Ultrapure Water Explained

Ultrapure Water is an article to provide a basic definition and background on high-purity water and related categories.

NOTICE: The References are used with copyright permission from their sources.

Ultrapure Water
The term “ultrapure water” (UPW) is used to refer to particular grades of water. UPW, also known as high-purity water, is associated with water treated for use in the manufacture of semiconductors (devices, flat panel display, solar panels), pharmaceuticals (traditional medicines, biopharmaceuticals, kidney dialysis, medical devices, and selected consumer products), power generation (critical and super critical boilers), laboratory water, and selected other applications. UPW (high-purity water) is an umbrella term that refers to several types of water that each has a different quality standard. Sometimes such water is referred to by the terms “deionized” or “demineralized.”
Treatment technologies associated with the final treatment of high-purity water include reverse osmosis, ion exchange (separate bed, mixed bed, and condensate polishing), electrodeionization (sometimes called continuous electrodeionization), and filtration steps like ultrafiltration microfiltration. In pharmaceutical and semiconductor applications, care is also given to sanitize or sterilize the water with methods such as ultraviolet light or ozone. Other technologies used prior to the final treatment include activated carbon for chlorine removal to protect membranes and cartridge or other filtration. These types of steps would be used for an incoming feedwater from a municipal source. For raw water from say a surface supply (e.g., river) or well water, other steps might include flocculation and coagulation, and multimedia filtration as an initial pretreatment step.
When one examines water treatment, an easy way to distinguish between types of treatment technologies are those that chemically adjust the water to eliminate or minimize concerns, and those that physically separate contaminants from the purified stream. Chemicals for control of scaling, fouling, or biofouling would be examples of treatments that chemically adjust the water. Technologies such as microfilters, reverse osmosis, and ion exchange are treatments that physically remove water contaminants. In the context of high-purity water, many of the purification technologies used aim to physically remove contaminants.
The next paragraphs briefly summarize the three major categories of high-purity water.
Semiconductor-grade water is considered by many to undergo the most rigorous treatment. This water is treated according to standards set either within a particular manufacturing plant, or according to organizations such as the International Technology Roadmap for Semiconductors (ITRS)(1)or American Society for Testing and Materials International (ASTM International). Balazs Analytical Laboratory (now known as Balazs NanoAnalysis, a unit of Air Liquide) also has published guidelines for treating semiconductor grade water. In microelectronics manufacturing, high-purity water is used for cleaning the semiconductor devices during different production steps. A semiconductor water treatment system is designed so that it will remove organics, dissolved minerals, and particles from the water—all of which can precipitate on microelectronic devices and potentially cause product defects. Care is also used with these water systems to ensure that the material of construction in treatment equipment and water distribution systems will not leach contaminants into the treated water. Accordingly, materials such as inert plastics like polyvinylidene fluoride (PVDF) are commonly used for piping systems.

Pharmaceutical-grade waters are treated so that they meet the standards set forth by pharmacopeias of which the United States Pharmacopeia (USP), European Pharmacopeia (EP), and Japanese Pharmacopeia (JP) are the most influential. In the United States, the U.S. Food and Drug Administration oversees pharmaceutical plant water systems, but bases their regulations according to those set forth through the USP. Different types of pharmaceutical-grade waters that are produced include Bacteriostatic Water for Injection, Purified Water, Sterile Purified Water, Sterile Water for Inhalation, Sterile Water for Injection, Sterile Water for Irrigation, and Water for Injection. For Purified Water, it is common that a treatment system will combine reverse osmosis with electrodeionization. Water for Injection treatment most commonly involves some form of thermal distillation. The production of these waters all fall under the standards set by the USP. Some of these waters are used as ingredients in medicines, while others may be used in the cleaning and preparatory steps for pharmaceuticals or say a medical device (2, 3).

