Water system integration can minimize both the freshwater consumption and wastewater discharge of a plant. In industrial applications, it is important to determine the contaminants to be considered, their limiting concentrations and whether the integration is feasible. In this paper, the water system in a catalyst plant is integrated. Using this industrial application, it is shown how to decide upon the contaminants, the water-using processes and the limiting concentrations. The optimal water reuse scheme is studied first. Because the limiting concentrations of each water-using process are estimated, and because the reused water may contain unexpected contaminants, experiments to determine the feasibility of water reuse are needed before the water system integration is completed. The experiments both examine the main contaminant, sodium ion and the effect of unexpected contaminants on the filtration rate and the crystallinity of the process. The final water integration reduces the freshwater consumption and wastewater discharge.