Defining Water Purity
Water purity is a relative term, as absolutely 100% pure water ($H_2O$) is virtually impossible to maintain, since it will begin absorbing gases like carbon dioxide from the air upon exposure. Instead, purity is measured by the removal of contaminants, which include inorganic ions, organic compounds, bacteria, and dissolved solids. Different applications require different levels of purity, and water is often classified into grades based on its intended use, especially in scientific and medical settings.
Ultrapure Water (Type I)
In a laboratory or industrial context, ultrapure water is considered the pinnacle of purification. It is not a single product but rather the result of a multi-stage purification process that can include pre-filtration, reverse osmosis, deionization, and UV oxidation. The result is water with an extremely high resistivity (over 18 MΩ-cm), indicating minimal ionic content. This grade of water is essential for highly sensitive applications like high-performance liquid chromatography (HPLC) and molecular biology experiments.
Common Purification Methods
Several methods are used to create different levels of purified water, each with distinct advantages and disadvantages.
Distillation
Distillation is one of the oldest and most reliable methods of purification. It involves boiling water and collecting the resulting steam, which then condenses back into a liquid. This process leaves behind non-volatile contaminants like minerals, heavy metals, bacteria, and viruses. While effective, distillation is energy-intensive and removes beneficial minerals along with the unwanted impurities.
Deionization (DI)
Deionization is a chemical process that removes mineral ions by passing water through ion-exchange resins. Positively charged ions (cations) are exchanged for hydrogen ions, and negatively charged ions (anions) are exchanged for hydroxyl ions. These then recombine to form water. This process is very efficient at producing water with low conductivity but does not effectively remove uncharged organic molecules or microorganisms.
Reverse Osmosis (RO)
Reverse osmosis uses pressure to force water through a semi-permeable membrane. This membrane allows water molecules to pass through while rejecting most dissolved solids, salts, and other contaminants. RO is a highly effective method used in both home water filtration systems and large-scale industrial desalination plants. A downside is the high volume of wastewater produced during the process.
Combined Methods
For the highest grades of ultrapure water, combinations of these methods are used. A typical process might start with reverse osmosis, followed by deionization and then polished with UV sterilization to create the purest water possible for critical applications.
Comparison of Purification Methods
| Feature | Distillation | Deionization (DI) | Reverse Osmosis (RO) |
|---|---|---|---|
| Purity | Very high, removes most minerals and contaminants. | Very high, specifically removes mineral ions. | High, removes 96-99% of contaminants. |
| Removes Minerals | Yes (99.9% removed) | Yes (Nearly all removed) | Yes (Significant reduction) |
| Removes Bacteria | Yes (Boiling kills pathogens) | No (Does not remove uncharged molecules) | Yes (Membrane blocks microorganisms) |
| Energy Use | High (Energy-intensive boiling) | Low to moderate (Uses resins) | Moderate (Requires pressure pump) |
| Wastewater | Minimal | Minimal (Requires resin regeneration) | High (Significant amount wasted) |
| Primary Use | Laboratories, medical equipment, humidifiers | Industrial processes, lab work requiring low conductivity | Drinking water purification, industrial processes |
The Purist Water for Drinking
While laboratory-grade ultrapure water is technically the purest, it is not ideal for drinking. The removal of essential minerals like calcium and magnesium can negatively impact health over the long term. For drinking, water purified by reverse osmosis is often preferred, as it removes harmful contaminants while some systems add back trace minerals for taste and health. In areas with safe public tap water, the best choice is often a simple filter to remove chlorine and improve taste, preserving beneficial minerals.
A Note on Rainwater
Rainwater is often considered the purest form of natural water, but this is a generalization. While it starts pure through the natural distillation process of the water cycle, it can pick up atmospheric pollutants as it falls and collects surface contaminants from roofs or storage tanks. Proper filtration and treatment are necessary to make harvested rainwater safe for drinking.
Conclusion
The question of what is the purest type of water has a complex answer that depends on context and application. For critical scientific and industrial purposes, ultrapure water produced by a combination of advanced filtration, deionization, and sterilization represents the highest achievable standard of purity. For everyday consumption, the purest water for drinking is one that is safe and free of contaminants but retains naturally occurring minerals essential for human health, such as water from a high-quality reverse osmosis system or properly filtered tap water. The key is aligning the purification method with the intended use to achieve the most appropriate level of purity.
