Reverse osmosis drinking water is water that has been purified by forcing it through a specialized membrane that blocks most dissolved contaminants, minerals, and impurities. The process removes 70% to 99% of common pollutants depending on the substance, producing some of the cleanest drinking water available from a home filtration system. It’s widely used both in bottled water production and in under-sink systems installed in homes with municipal or well water.
How Reverse Osmosis Works
Osmosis is a natural process: when two bodies of water with different concentrations of dissolved substances are separated by a membrane, water naturally flows toward the more concentrated side. Reverse osmosis flips this by applying pressure to the concentrated (dirty) side, pushing water molecules through the membrane while leaving contaminants behind.
The membrane itself has pores small enough to let water molecules pass through via hydrogen bonding while physically blocking larger particles. Most organic substances with a molecular weight over 100 get sieved out, including bacteria, viruses, and other particulates. Dissolved salts and metal ions are rejected by a different mechanism: charged ions are electrically repelled from the membrane surface, with higher-charged ions repelled more strongly. This combination of physical filtering and electrical repulsion is what makes RO effective against such a wide range of contaminants.
What It Removes
A residential RO system is required to reduce total dissolved solids (TDS) to earn certification under the NSF/ANSI 58 standard, which is the main benchmark for home RO units. Beyond that baseline, certified systems can also be tested and verified for reduction of specific contaminants including lead, arsenic, fluoride, nitrates, cadmium, chromium, barium, copper, radium, selenium, and volatile organic compounds.
Real-world performance varies based on the specific system, water chemistry, and filter condition. In a study of private wells with high arsenic contamination, RO filters reduced arsenic concentrations by about 78% to 79% on average under actual household conditions. Laboratory testing of RO devices has shown arsenic reduction ranging from 70% to 99%, with the spread depending on the manufacturer, the starting concentration, water pH, and how much water had already been filtered through the unit. Similar variability applies to other contaminants, which is why checking for NSF/ANSI 58 certification matters: it confirms the system has been independently tested for the specific claims on its label.
The Mineral Question
Because RO membranes don’t distinguish between harmful contaminants and beneficial minerals, the process strips out calcium and magnesium along with everything else. This is the most debated health consideration around RO water. A review published in the National Library of Medicine found that even in industrialized countries with otherwise adequate diets, food alone may not fully compensate for the absence of calcium and especially magnesium in drinking water.
The same research linked long-term consumption of soft (low-mineral) water to a somewhat higher risk of cardiovascular disease, with magnesium being the more likely protective factor in hard water. Other associations noted in the literature include higher fracture risk in children drinking low-calcium water, and possible connections to certain pregnancy complications and neurodegenerative diseases, though these links are less firmly established.
For most people eating a varied diet and getting minerals from food, the practical risk is likely small. But if RO water is your primary source and your diet is already low in calcium or magnesium, adding minerals back is worth considering.
pH and Taste
RO water tends to be slightly acidic compared to tap water. This happens because removing the bicarbonate ions that normally buffer water’s pH leaves it with less resistance to acidity. The result is water that typically sits below neutral pH 7, sometimes noticeably so. Some people describe the taste as flat or slightly sour compared to mineral-rich tap water.
Many RO systems now include a remineralization stage to address this. These filters use calcite (naturally occurring calcium carbonate) that slowly dissolves into the purified water, raising the pH to around 8.5 to 9.5 while adding back some calcium. Beyond improving taste, this step has a practical benefit: slightly acidic water can leach copper and lead from metal plumbing, so raising the pH before the water reaches your pipes and fixtures reduces that risk.
Water Waste and Efficiency
RO systems produce wastewater as a byproduct. The rejected contaminants have to go somewhere, and they flow out in a concentrated stream of water that goes down the drain. A typical residential unit uses 4 to 5 gallons of water for every 1 gallon of purified water it produces, making the waste ratio roughly 4:1 to 5:1. That means your water bill will reflect more usage than what actually comes out of your RO faucet.
Some newer systems advertise improved efficiency ratios, and the NSF/ANSI 58 standard now includes an efficiency rating as part of certification. If water waste concerns you, compare the recovery ratings of different models before purchasing. Some homeowners also route the wastewater to garden irrigation or other non-drinking uses to offset the loss.
Maintenance and Filter Replacement
An RO system isn’t a set-it-and-forget-it appliance. A typical under-sink unit has three main components that need periodic replacement on different schedules:
- Sediment filter: every 6 to 12 months. This pre-filter catches sand, silt, and rust particles before they reach the membrane.
- Carbon filters: every 6 to 12 months. These remove chlorine and organic chemicals that would otherwise damage the RO membrane.
- RO membrane: every 2 to 3 years. This is the core of the system and the most expensive component to replace.
Skipping filter changes doesn’t just reduce water quality. A clogged sediment or carbon filter forces the membrane to work harder and shortens its lifespan, and a degraded membrane lets contaminants pass through that it would normally block. If your water tastes different or your system’s flow rate drops noticeably, those are signs a filter is overdue. Keeping up with the replacement schedule is the single most important factor in whether your RO system actually delivers the clean water it promises.
Who Benefits Most From RO Water
RO makes the most sense when your water source has a known contamination problem. If you’re on a private well with elevated arsenic, nitrates, or other pollutants, a point-of-use RO system is one of the most effective and affordable solutions. It’s also useful in areas where municipal water has high TDS, heavy metals, or taste issues that a basic carbon filter can’t handle.
For homes already receiving clean, well-treated municipal water, RO is less of a necessity and more of a preference. The tradeoff is paying more for water (both the system cost and the wasted water), losing beneficial minerals, and committing to regular maintenance, in exchange for an extra layer of purification. Whether that tradeoff makes sense depends on your local water quality, which you can check through your utility’s annual consumer confidence report or by testing your water independently.