RO water is not distilled water. They share similarities, especially in producing clean water with very few dissolved minerals, but they rely on completely different purification methods and yield slightly different results. Understanding what sets them apart matters if you’re choosing between them for drinking, appliances, or medical devices.
How Each Purification Method Works
Reverse osmosis pushes water under high pressure through a semipermeable membrane, typically made of polyamide. Water molecules pass through while dissolved salts, heavy metals, bacteria, and other contaminants get trapped on the other side. Most home RO systems also include additional sediment and carbon filters in a multi-stage setup.
Distillation takes a fundamentally different approach. Water is boiled into steam, and that steam is collected and cooled in a separate container until it condenses back into liquid. Because impurities that don’t evaporate at water’s boiling point stay behind in the original container, the condensed water is extremely pure. It’s the same basic principle behind rainfall: evaporation naturally separates water from everything dissolved in it.
Purity Levels Are Close but Not Identical
Distilled water typically measures between 0 and 10 parts per million (ppm) of total dissolved solids. RO water lands a bit higher, generally between 10 and 50 ppm. Both are far cleaner than typical tap water (which can range from 100 to over 500 ppm), but distillation edges ahead in raw purity because it leaves behind virtually everything that doesn’t turn to steam.
That small gap in dissolved solids also shows up in pH. RO water tends to be slightly acidic, around 6.0 to 6.5 on the pH scale, because removing mineral buffers allows carbon dioxide in the air to dissolve into the water and lower its pH. Distilled water starts near a neutral 7.0 but drifts slightly acidic for the same reason once exposed to air. In practice, neither is acidic enough to cause health concerns.
What Each Method Removes Best
Both methods are effective against heavy metals, salts, and bacteria. Where they differ is in the margins. Distillation is particularly strong against bacteria, viruses, and organic compounds because the boiling process kills or separates out nearly all biological contaminants. RO membranes achieve virus removal in the range of 4.1 to 7 log reduction values (meaning they block 99.99% or more of viruses), which is excellent, but distillation’s thermal process is inherently more reliable against pathogens since nothing living survives the boiling step.
RO, on the other hand, handles a broader spectrum of dissolved solids in a single pass and can be paired with specialized carbon filters to catch volatile organic compounds that might actually carry over during distillation (since some chemicals have boiling points close to water’s). Neither method is perfect on its own, but both produce water far cleaner than what comes out of a standard faucet filter.
Energy and Water Waste
The two methods consume resources in very different ways. Distillation is energy-intensive: you’re boiling water for an extended period, which adds up on your electricity or gas bill. A countertop distiller can take four to six hours to produce a single gallon.
RO systems use much less energy (they only need enough pressure to push water through the membrane), but they waste water. For every gallon of purified water, a typical home RO system sends two to four gallons down the drain as reject water carrying the concentrated contaminants. Industrial-scale RO plants recover more, and energy recovery systems can cut their power consumption by roughly 35%, but for a home unit, water waste is the main tradeoff.
When It Matters Which One You Use
For most drinking purposes, RO and distilled water are interchangeable. Both taste flat compared to mineral water because the minerals that give water flavor have been removed. If you find either one unappealing, adding a remineralization filter or trace mineral drops restores some calcium, magnesium, and bicarbonate, which improves both taste and nutritional value. Many home RO systems now include a remineralization stage for exactly this reason.
For certain appliances and medical devices, the distinction matters more. CPAP machine manufacturers specifically recommend distilled water in humidifier chambers. Even the trace minerals left in RO water (those 10 to 50 ppm of dissolved solids) can build up as mineral deposits inside the machine over time. The same logic applies to steam irons, autoclaves, and laboratory equipment where any mineral residue causes problems. RO water works as a short-term substitute, but you’ll need to clean the equipment more frequently.
For car batteries, aquariums, and home brewing, RO water is often preferred because you can control exactly which minerals get added back. Starting with a blank slate of low-mineral water lets you customize the chemistry for whatever you’re doing.
The Mineral Question for Drinking
Both RO and distilled water strip out beneficial minerals like calcium and magnesium along with contaminants. A narrative review published in Cureus examined the long-term effects of drinking low-mineral water and found that remineralizing RO water is recommended to restore calcium and magnesium levels for both taste and health. The most common approach involves passing the water through a calcium carbonate filter with added CO2, though simpler options like alkaline pitchers and mineral drops also work.
If your diet already includes plenty of mineral-rich foods (dairy, leafy greens, nuts), the minerals missing from your water are a small fraction of your total intake. But if you’re relying heavily on water as a mineral source, or if you drink large volumes of either RO or distilled water daily, adding minerals back makes sense.
Quick Comparison
- Purity: Distilled water (0 to 10 ppm) is slightly purer than RO water (10 to 50 ppm)
- Method: Distillation boils and condenses; RO forces water through a membrane
- Speed: RO produces water on demand; distillation takes hours per gallon
- Waste: RO wastes water; distillation wastes energy
- Pathogen removal: Distillation is more reliable against viruses and bacteria
- Best for appliances: Distilled water is preferred for CPAP machines, steam irons, and lab equipment
- Best for drinking: Either works well, especially with remineralization