High Purity Applications

Highly purified water (HPW), high purity water or ultrapure water is necessary for various applications including dialysis, pharmaceutical and electronics rinse water. Highly purified water is a commonly used term to emphasize the fact that the water is treated to the highest levels of purity, including the removal of: organic and inorganic compounds; dissolved and particulate matter; volatile and nonvolatile, reactive and inert; hydrophilic and hydrophobic; and dissolved gases.

While each industry uses what it calls “highly purified water”, the quality standards vary, meaning that the highly purified water used by a pharmaceutical plant is different than that used in an electronics plant, semiconductor fabrication plant or a power station. For example, pharmaceutical facilities will use highly purified water as a cleaning agent as well as an ingredient in products, so they require water free of endotoxins, microbials and viruses. Electronics and semiconductor plants use highly purified water as a cleaning agent, so it is important that the water not contain dissolved contaminants that may precipitate or particles that may become lodged on the circuits and cause microchip failures. The power industry, on the other hand, uses highly purified water as a source to make steam to drive steam turbines.

A number of organizations and groups develop and publish standards associated with the production of highly purified water. Pharmaceutical plants follow water quality standards as developed by pharmacopeias, including the United States Pharmacopeia (USP), European Pharmacopeia (EP), British Pharmacopeia and Japanese Pharmacopeia. The most widely used requirements for highly purified water quality for electronics and semiconductor fabrication are documented by the American Society for Testing and Materials International (ASTM International) and Semiconductor Equipment and Materials International (SEMI).

A typical highly purified water system has three stages: a pretreatment stage to produce purified water, a primary stage to further purify the water, and a polishing stage, often the most expensive part of the treatment process. The usual treatment scheme comprises of reverse osmosis, electrodeionization and then ultrafiltration. Today, ion exchange (IX) and electrodeionization (EDI) are the primary deionization technologies associated with highly purified water production, in most cases following reverse osmosis (RO). Depending on the required water quality, highly purified water treatment plants often also feature degasification, microfiltration, ultrafiltration and ultraviolet irradiation. Ultrafiltration (UF) is commonly used after EDI to remove microbial contamination that may get generated due to the presence of anion resin inside the EDI unit. Reduction values for a 10,000 Dalton (Da) molecular weight cut-off (MWCO) membrane have been summarized in Table 1.