The distillation system consists of a series of interconnected columns and works according to a multistage principle. Pre-treated feed water enters into the first column that contains a double tube heat exchanger. Using external energy the feed water evaporates in the heat exchanger. In order to separate any water droplets that might harbour contaminants like salts and endotoxins, the column has an expansion chamber with demistor. The tubes of the heat exchanger are connected with the expansion chamber. While the steam moves further upwards in the chamber, unevaporated water droplets fall back into the lower section of the column by gravity and due to the specific column design with demistor. In this way they are separated from the steam. The non-evaporated water enters the next column where it is heated again in a heat exchanger. For heating steam from the previous stage is used. While another portion of the water evaporates, some of the steam condenses into WFI. This process is repeated in each column. The more columns, the more energy-efficient the system. In the last stage the steam is condensed in a double tube heat exchanger and cooled to 85 to 95 °C. The concentrated contaminants are directed to drain by a time controlled blow-down valve. As an important quality characteristic, the conductivity of the WFI is continuously monitored. If required, the distillation system can be designed for combined generation of WFI and pure steam at the same time.