Reverse osmosis water: what it is and where to refill it
Short answer: reverse osmosis (RO) pushes water through a very fine membrane that holds back most dissolved substances. It's the most common treatment at self-serve refill machines. The label tells you the process a machine uses, not a lab result for that particular machine.
How it works
According to the CDC's guide to home water treatment, reverse osmosis pushes water from the side with more dissolved substances through a filter to the side with fewer. The membrane's pores are about 0.0001 micron. Many systems add filters before and after the membrane.
What it removes, per the CDC
- Germs: parasites, bacteria and viruses.
- Some chemicals: including lead, copper, chromium, chloride and sodium. It may also reduce arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate and phosphorus.
- The CDC's advice: check a system's label for the specific chemicals it removes.
The stages in a typical system
Most RO systems combine several steps: a sediment filter, a carbon filter, the RO membrane itself, and often a final "polishing" filter. Some add minerals back afterward (that's how many alkaline options are made). The CDC notes that water may go through filters before and after the membrane. Primo says its self-service machines use sediment and carbon filtration, RO and a final disinfection step.
The downsides
- Wastewater: the EPA's WaterSense program says a typical home under-sink RO system sends five gallons or more of reject water down the drain for every gallon it treats, and inefficient units up to 10. WaterSense-labeled systems must keep it to 2.3 gallons or less. (That's for home systems. Refill machines are run by their operators.)
- Fewer minerals: RO removes most dissolved minerals. Some people prefer the taste of water with minerals left in or added back.
- Upkeep: filters and membranes need regular replacement. At a refill machine, that's the operator's job, which is why the machine check matters.
What that means at a refill machine
- The process is good at lowering dissolved minerals, so RO water tastes plain to many people. That's why it's popular for coffee, tea and cooking.
- A machine's results depend on maintenance. A 2026 study of US kiosks found that some machines picked up lead from plumbing parts after the RO step. See our 30-second machine check.
- A low TDS reading shows low dissolved solids. It can't tell you everything. See what TDS tells you.
Is reverse osmosis water good for you?
Our pick: yes, RO is our everyday drinking water pick. The CDC says it removes parasites, bacteria, viruses and some chemicals, and it's the most widely available water in our directory: more than 21,000 stations list reverse osmosis. It does take out most minerals too, and food is where most of your minerals come from. If you have a specific health concern, talk to your doctor. If you like some minerals back, many machines offer an alkaline option made by adding minerals back to RO water.
Where to refill
Reverse osmosis is the water type listed most often on PureWaterFinder: grocery machines, drive-up kiosks and water stores. Find it near you: Denver · Colorado Springs · Austin · Tampa · Los Angeles · All cities. Cost: listed refill prices.
General information, not medical advice.
Frequently asked questions
- Is RO water the same as distilled?
- No. Both lower dissolved solids, by different methods. See distilled vs. RO vs. spring.
- Is RO "purified" water?
- Many machines label it that way. The label names the treatment, not a test result.
- Does RO remove fluoride?
- The CDC says RO may reduce fluoride.
- Should I buy a home system instead?
- If you refill often, maybe. Weigh the upfront cost, filter changes and wastewater (see above). If you just need plain filtered water now and then, a refill machine avoids all three. We priced it out in is reverse osmosis worth it?
