The use of water in industrial processes has become an important issue over the last 20 years, and then the optimization of water reuse considering multiple contaminants has gained great attention. This paper presents a new approach for the algorithmic procedure Water Sources Diagram (WSD), which is developed to be applied in large multiple contaminants water allocation problems (WAPs). In the WSD procedure, the reference contaminant definition is a key step and often leads to cumbersome calculations in following steps to get the final network from an intermediate structure, which is obtained based on the reference contaminant. The proposed new approach reduces the importance of the reference contaminant choice, simplifying the following calculations by adopting a decomposition strategy. It is applied in two large process plant examples, and the results show a good performance of the new approach.