The quality of water used for metal cleaning processes is extremely important. The level of cleanliness required will differ depending on the applications and purposes of the subject products to be cleaned. For products requiring precision cleaning such as electronic equipment, optical components and semiconductors, any impurities, however minute in quantity within the cleaning water will affect the product accuracies and yield, so the water must be of in extremely high purity. This is called "Ultra Pure Water". An example of such water is of quality: Electrical resistivity 18MΩ•cm, Particulate count (>0.2μm) 20/mL or less, Viable bacteria 0.2/mL or less.

Ultra pure water production system is shown in [Fig.1] below.

Firstly, the source water from wells and city tap is filtered to remove the relatively larger particles, then pumped to a reverse osmosis system where ions are removed and loads on the ion exchange tower are reduced, as well as other soluble organic compounds. Then the air removal system removes carbon dioxide. The mixed bed deionizer, an ion exchange tower raises the water's electrical resistance up to the standard value. The pure de-ionized water obtained here is sent to the primary pure water tank, ultraviolet germicide system, polisher (resin ion exchange tower) to remove remaining minute amounts of ions, ultra-filtration system to remove submicron particulates, then supplied to the use point (where the water is actually used).

The ultra pure water, as produced as such, stored in air can be contaminated with carbon dioxide and oxygen as well as various germs. The basic rule is to use the water on-line and not store, but for unavoidable cases where storage is necessary such as for the primary pure water storage tanks, ultraviolet germicide lamps are used for sterilization and nitrogen gas is used to isolate the water from the air.