A new method, named inflection point method (IPM), is proposed for designing single-contaminant water networks. On the basis of mass balance, a few equations are developed to calculate the targets of water networks design, including the minimum freshwater flowrate (F-w), the minimum regenerated water flowrate (F-R) under the condition of the minimal Fw, and the relationship between the freshwater flowrate (F-w) and the minimum regenerated water flowrate (F-R) accordingly. The targets of water networks design determined by IPM based on mass balance are global optimum solutions. Therefore it could be used to appraise water networks design result obtained via other design methods, for instance water pinch analysis and mathematical programming method. Furthermore, the operation cost of water networks could be calculated by the relationship between F-w and F-R, and then the trade-off between cost and freshwater consumption could be determined prior to the networks design. (C) 2014 Elsevier Ltd. All rights reserved.