Understanding Distilled Water
Distilled water is produced by heating water to its boiling point, collecting the resulting steam, and condensing it back into a liquid. This process leaves nearly all impurities, including minerals, heavy metals, and most organic compounds, behind. While highly pure, distillation is energy-intensive and can leave behind volatile organic compounds (VOCs) that evaporate with the steam if not followed by additional filtration. The resulting water has a flat taste due to the removal of beneficial minerals.
The closest alternatives to distilled water
For many applications, several alternatives can achieve a level of purity comparable to, or even exceeding, that of distilled water. The two primary contenders are reverse osmosis and deionized water.
Reverse osmosis (RO) water
Reverse osmosis forces water through a very fine, semi-permeable membrane at high pressure. This membrane allows water molecules to pass through while trapping and flushing away dissolved solids, inorganic minerals, bacteria, and many chemical contaminants. A typical RO system can remove 95-99% of dissolved salts and other impurities. While highly effective, RO alone may not remove all volatile chemicals, so systems often incorporate carbon pre- and post-filters.
Deionized (DI) water
Deionized water is produced by running water through ion exchange resins that attract and replace mineral ions with hydrogen ($H^+$) and hydroxyl ($OH^-$) ions. These ions then combine to form pure water ($H_2O$). The process is highly effective at removing ionized minerals and salts, resulting in water with extremely low conductivity and a TDS of zero. However, DI alone does not effectively remove uncharged particles, bacteria, or viruses.
Comparison of Water Purity Methods
To better understand the differences and similarities between these water types, the following table compares their purification methods, purity levels, and primary uses.
| Feature | Distilled Water | Reverse Osmosis (RO) Water | Deionized (DI) Water | Ultrapure Water (RO + DI) |
|---|---|---|---|---|
| Purification Method | Boiling and re-condensing steam | Pressurized water forced through a semi-permeable membrane | Passing water through ion-exchange resins | Combination of RO followed by DI |
| Effectiveness (Contaminants Removed) | Removes minerals, heavy metals, bacteria, viruses | Removes 95-99% of dissolved solids, salts, and chemicals | Removes mineral ions and salts (charged particles) | Removes nearly all ions, dissolved solids, bacteria, and organics |
| Purity Level (TDS) | Often 0 ppm TDS | Very low TDS (<50 ppm) | 0 ppm TDS at its peak efficiency | The highest level of purity, virtually 0 TDS |
| Removes Uncharged Contaminants? | Yes, most are left behind in the boiling chamber | Yes, depending on filter stages | No, bacteria and organics are not removed | Yes, effectively removes all |
| Energy Efficiency | High energy consumption due to boiling | Lower energy use than distillation | Low energy, but resin regeneration requires chemicals | Moderate energy, but can be highly efficient |
| Best For | CPAP machines, humidifiers, car batteries | Drinking water, cooking, aquariums | Industrial processes, lab work, cosmetic production | High-tech manufacturing, advanced lab research |
Other water types and their limitations
Some might consider simple purified water or filtered water, but they don't achieve the same level of purity. The term "purified water" is broad and can involve many methods, but generally, it must contain no more than 10 ppm TDS. However, this is still a higher concentration of dissolved solids than properly distilled, RO, or DI water. Standard filtered water, typically passed through activated carbon, removes chlorine and improves taste but does not significantly lower the TDS or remove minerals.
Combining purification methods for maximum purity
In scenarios demanding the absolute highest level of water purity, such as in scientific research or specific manufacturing processes, combining purification methods is a standard practice. A common setup involves using a reverse osmosis system as a primary purification step, followed by a deionization process. This creates "ultrapure" water by first removing the majority of contaminants via the RO membrane and then polishing the water with DI resins to remove any remaining ionic impurities. This multi-stage process yields a final product with purity that surpasses what a single method can achieve, especially for applications sensitive to even trace amounts of contaminants.
How to choose the right water
Choosing the right water type depends entirely on its intended use. For household appliances that are sensitive to mineral buildup, like humidifiers, steam irons, and CPAP machines, distilled water is the classic recommendation. For everyday drinking and cooking, an RO system provides excellent purity and taste without the high energy cost of distillation. In industrial or laboratory settings requiring the removal of all ionic impurities, DI water is the go-to, often used in tandem with other filtration methods for optimal results. For most home users seeking water comparable to distilled water, a modern RO system offers a practical and efficient solution for high-quality, low-TDS water.
Conclusion: Reverse osmosis and deionized water are the closest rivals
While distilled water is the benchmark for purity, removing nearly all contaminants by boiling and condensation, reverse osmosis and deionized water are exceptionally close alternatives, and in some cases, can even be purer. For household needs like drinking and cooking, an RO system offers the best balance of efficiency, cost, and high purity, while deionized water is crucial for industrial applications requiring ionic purity. The combination of both methods creates ultrapure water, achieving a level of contaminant removal that exceeds standard distillation and making it the cleanest water available for specialized use. Ultimately, the best choice depends on balancing the specific purity requirements of the task at hand with the convenience and cost of the purification method.
