In this paper, the application of the water networks with internal mains is introduced. A design methodology for multiple-contaminant water networks with single internal water main is presented. A new concept of "water-saving factor" is proposed. Emphasis is placed on the location of the first internal water main, which is related to the maximum water-saving potential. The paper is accompanied by several case studies to illustrate the methodology. According to these case studies, water networks with just one internal water main determined by the presented method can obviously reduce water consumption, approaching the minimum water consumption target.