What is Ultra Pure Water (UPW)?
Ultra pure water, often referred to as UPW, is water that has been treated to remove almost all impurities, including minerals, salts, organic compounds, and dissolved gases. The purity is measured by its electrical resistivity, with the highest grade reaching 18.2 MΩ⋅cm at 25 °C. This level of purity is achieved through a multi-stage purification process, which may include:
- Pre-filtration
- Reverse osmosis (RO)
- Deionization (using ion-exchange resins or electrodeionization)
- Ultraviolet (UV) irradiation
- Microfiltration or ultrafiltration
UPW's incredible purity is essential for industries where even trace contaminants can interfere with processes or compromise product quality. Key applications include semiconductor manufacturing, power generation, and certain laboratory analyses like high-performance liquid chromatography (HPLC) and mass spectrometry (MS).
What is Sterile Water?
In contrast to UPW, sterile water is defined by its complete freedom from living microorganisms, including bacteria, viruses, fungi, and bacterial spores. The key distinction is that while sterile water must be highly purified, the critical step is sterilization, not just purification. Sterilization is a process designed specifically to kill or remove all microbial life. Common methods for sterilizing water include:
- Autoclaving: Using superheated, high-pressure steam to destroy all microbial life.
- Microfiltration: Filtering the water through a membrane with extremely small pores (typically 0.22 microns) to physically remove microorganisms.
- Gamma Irradiation: Using radiation to disrupt the DNA of living organisms.
Crucially, sterile water must be packaged in sealed, sterile containers immediately after sterilization to prevent re-contamination. Once a sealed container of sterile water is opened, it is no longer considered sterile due to exposure to airborne microorganisms.
The Surprising Survival of Bacteria in UPW
A common misconception is that UPW is so pure that bacteria cannot survive in it. This is false. While the nutrient-poor environment of UPW slows down bacterial growth, certain types of hardy bacteria, often referred to as oligotrophic bacteria, can not only survive but also reproduce. They can subsist on trace amounts of organic compounds leached from the system's piping and storage materials.
- Biofilm Formation: In UPW systems, bacteria can form biofilms on the inner surfaces of pipes and tanks. These biofilms can act as a persistent source of contamination.
- System Integrity: Contamination often enters through the feed water or improper handling and storage. Maintaining a system with strict operational procedures and final-stage purification is crucial to controlling bacterial levels.
Therefore, even the most rigorously produced UPW is not guaranteed to be free of all microorganisms, and its sterility cannot be assumed. For applications demanding absolute biological safety, additional sterilization is always required.
Comparison: Ultra Pure Water vs. Sterile Water
| Feature | Ultra Pure Water (UPW) | Sterile Water |
|---|---|---|
| Primary Goal | Remove all chemical and physical contaminants | Eliminate all living microorganisms |
| Purity Standard | Defined by electrical resistivity (e.g., 18.2 MΩ⋅cm) and low Total Organic Carbon (TOC) | Defined by the absence of living microbes (e.g., bacteria-free) |
| Production Process | Multi-stage purification (RO, DI, UV, filtration) | Purification followed by a specific sterilization step (autoclaving, filtration) |
| Microbe-Free Status | Not guaranteed. Prone to re-contamination upon exposure. Bacteria can survive and grow. | Guaranteed only when packaged aseptically and sealed. No longer sterile once opened. |
| Risk of Re-contamination | High, especially with long-term storage or improper handling. | Minimal in a sealed, intact container. High once opened. |
| Common Uses | Semiconductor manufacturing, trace analysis, power generation. | Injections, wound irrigation, cell culture media, pharmaceutical preparations. |
| Medical Use | No. Must not be used for medical purposes unless sterilized separately. | Yes. Essential for all applications requiring biological safety. |
Why the Distinction Matters
For many laboratory and industrial applications, UPW is sufficient and necessary. In the semiconductor industry, for example, the concern is chemical purity to prevent defects, not biological contamination that would be dangerous for a person. However, in medical and pharmaceutical settings, sterility is non-negotiable.
Using un-sterilized UPW for an injection or wound irrigation could introduce bacteria directly into the body, causing serious infection or sepsis. This is why medical water products, such as Sterile Water for Injection, USP, must be explicitly labeled as sterile and processed to meet extremely rigorous standards. Even for seemingly benign applications like nasal rinsing, health authorities recommend using water that is either sterile or boiled and cooled to prevent potentially deadly infections.
Conclusion
While ultra pure water represents the pinnacle of chemical purity, it is not inherently sterile. The two are distinct qualities achieved through different processes to serve different, critical purposes. UPW is about the absence of chemical contaminants for industrial and analytical precision, whereas sterile water is about the complete absence of living microorganisms for biological and medical safety. For any application where microbial contamination is a risk, always use water that has been specifically processed and guaranteed as sterile. Simply relying on ultra-purity is a dangerous mistake. For non-medical applications, understanding the production methods and limitations of your water source is key to achieving reliable and safe outcomes.
Visit the U.S. Pharmacopeia website for official standards on water for pharmaceutical use
The Importance of Correct Water Grade Selection
Choosing the wrong water grade for a task can have serious consequences, from invalidating scientific results to causing patient harm. A lab technician using non-sterile UPW for a cell culture experiment may find their culture overrun by microorganisms, ruining weeks of work. A medical professional using non-sterile water for irrigation could cause a secondary infection in a patient's wound. In contrast, using expensive, sterile water for a non-critical task like a steam iron is wasteful. Therefore, it is essential to always match the water's properties to the application's specific requirements. Knowing when purity is enough and when sterility is mandatory is a fundamental rule in both scientific and medical fields.
