location of treatment plants, number and location of pumps/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 design combinations. A hydraulic/process simulation model has been developed and is described, which will be used in combination with an integrated optimisation engine to allow a range of design possibilities to be explored. The model includes a computational module for wastewater treatment trains, a computational/optimisation module for reclaimed water distribution system, and a knowledge base. The computational modules are used to calculate the performance of user-defined reuse system alternatives, utilizing the information contained in the knowledge base that includes rules for generation of treatment trains, design, cost and evaluation criteria information.