Fresh water is becoming an ever more valuable and scarce resource, and any method or approach that can contribute to the saving of fresh water resources is therefore beneficial. Dual water supply systems are water supply distribution systems employing two sources, consisting of one fresh water system for potable use, and another system of either seawater, untreated raw fresh water, or treated / reclaimed wastewater for toilet flushing purposes. The purpose of this book is to discuss the engineering and cost aspects of dual water supply systems drawing on the author's experience obtained in Hong Kong, where dual water supply systems have been used for fifty years. The book is suitable for use as a text book or reference book at undergraduate and postgraduate levels. University undergraduate students and postgraduate students in water science, civil engineering, environmental engineering and environmental science or management will be the principal audiences. Practicing engineers, managers and other practitioners in these fields will also find this an invaluable reference source.