Demineralized water or demi water is water of which the minerals or salts are removed. It is used in applications where water with low salt content or low conductivity is required. Some examples are:
For every application a required conductivity is used. However some conductivity will always remain, because of the acid / base equilibrium of water. The best quality of demi water has resistance of approx. 18,2 MegaOhm·cm or a conductivity of 0.055 microSiemens per cm.
Deionized water can be produced via ion exchange with cation and anion resins. The resins need to be regenerated with acid and caustic. To reduce the cost for regeneration for larger systems the water going into the demi installation is pretreated with a reversed osmosis unit, which reduces the total salt content with more than 90% and thus reduces the cost for regenerants. After the cation and anion exchangers mixed bed ion exchangers are used to reduce the conductivity even more.
In some cases reversed osmosis (up to 99,8% salt removal) will be sufficient.
Common applications for demi water is the use for ultra pure water, which has even higher standards of pollution. In this case rest organics are removed together with all biological components. The best qualities of ultra pure water are used in the microchips industry and in the pharmaceutical industry.
Demi water is quite aggressive for metals, even for stainless steel. So in many cases plastic materials are used to transport the demi water.