Power plant water. Many electrical generating stations use high-purity water in critical and super-critical (above 3,200 psig) boilers (nuclear or fossil powered). In some instances, utilities will set guidelines for the water treatment. Otherwise, organizations such as Electric Power Research Institute (EPRI), utility user groups, power generation equipment manufacturers, and the American Society of Mechanical Engineers (ASME) are examples of those who have developed standards for water treatment in power plants. Not all power plants are required to use high-purity water. For some lower-pressure boilers, even softened water may be acceptable. In power stations, treated water may be used for steam generation, as well as for air pollution control systems, and in cooling water. Types of treated water associated with power plants include high-purity water, boiler water for lower pressure boilers, and cooling water. Treatment technologies associated with power plant water include reverse osmosis, ion exchange, softening, electrodeionization (an emerging approach), and condensate polishing (4, 5).
Thermoelectric generation plants are a major user of water in the United States, and worldwide. Estimates from the U.S. Geological Survey (USGS) are that about 195 billion gallons of water were used daily (Bgal/d) in 2000 to produce electricity in the United States. This figure excludes hydroelectric power. Of that amount, about 59.5 Bgal/d comes from saltwater sources, and the remaining 136 Bgal/d is from freshwater. Of the total amount of water used by the power industry, 99% comes from surface water (cited in Reference 4).
Other Types of Treated Water
Within the water water treatment segment, there are a number of classes of treated water outside of UPW. As already discussed, high-purity water is a term that refers to the purist types of treated water. Briefly, here are some other categories that are also treated.
Municipal drinking water. This type of water is treated to meet standards set by regulators. In the United States, those standards are set by the U.S. Environmental Protection Agency (EPA). The EPA guidelines require drinking water utilities to monitor for waterborne microorganisms of which Cryptosporidium is one example, and for the presence of dissolved contaminants (6) that are considered harmful for human health.
Industrial water. This is a broad category of water used at industrial facilities. Applications can include boiler water, and process water. Examples would include power plants, industrial boilers, pulp and paper, petrochemical and oil refineries. The water may be treated by deionization technologies, or water softening. A subcategory of this type of water would be that used by the food and beverage industries.

Cooling water. This is water used in cooling towers and cooling systems. Sometimes it includes the reuse of waste streams from the industrial plant. The water may be used for HVAC systems or for cooling process streams. This water is often treated chemically for biocontrol, and fouling and scale control. Cooling systems can vary from large sets of cooling towers often associated with power plants or industrial facilities, to a small HVAC cooling tower seen on the roof top of a building. The sophistication and amount of treatment often varies, depending on the enduse.

Desalinated water is water from seawater or brackish water sources that is used for drinking water, or for use at power plants or industrial facilities along coastal areas. Common treatment technologies are either reverse osmosis or different forms of thermal distillation.

Reclaimed and reused water. This type of water comes from industrial or municipal wastewater sources. Some industrial wastewaters may still be clean enough for use in say a cooling tower or even for irrigation without needing to be treated. Other waters must first be treated prior to reuse—either as a feedwater source in an industrial facility, or say as municipal irrigation water. In addition to irrigation, municipalities will also use treated wastewater to inject to replenish groundwater supplies or to protect against saltwater intrusion in coastal regions. It should be noted that in all instances these types of waters are treated to be clean enough for potential reuse, and not to meet regulatory requirements for discharge.

Home drinking water. While not different than municipal drinking water, it should be noted that a whole industry has developed for treatment systems to improve the quality of water used in homes. There are whole-house systems like a water softener that treats all incoming water, and then there are point-of-use filters at kitchen sinks. In some instances, homeowners will even contract with a supplier who will place a large bottled water dispenser.

Produced water is treated water used in oil and gas drilling and production.

Common Water Sources
UPW is obtained primarily from treating municipal water, or occasionally reclaimed water, although in some cases it also comes from purifying other water sources, which are briefly identified in this section.

Groundwater. This is well water used either at a home, municipality, or industrial plant to provide for the respective needs.

Surface water. Common examples include rivers, lakes, or streams.

Seawater. Oceans or seas used as a water source along coastal regions. Must be treated by desalination technologies prior to use.

Brackish water. Inland water supplies with higher salt content that must be removed prior to use.

Main Menu

See Also

Main Menu