Reverse Osmosis (RO) water and Distilled Water (DW) are both highly purified, leading many to assume they are interchangeable. While tap water contains varying levels of dissolved solids, minerals, and contaminants, both RO and DW processes significantly reduce these impurities. However, the distinct purification methods yield products with subtle but important differences in purity and composition. Understanding these differences is necessary to determine if one can truly be substituted for the other in specific household or industrial applications.
Understanding How Each Water is Purified
Distilled water relies on a process that mimics the natural water cycle, known as phase change. Water is boiled until it converts entirely into steam, leaving behind virtually all non-volatile substances like heavy minerals, salts, and dissolved solids. The steam is then collected and cooled in a separate container, condensing back into its liquid form as pure water. The resulting water is essentially pure H₂O because only the water molecules transform into vapor.
Reverse osmosis is a pressure-driven membrane filtration process rather than a thermal one. Source water is forced under pressure through a semi-permeable membrane with extremely tiny pores, measured at about 0.0001 microns. These microscopic pores allow water molecules to pass through while blocking larger molecules and ions, such as dissolved salts and contaminants. A standard RO system is often multi-stage, typically including pre-filters like sediment and activated carbon filters to remove larger particles and chemicals before the water reaches the membrane.
Key Differences in Purity and Composition
The fundamental difference between the two lies in their final purity, which is often measured by Total Dissolved Solids (TDS). Distilled water is the gold standard for purity, consistently achieving a TDS level of 0.5 parts per million (ppm) or less. This near-zero TDS content means the water is almost entirely stripped of all inorganic and organic substances.
Reverse osmosis water, while extremely clean, typically retains a small fraction of the original TDS because the membrane removes a percentage of impurities, not all of them. A properly functioning RO system reduces the influent TDS by 85% to 99%. This results in a final product that usually has a TDS between 10 and 50 ppm, depending on the source water quality and system efficiency.
The two methods also differ in their effectiveness against various types of contaminants, particularly gases and volatile organic compounds (VOCs). Distillation is highly effective at removing non-volatile substances, but it may not eliminate VOCs that boil at or below the temperature of water unless the system is vented. Conversely, the activated carbon filters used in most RO systems are highly effective at reducing VOCs and chlorine.
Both types of purified water tend to have a slightly acidic pH because they lack the buffering minerals found in tap water. When exposed to air, the water absorbs carbon dioxide, which creates a mild carbonic acid. Some RO systems address this by incorporating a re-mineralization stage, which adds beneficial minerals like calcium and magnesium back into the water, resulting in a more neutral pH and improved taste.
Interchangeability for Common Household Uses
In many cases, the slight difference in purity between RO and DW is negligible, allowing for interchangeability in general applications. For drinking and cooking, RO water is widely preferred because it is clean yet often retains or has re-added minerals, giving it a more palatable taste than the flat flavor of distilled water. Distilled water is perfectly safe to drink, but its total lack of minerals is often perceived as flavorless.
However, for applications requiring the absolute lowest possible mineral content, distilled water is the necessary choice. Appliances such as steam irons, humidifiers, and Continuous Positive Airway Pressure (CPAP) machines are designed to operate using water that leaves no residue. Even the small amount of TDS found in RO water can lead to mineral buildup, or scaling, which can damage internal components and reduce the lifespan of the equipment.
In industrial and automotive contexts, such as topping off batteries or mixing with engine coolants, the high purity of distilled water is non-negotiable. Mineral deposits from RO water could interfere with chemical processes within a battery or cause corrosion within a cooling system. Conversely, for large-scale uses like hydroponics or aquariums, RO water is frequently utilized because its purification level is sufficient and the system is more practical for continuous, high-volume production. In these specialized environments, the water often requires careful re-balancing with nutrients or minerals regardless of the purification method.