The simultaneous design methodology has been developed, as a general design framework which can be applied to combined water-and-energy systems. The new approach is based on the mathematical optimisation of combined superstructures to design both water network and heat exchanger networks simultaneously, and to deal with large-size multiple contaminant problems. Design interactions between two subsystems and its economic trade-offs have been systematically investigated, and stream merging and generation of separate systems have been also incorporated to the design method which provides the cost-effective and environmental-benign design of heat-integrated water systems. (C) 2007 Elsevier B.V. All rights reserved.