The American Society for Testing Materials (ASTM) defines specifications for highly pure water of various grades based on specific applications. Ultra pure water is essential for critical industries such as healthcare, laboratories, pharmaceuticals, and semiconductors. However, the standards for water quality can vary significantly depending on the intended use.
Purification Technologies
There are various types of purification technologies, such as:
- Micron Filtration: Micro porous screen filters that act as sieves, creating a physical barrier based on pore size to prevent the passage of particles while allowing water to flow through.
There are three categories of micron filters:
- Depth Filters: Depth filters are compressed matrices of fibrous materials that use random adsorption or entrapment to retain particles, making them effective for filtration in various industries.
- Surface Filters: Multiple layers of media that trap particles larger than the spaces within the filter matrix, primarily accumulating on the surface of the filter.
- Screen Filters: Microporous screens that remove all particles greater than the specified pore size.
- Activated Carbon: Activated carbon is used in laboratory water purification as a pre-treatment method. It helps remove free chlorine and chloramines from the feed water to RO membranes. This prevents membrane damage caused by oxidation and eliminates trace organic impurities from pure water. Additionally, activated carbon’s affinity for organics makes it useful in vent filters, protecting reservoirs of purified water.
- Ultraviolet (UV) Radiation: Ultraviolet radiation is a form of electromagnetic radiation with wavelengths ranging from 10 to 400 nm, shorter than visible light but longer than X-rays. UV radiation is widely employed as a germicidal treatment for water. It prevents micro-biological growth and removes organic compounds by oxidizing them and eventually converting them to carbon dioxide. UV light at 185 nm possesses strong oxidizing properties, breaking down large organic molecules into smaller ionized components. Downstream ion exchange resin is then used to remove these ionic species. One major advantage of UV treatment is its low maintenance requirements, typically limited to annual lamp replacement.
- Reverse Osmosis (RO): Reverse osmosis is a cost-effective method capable of removing 95% to 99% of contaminants. RO membranes act as selective barriers, rejecting particles, bacteria, dissolved ions, and organic compounds. They prevent the passage of contaminants in water larger than approximately 0.1 nm in effective diameter. Typically, RO membranes remove well over 90% of ionic contamination, most organic compounds, and effectively all particulates. However, the removal efficiency for non-ionic contaminants with molecular weights below 100 Dalton can be lower. The removal increases as the molecular weight rises, and theoretically, molecules with molecular weights exceeding 300 Dalton—including particles, colloids, microorganisms, and large biologically-active molecules—will be completely rejected. Dissolved gasses are not removed. RO membranes, usually made of thin film polyamide, are stable over a wide pH range. However, they can be damaged by oxidizing agents like chlorine and fouled by organics or colloids, necessitating pre-treatment to protect the membranes.
- Ion Exchange Resin: Ion exchange is a deionization process used in laboratory water purification. Water is passed through one or more beds of ion exchange resin, where impurity ions are taken up on the resin bed and are exchanged with hydrogen and hydroxyl ions, purifying the water. This process is highly effective and can reduce ionic levels in treated water to sub-ppt (parts per trillion) levels. However, ion exchange resin beds have a finite capacity, and when they approach their full capacity, they begin to release weakly held ions. Ion exchange resin beds are also utilized after exposing water to short-wavelength UV light to remove charged organic molecules produced and decrease Total Organic Carbon (TOC) levels to low ppb (parts per billion) concentrations. Ion exchange is typically part of a ‘polishing’ loop, including UV treatment and filtration, through which water is repeatedly circulated to maintain quality.
- Electrodeionization (EDI): Electrodeionization is an electrically-driven water treatment technology that employs electricity, ion exchange membranes, and resin to remove ionized species from water. EDI eliminates ions and other charged species such as salts and organic acids. The EDI module consists of chambers filled with ion exchange resins separated by ion-exchange membranes. Essentially, the EDI module acts as an ion exchange bed that is continuously regenerated electrically. EDI is often used after reverse osmosis, and in the case of very hard water, in conjunction with degassing to remove carbon dioxide.
- Ultrafiltration (UF): Ultrafiltration (UF) membrane functions as a molecular sieve. It separates dissolved molecules on the basis of their size–often reported as the “molecular weight It ensures complete removal of particulate matter bacteria and reduces endotoxins, pyrogen, DNase from the treated water
Conclusion
In conclusion, the specifications for ultra pure water of different grades, as defined by the American Society for Testing Materials (ASTM), play a crucial role in ensuring water quality for critical applications. Various purification technologies are employed to meet these stringent standards in industries such as healthcare, laboratories, pharmaceuticals, and semiconductors. By adhering to the defined specifications and employing suitable purification technologies, industries can achieve high purity water suitable for their critical processes. Consistently meeting these standards ensures the integrity, reliability, and safety of products and processes in healthcare, research, manufacturing, and other industries that rely on high purity water.
