Consumers seeking highly purified water often choose between distilled water (DW) and reverse osmosis (RO) water. Both methods significantly reduce contaminants, offering a cleaner alternative to standard tap water. While both aim for high purity, the physical and chemical principles employed by each system are fundamentally different. Understanding these distinct approaches helps determine which type of purified water is more suitable for a particular need.
Defining the Production Processes
Distilled water is created through a process mimicking the Earth’s natural water cycle, relying on a phase change to separate water from impurities. Source water is heated to its boiling point, converting it into steam and leaving behind all non-volatile substances like minerals and heavy metals. This purified vapor is then collected and cooled in a separate chamber, where it condenses back into liquid water, resulting in near-perfect purity.
Reverse osmosis (RO), in contrast, is a physical filtration method utilizing pressure and a specialized barrier. Water is forced through a semi-permeable membrane containing microscopic pores. These pores allow water molecules to pass through but block most dissolved solids and contaminants. This process continuously separates the input water into purified permeate water and concentrated reject water carrying the trapped impurities.
Purity Levels and Contaminant Removal
Distilled water removes virtually all Total Dissolved Solids (TDS), often achieving 99.9% purity or higher. The boiling process effectively eliminates inorganic solids, minerals, bacteria, and viruses. However, distillation may not remove all volatile organic compounds (VOCs) or chemicals with a lower boiling point than water, as these can vaporize and recondense in the final product.
Reverse osmosis systems are highly effective, typically removing 90% to 99% of TDS and a broad spectrum of contaminants, including lead, salts, and fluoride. The effectiveness of RO depends on the membrane’s quality and system maintenance, resulting in a slightly lower percentage of impurity removal than distillation. Standard RO membranes may struggle to block certain very small molecules or dissolved gases, sometimes requiring additional carbon filtration stages. The resulting RO water is considered purified water, which the U.S. Environmental Protection Agency permits to contain up to 10 parts per million of dissolved solids.
Implications for Human Consumption
When considering drinking water, the high purity of both types raises concerns about mineral content and taste. Distilled water is nearly devoid of all minerals and is often described as having a flat or bland taste, which can discourage regular consumption. The lack of dissolved solids also means distilled water can become slightly acidic, as it readily absorbs carbon dioxide from the air.
Reverse osmosis water, particularly from modern home systems, is often the preferred choice for daily drinking and cooking. While the RO process removes beneficial minerals like calcium and magnesium, many systems incorporate a post-filter stage to reintroduce a balanced blend of these minerals. This remineralization improves the water’s taste and provides a healthier profile, making it more palatable than distilled water. Consuming highly demineralized water, such as distilled water, may not pose an immediate health risk since the body obtains most minerals from food. However, some experts suggest that highly demineralized water could potentially cause an electrolyte imbalance if consumed excessively without sufficient dietary mineral intake.
Specialized Non-Drinking Uses
The near-absolute purity of distilled water makes it uniquely suited for applications where any dissolved solid is problematic. Distilled water is the industry standard for use in medical equipment like autoclaves, laboratory experiments, and lead-acid batteries. Its zero-TDS content prevents the formation of mineral scale and deposits, maintaining the efficiency and lifespan of devices such as steam irons, humidifiers, and CPAP machines.
Reverse osmosis water is often employed in high-volume commercial and specialized domestic settings. RO water is commonly used in hydroponics and aquariums, as its low contaminant level allows precise control over the nutrient or mineral composition added back into the water. Furthermore, its production speed is significantly faster than the slow, energy-intensive process of distillation. This makes RO a more practical choice for continuous supply in many industrial processes or for general household use.