Reverse osmosis (RO) pushes water through a membrane with pores small enough to hold back most dissolved substances. It is the most thorough filter you can put on a kitchen tap. It is also more than many homes need, so start with what is in your water.
When RO is worth it
- PFAS. EPA lists reverse osmosis, along with activated carbon, as effective home treatment for PFAS, and suggests choosing a filter certified to NSF/ANSI 53 or 58 for PFAS reduction.
- Lead, arsenic, nitrate, fluoride, uranium and high dissolved solids (TDS), which a basic carbon filter does not remove reliably.
- Well water with any of the above. See arsenic and nitrate in wells.
If your only complaint is chlorine taste, a certified carbon filter is cheaper and simpler. Check your city's PFAS, lead and hardness results on our city pages first.
The three types
- Under-sink with a tank. The classic design. Water is filtered slowly into a pressurized storage tank under the sink and comes out of its own faucet. Reliable and no electricity, but the tank takes up much of the cabinet.
- Tankless under-sink. A built-in pump filters water on demand. Smaller, faster flow, and usually less waste water, but it needs an electrical outlet under the sink.
- Countertop. You fill a reservoir by hand and empty a waste tank. No plumbing and no drilling, so it suits renters and anyone who cannot modify the sink. Smaller capacity and more chores.
What to check on the spec sheet
- NSF/ANSI 58 certification, and the specific contaminants on the listing. Certification is per contaminant, so a system certified for TDS is not automatically certified for PFAS or arsenic. Look the model up on the certifier's website rather than trusting the box.
- Waste water ratio. Every RO system sends some water down the drain to flush the membrane. Older tank designs often waste several gallons per gallon filtered; many newer tankless models claim far less. Compare the stated ratio.
- Water pressure. Tank systems rely on house pressure. In a low-pressure home they fill slowly and waste more; a booster pump helps. Tankless units have their own pump.
- Replacement costs. Pre-filters typically need changing every several months and the membrane every couple of years. Price the full set before buying.
Chloramine and hard water shorten membrane life
Chlorine and chloramine damage the common thin-film RO membrane, so every system has a carbon pre-filter in front. Chloramine needs more carbon contact time than chlorine. If your city uses it (see our chloramine guide), change the carbon pre-filter on time or choose a catalytic carbon pre-filter. In very hard water, scale builds up on the membrane; a softener upstream extends its life.
Minerals and taste
RO removes calcium and magnesium along with everything else, which some people find makes the water taste flat. Most of the minerals in a normal diet come from food, and remineralizing stages are mainly about taste. Add one if you prefer it.
Test after you install
A cheap TDS meter shows the membrane is working: filtered water should read far lower than the tap. For lead, arsenic or PFAS, a lab test of the filtered water is the only real confirmation.