Ultra Pure Water (UPW) represents the highest grade of water purification achievable. This highly refined substance is stripped of virtually all contaminants, including dissolved gases, organic compounds, particulate matter, and dissolved ions. Achieving this level of purity requires sophisticated, multi-stage processing technology. The resulting water is a necessity for modern high-technology industries where even trace impurities can lead to devastating manufacturing failures. UPW is manufactured not for consumption, but to serve highly specialized industrial and scientific processes.
Quantifying Extreme Purity: Standards and Unique Properties
The defining metric for Ultra Pure Water is its electrical resistivity, which measures the water’s resistance to an electrical current. UPW must achieve a theoretical maximum resistivity of 18.2 megaohm-centimeters (MΩ·cm) at 25 degrees Celsius, indicating an extremely low ion count. This standard is vastly different from deionized (DI) water, which typically reaches values only around 1 to 10 MΩ·cm.
Beyond ionic content, the purity of UPW is also measured by its Total Organic Carbon (TOC) content, which tracks dissolved organic molecules. Modern UPW systems typically limit TOC to less than 1 part per billion (ppb), a level achieved through powerful oxidation processes. Specifications for microbiological purity require the water to be essentially sterile, with bacteria counts below 1 colony-forming unit per 10 milliliters. Furthermore, the water must be nearly free of particulate matter, often requiring fewer than 10 particles per liter larger than 0.2 micrometers.
A unique characteristic of UPW is its aggressive nature. Because the water molecules have been stripped of almost all dissolved substances, they possess a strong tendency to return to a state of chemical equilibrium. This makes the water highly corrosive, as it actively seeks to dissolve ions and gases from any material it contacts, including storage tanks and piping. Maintaining the required purity is a continuous challenge, necessitating specialized, non-contaminating materials like high-purity plastics for the entire delivery system. UPW must be used almost immediately after it is produced, or continuously recirculated and polished.
The Multi-Stage Purification Train
Producing Ultra Pure Water requires a meticulous, multi-step sequence known as a purification train, beginning with extensive pretreatment of the source water. This initial phase often involves softening, multimedia filtration, and activated carbon beds to remove large particles, chlorine, and other common contaminants. The first major purification step is Reverse Osmosis (RO), where water is forced under high pressure through a semipermeable membrane, effectively removing 95 to 99 percent of dissolved salts, bacteria, and larger organic molecules.
In many high-demand systems, a two-pass Reverse Osmosis system is employed, feeding the permeate from the first RO stage into a second set of membranes to achieve greater salt rejection. Alternatively, continuous electrodeionization (CEDI) is sometimes used immediately after the RO stage, using electricity to continuously regenerate the ion exchange material. These steps are necessary to reduce the load on the final polishing resins, maximizing system performance and longevity.
Following the RO stage, the water must undergo further purification to eliminate remaining trace ionic impurities. This is primarily achieved using large beds of mixed-bed ion exchange (DI) resins, which chemically swap trace ionic contaminants for hydrogen and hydroxide ions. This is a highly effective method for reaching the 18.2 MΩ·cm resistivity standard. However, the DI resins do not effectively remove small, non-ionic substances like bacteria or organic compounds.
To address non-ionic contaminants, the system incorporates intense ultraviolet (UV) light irradiation. UV oxidation targets and breaks down small, non-polar organic molecules into charged fragments that the DI resins can then capture. Another UV stage is used for sterilization, destroying any microbes that may have entered the system. This dual-purpose UV treatment maintains the required low levels of both Total Organic Carbon (TOC) and biological contaminants.
The final stages of the purification train often include membrane degasifiers to remove dissolved gases like oxygen and carbon dioxide, which can rapidly lower the water’s resistivity. Ultrafiltration (UF) membranes are also employed as a final barrier to capture any remaining submicron particles or microbial fragments before delivery to the point of use. Because UPW degrades rapidly upon exposure to air and piping materials, the entire system is designed to continuously circulate the water back through the polishing loops, ensuring the purity standard is maintained.
Essential Industrial and Scientific Applications
The most demanding consumer of Ultra Pure Water is the semiconductor industry, specifically in the fabrication of microchips. In this highly sensitive process, components are routinely washed with UPW to rinse away chemical residues between manufacturing steps. Since modern chip features can be smaller than 10 nanometers, even a single particle or a trace ion in the water could cause a short circuit or defect, rendering the microchip useless.
Pharmaceutical and medical device manufacturers use UPW for both cleaning delicate equipment and as a component in certain formulations. The water ensures that cleaning processes introduce no contaminants that could compromise product quality or drug stability. Power generation facilities, particularly those with high-pressure boilers, use UPW to prevent scaling and corrosion inside the turbines and piping systems. Introducing even moderately pure water into these systems would lead to mineral deposits, reducing efficiency and eventually causing equipment failure.