Many assume purified water is free of all contaminants, including microorganisms. However, a key distinction exists between “purified” and “sterile” water. This difference is important for various applications, from medical procedures to daily household uses. This article clarifies purification and sterilization, explaining why purified water is not sterile.
What is Purified Water?
Purified water undergoes processes designed to remove chemical impurities and dissolved solids. Common methods include distillation, where water is boiled and condensed, leaving behind non-volatile substances. Another widely used technique is deionization, which removes charged ions like minerals through ion-exchange resins. Reverse osmosis employs a semi-permeable membrane to filter out dissolved salts and larger molecules, effectively removing particles down to approximately 0.0001 microns.
These processes reduce inorganic compounds, heavy metals, and organic chemicals. While achieving high chemical purity, these methods do not eliminate all microscopic life. Viruses or bacterial spores can remain after certain purification steps, depending on the method.
What is Sterile Water?
Sterile water is defined by the complete absence of all viable microorganisms, such as bacteria, viruses, fungi, and spores. Achieving this requires rigorous sterilization methods targeting biological contaminants. Autoclaving is a widely recognized method, involving heating water to high temperatures, typically 121 degrees Celsius (250 degrees Fahrenheit), under pressure for a sustained period, often 15 to 20 minutes. This intense heat and pressure effectively denature proteins and destroy microbial structures.
Filtration through membranes with small pore sizes, like 0.2 micron filters, physically blocks bacteria and larger microorganisms. Irradiation, using ultraviolet light or gamma rays, sterilizes water by damaging microbial DNA, preventing replication. Standards for sterile water are stringent, requiring validated processes to ensure no living organisms.
Why Purified Water Isn’t Sterile
Purified water is not sterile because its primary goal is removing non-living contaminants, not eradicating all living microorganisms. Processes like reverse osmosis or deionization effectively remove dissolved solids and chemical impurities. However, they may not eliminate all bacteria, viruses, or microbial spores. Some filters might not capture the smallest viruses or spores, or recontamination can occur.
The difference lies in their objectives: purification aims for chemical purity, while sterilization targets biological purity. Microorganisms are resilient and can survive conditions that eliminate chemical contaminants. While chemically clean, purified water can still harbor microscopic life forms, rendering it non-sterile.
When Sterility Matters
The distinction between purified and sterile water is important where even minimal microorganisms pose a risk. In medical settings, sterile water is necessary for procedures like wound irrigation, diluting medications, or preparing intravenous solutions. Using non-sterile water could introduce pathogens, leading to serious infections. Contact lens solutions must be sterile to prevent eye infections. Water for immunocompromised infants’ formula also requires sterility to protect developing immune systems.
In laboratories, sterile water prevents microbial contamination that could compromise experimental results or cell cultures. Conversely, purified water is sufficient for many everyday uses, such as drinking, household cleaning, or industrial processes. The risks of microbial contamination dictate whether purified water’s chemical purity is adequate or if sterile water’s absolute absence of living organisms is required.