Refinery is a process where crude oil is transformed and refined into useful products like fuel or asphalt. Before the oil goes through the distillation process, it needs to be in good quality. For that purpose, the oil needs to be treated both mechanically and chemically.
This oil quality improvement process also requires water. Water is used to wash the oil, to transfer heat, etc. The water that is used has to be in good quality, so corrosion, scales, and other problems will not occur on refinery equipment. To improve water quality, both mechanical and chemical treatments are required.