The quest for the purest water can be confusing, with marketing terms like 'purified' and 'filtered' sometimes obscuring the real science. The answer depends on whether you are seeking scientific, laboratory-grade purity or the purest, most contaminant-free water available for everyday drinking.
The Scientific Definition of Purest Water: Ultrapure Water
In a laboratory or industrial setting, the definition of pure water is extremely strict and is based on a number of parameters, primarily resistivity, Total Organic Carbon (TOC), and bacterial content. Ultrapure water, often called Type I water, is the absolute purest water that can be produced with current technology. It is not sold on the consumer market for drinking and would be unhealthy to consume exclusively due to its lack of minerals.
What is Ultrapure (Type I) Water?
This grade of water is produced through a multi-stage process that typically includes reverse osmosis (RO), deionization (DI), ultraviolet (UV) light treatment, and microfiltration. Its properties are carefully controlled to meet stringent standards:
- Resistivity: The ability to resist electrical current, measured in megaohms per centimeter (MΩ·cm). Ultrapure water has a resistivity of 18.2 MΩ·cm, indicating a near-total absence of ions.
- Total Organic Carbon (TOC): A measure of organic contaminants. Type I water must have very low TOC levels, often less than 5 parts per billion (ppb).
- Microbes: It is also filtered to remove bacteria and other microorganisms.
Common Purification Methods and Their Purity Levels
For consumers, different methods of purification result in varying degrees of purity. Understanding these processes is key to determining which water is best for your needs.
Distillation: An Ancient Method
Distillation involves boiling water into steam and then condensing it back into a liquid, leaving most impurities behind. This process is highly effective at removing minerals and non-volatile compounds, but volatile organic compounds (VOCs) can sometimes carry over with the steam. Distilled water is available commercially, but because it is completely stripped of minerals, it often tastes flat. It's often used for appliances like CPAP machines and steam irons, not for regular drinking.
Reverse Osmosis (RO): A Modern Standard
Reverse osmosis forces water through a very fine, semi-permeable membrane at high pressure. This process effectively removes up to 99% of dissolved salts, particles, bacteria, viruses, and other contaminants. RO systems are a popular option for home water purification, providing a high level of purity. However, since it also removes beneficial minerals, many RO systems for drinking water include a remineralization stage to improve taste and reintroduce healthy minerals.
Deionization (DI): Removing Mineral Ions
Deionization is a process that uses ion-exchange resins to remove mineral ions, but it does not effectively remove organic molecules or microorganisms. Water is pre-filtered (often with an RO system) before undergoing DI to achieve very low levels of ionic contaminants, useful for many lab applications but not safe to drink without further treatment.
What About Bottled Water on the Market?
The bottled water industry offers a wide range of options, and not all are equally pure. The level of purity depends on the source and processing.
The Bottled Water Landscape
- Purified Water (e.g., Aquafina): This is typically tap water that has been purified using reverse osmosis and other methods. Aquafina, for example, is purified tap water with a very low TDS.
- Fortified RO Water (e.g., Dasani, Smart Water): These brands use reverse osmosis but then add a blend of minerals back for taste and marketing purposes.
- Natural Spring/Artesian Water (e.g., Fiji, Voss): These waters come from a specific underground source. Their purity depends on the filtration provided by the rock and earth, and they contain naturally occurring minerals. Their Total Dissolved Solids (TDS) can be higher than purified brands.
- Alkaline Water (e.g., Icelandic Glacial): Some bottled waters are naturally alkaline due to filtration through volcanic rock, while others are artificially made alkaline through added minerals.
Purity Comparison: A Quick Table
| Water Type | Purification Method(s) | Purity Level (Relative) | Ideal Use | Notes |
|---|---|---|---|---|
| Ultrapure (Type I) | RO, DI, UV, Filtration | Highest (18.2 MΩ·cm) | Sensitive lab applications | Not for drinking; corrosive. |
| Distilled Water | Distillation | Very High (near-zero TDS) | Appliances, humidifiers | Lacks minerals, flat taste; not ideal for regular drinking. |
| Reverse Osmosis (RO) | Membrane Filtration | High (95-99% removal) | Drinking (often remineralized), cooking | Very effective, but some systems use a lot of water. |
| Filtered Tap Water | Activated Carbon, etc. | Varies | Improving taste and removing chlorine | Depends heavily on the filter type and source water. |
How to Measure Water Purity
For consumers, the most common way to get a quick measurement of water purity is using a Total Dissolved Solids (TDS) meter. This device measures the electrical conductivity of water, which is directly related to the amount of dissolved inorganic matter present. A lower TDS reading indicates fewer dissolved solids, but it does not measure other contaminants like bacteria, viruses, or organic chemicals. Laboratory testing is required for a complete analysis of water quality.
Conclusion: Finding Your Definition of Purity
Ultimately, the concept of "purest water" depends on context. The scientifically purest water, ultrapure water, is for industrial use only and not safe for drinking. For everyday consumption, a high-quality reverse osmosis system is a practical way to achieve a very high level of purity in your home. Some remineralized RO systems and natural spring waters offer a balance of high purity and desirable mineral content. When choosing, it's important to consider your specific needs and the contaminants you wish to remove. For those concerned about general safety standards, organizations like the World Health Organization provide comprehensive guidelines. World Health Organization.
Understanding Water Purity Levels
- TDS is a limited indicator: While a TDS meter is useful for measuring total dissolved minerals and salts, it cannot detect biological contaminants like bacteria or organic chemicals.
- Remineralization is important: Water purified to a high degree (like RO or distilled) is often remineralized for drinking purposes to improve taste and replace lost minerals.
- Source matters for bottled water: Not all bottled water is the same; spring waters contain natural minerals, while purified bottled waters start from a municipal source.
- Lab-grade water is an extreme: The purest grades of water are a product of extreme purification processes for sensitive applications and are not meant for human consumption.
- Combined methods yield highest purity: Systems that combine multiple purification stages, such as RO, DI, and UV, achieve the highest non-lab-grade purity for drinking water.
Key Takeaways
- Ultrapure Water is Not for Drinking: The purest water, with a resistivity of 18.2 MΩ·cm, is a laboratory-grade product and is not intended for consumption.
- Distilled Water Lacks Minerals: Distilled water, created by boiling and condensing, is very pure but lacks beneficial minerals and can taste flat, making it less ideal for regular drinking.
- Reverse Osmosis (RO) Offers High Purity: RO systems can remove 95-99% of contaminants, making them a popular and effective method for creating very pure drinking water at home.
- Bottled Water Purity Varies: Different bottled water brands have varying purity levels based on their source (spring vs. purified tap water) and processing (e.g., remineralization).
- Measure Purity with Care: While a TDS meter can give a quick reading of dissolved solids, it doesn't measure all contaminants, and specialized lab tests are needed for full analysis.
