An RO water purifier uses reverse osmosis to force water through a semipermeable membrane, removing dissolved solids, heavy metals, and many chemicals.
If you’ve shopped for drinking water systems, you’ve seen RO purifiers advertised as the serious option. The technology behind them is straightforward: a pump pushes water against a semipermeable membrane, and only water molecules and a few other tiny particles make it through. The water that passes is the purified product; the rest, carrying the contaminants, flushes away as reject water. The U.S. EPA describes these as point-of-use systems, meaning they treat water at a single tap or faucet rather than the whole house. That distinction matters when you’re deciding what to buy.
The EPA outlines that RO can reduce a wide range of contaminants, including lead, copper, chromium, arsenic, volatile organic compounds (VOCs), PFAS, and radionuclides. It also removes dissolved solids like salt and minerals. The trade-off is water waste and ongoing maintenance, which this guide covers below.
The Core Filtration Process
Reverse osmosis is pressure-driven separation. Water is pushed through a thin, semipermeable membrane with pores so small that most contaminants simply can’t fit through. The treated water that emerges is called permeate or product water; the concentrated waste stream is called concentrate or reject water.
The membrane doesn’t target one specific pollutant. The EPA notes that RO and nanofiltration (NF) systems are effective for mixtures of contaminants but don’t selectively remove just one. That’s why a typical RO purifier is more than just the membrane. Most systems are three, four, or five stage setups: sediment and carbon filters remove larger particles and chlorine, the RO membrane handles dissolved solids, and a final polishing filter often improves taste before the water reaches your glass.
What RO Removes And What It Can’t
RO purifiers are hired for their broad reach. The EPA lists the following among contaminants these systems can effectively reduce:
- Lead, copper, chromium, and arsenic
- Volatile organic compounds (VOCs)
- PFAS and other synthetic organic chemicals
- Radionuclides and dissolved solids
- Salts and other inorganics
That is an impressive range, but there are clear limits. The RO membrane alone does not reliably remove all microorganisms; the EPA notes these systems are designed for water that is already considered microbiologically safe. Also, because the membrane acts like a filter, it reduces minerals you might actually want for taste or nutrition. Many people add a remineralization stage for that reason.
The gallon-for-gallon trade-off is the other big caveat. Older EPA guidance states RO units use roughly three times as much water as they treat — meaning for every gallon of purified water, about two gallons go down the drain as reject water. Those numbers have improved with modern efficiency standards, but some waste is unavoidable with this technology.
Cost, Maintenance, And The WaterSense Difference
A residential point-of-use RO system typically costs between $150 and $1,000 upfront, according to EPA guidance, not including ongoing maintenance. That price range covers under-sink models, faucet-mounted options, and countertop units.
The ongoing cost is recurring. Cartridges and the RO membrane itself need regular replacement — typically a few times per year, depending on your water quality and the system’s design. If you skip these changes, performance drops and your water quality suffers.
If you want a system that’s efficiency-certified, look for the WaterSense label. The EPA’s WaterSense specification for point-of-use RO systems, which took effect for participating products on November 14, 2024, requires an efficiency rating of at least 30 percent. That’s a significant step up from older models. Before buying, always check for certification under NSF/ANSI 58 (the standard for RO systems) or NSF/ANSI 53 (for specific contaminant reduction like PFAS). The EPA recommends confirming on the manufacturer’s website or the NSF listing that the system is certified for the specific contaminants you’re worried about.
| Feature | Typical RO Purifier |
|---|---|
| Upfront cost range | $150–$1,000 |
| Reduces | Lead, arsenic, chromium, PFAS, VOCs, dissolved solids |
| Requires | Ongoing filter and membrane replacement |
| Water efficiency | WaterSense-labeled systems must be at least 30% efficient |
| Best for | Point-of-use drinking and cooking water |
| Not for | Whole-home treatment or washing/flushing water |
When you’re ready to compare specific models, our tested roundup of the best commercial RO water purifiers breaks down the top performers by capacity.