Methods of process integration could be classified in to two categories: the mathematical and conceptual method. The former is outstanding to solve the overall simultaneous synthesis problems, but sometimes, complex calculations will be needed. The conceptual method also could be employed to solve the problem about simultaneous optimization of water and energy but few of them could deal with the multiple contaminants problems. In this paper, a new heuristic approach is proposed to optimize simultaneously the water allocation and heat-exchange network (WAHEN) with both single and multiple contaminants in a fixed flowrate (FF) system. In order to build up this approach solidly, a principle of the single temperature -peak design is proved through pinch analysis which discloses the interactions between water allocation network (WAN) and energy exchange network (HEN). When WAN only has a single temperature-peak for each sub-stream, the heat recovery problem of this system could be a threshold problem requiring less energy. As a trade-off between water and energy consumption is established in this design principle, the water and energy consumption could be optimized at one step. Based on this single-temperature-peak design, a novel heuristic approach including two main designing steps, the design of WAN and HEN, is established. A graphical method is employed to design the original water allocation network to ensure it to be a single-temperature-peak type. Next, based on this WAN, WAHEN structures are further generated, which employs a method ensuring that the total energy consumption equals to the minimal value calculated by the first step. Graphical visualization is the advantage of this methodology, and it does not need complex mathematical calculations. Four literature examples are employed to check this new method. That the obtained optimization results are better than those of other works proves its effectivity and advantage. Currently, this paper mainly focuses on the water reusing network, not the total network which will be investigated in our next work. (C) 2016 Elsevier Ltd. All rights reserved.