A water main is an interface between water sources and sinks to reduce the complexity of the Total Site Integration for water reuse. The selection of the contaminant concentration of water main can significantly affect the site freshwater use and wastewater generation. This paper proposes a hierarchical approach to the selection of the inter-plant water main concentration for minimising the water utilities. The algorithm first performs Plant-Level Water Integration, exposing to the site level only the residual water sources and sinks. The case study shows that, for the water main inlet concentration increase from 100 ppm to 800 ppm, the minimum freshwater input increases from 127.5 t/h to 133.3 t/h, and the wastewater generation increases from 132.5 to 138.4 t/h. The trend, in this case, can be explained by the fact that the water sinks require cleaner water than the water main can offer. The conclusion is that the selection of the water main contaminant concentration requires balancing the availability from water sources and the demand by water sinks at the site level.