Water terminology can be confusing, especially with the variety of labels used on bottled products and purification systems. While both purified water and distilled water undergo extensive processing to remove contaminants, they are not the same product. These terms are often assumed to be interchangeable, but they differ significantly in their production methods and final chemical composition. Understanding these differences is necessary to determine which type of water is appropriate for a particular use.
The Fundamental Distinction in Purity
The primary difference between these two types of water lies in the standard of purity they achieve. Purified water is defined as water treated to meet stringent regulatory standards, such as those set by the U.S. Pharmacopeia (USP) or the EPA. To earn the “purified” label, the water must have a Total Dissolved Solids (TDS) concentration no greater than 10 parts per million (ppm). This standard ensures the removal of chemicals, pathogens, and various contaminants.
Purified water is a broad designation indicating a minimum level of cleanliness, regardless of the method used. Although contaminants are removed, trace minerals may still be present in the final product, or they may be intentionally added back for flavor. In contrast, distilled water is a specific type of purified water that represents an extreme level of demineralization.
Distillation involves boiling water into steam and then condensing the steam back into a liquid. This process is highly effective at separating water molecules from nearly everything else, including minerals, salts, and non-volatile chemicals. Consequently, distilled water typically has an ultra-low TDS, often falling in the range of 1 to 5 ppm or even lower. This makes it closer to pure H₂O than most other types of water.
Processes That Define Each Water Type
The purity profile of each water type is a direct result of the manufacturing methods employed. Distillation is a thermal separation process that capitalizes on the differing boiling points of water and its dissolved impurities. When water is boiled, it vaporizes into steam, leaving behind substances like heavy metals, salts, and minerals. The resulting steam is then collected and cooled in a separate container, condensing back into exceptionally clean water.
While highly effective, distillation may not remove certain volatile organic compounds or pesticides that can vaporize along with the water. Achieving the “purified” standard often requires a combination of sophisticated filtration techniques. One of the most common methods is Reverse Osmosis (RO), which forces water through a semi-permeable membrane to trap larger molecules and dissolved solids.
Other techniques frequently used for purification include deionization (DI), which uses ion exchange resins to remove charged ions. Extensive carbon filtration targets chlorine and organic compounds that affect taste and odor. The use of multiple methods, such as RO followed by deionization and ultraviolet (UV) light disinfection, ensures that purified water meets the specified low-TDS requirement.
Practical Applications and Health Considerations
The distinct compositions of purified and distilled water lead to different practical applications. Distilled water’s near-total absence of minerals makes it the preferred choice for sensitive equipment. Appliances like steam irons, humidifiers, and CPAP machines rely on mineral-free water to prevent the accumulation of scale. Distilled water is also indispensable in laboratories and industrial settings where trace amounts of dissolved solids could interfere with chemical reactions or contaminate high-precision manufacturing processes.
Purified water, on the other hand, is the most common choice for daily consumption and cooking. While both types are safe to drink, the thorough mineral stripping in distilled water often gives it a noticeably “flat” taste. Purified water may retain some naturally occurring minerals or have them added back specifically to improve flavor and palatability.
Regarding health, drinking distilled water is generally not considered unsafe, but sustained, long-term consumption is not recommended for the general public. The lack of electrolytes and minerals, such as calcium and magnesium, means the water does not contribute to the body’s daily mineral intake. Purified water meets high safety standards while potentially retaining or reintroducing beneficial minerals. It is a healthy and suitable option for consistent daily hydration.