Reverse osmosis at the kitchen tap, for city-water homes

What a drinking-water RO system is

An RO system pushes water through a very fine semi-permeable membrane. Most systems put one or more filters before and after the membrane, typically a sediment filter and carbon to protect the membrane and polish taste.^[source] Treated water collects in a small storage tank (or, on tankless models, flows straight through) and comes out of a dedicated faucet at your sink. The water that doesn't pass the membrane carries what it rejected down the drain.

Because it treats one tap, it's a point-of-use (POU) system. It's the most common treatment city-water households choose, because the kitchen tap is where nearly all drinking and cooking water comes from. For the full picture across city and well water, see our main reverse osmosis page.

Who it makes sense for on city water

• You don't like the taste or smell of the disinfectant and want something better than a pitcher. (If the smell bothers you in the shower too, look at whole-home filtration instead of, or as well as, RO.)
• Your utility reported PFOA or PFOS and you'd like to reduce them in your drinking water. Our PFAS filtration page lists which local utilities reported what.
• A certified-lab test at your tap found lead from your home's plumbing.
• You buy a lot of bottled water and want to stop. Our RO vs. bottled water tool lays out the comparison.

If none of these apply, you may not need anything. Charlotte Water, for one, reported zero violations for 2025.^[source]

NSF/ANSI 58: what the certification does and doesn't tell you

NSF/ANSI 58 is the standard for reverse osmosis drinking water systems.^[source] The key detail is that certification is per contaminant, per model. The NSF listing for each certified system names the specific claims it earned, such as Lead Reduction, TDS Reduction or Arsenic (Pentavalent), and a system is only certified for the claims printed there.^[source] For PFAS, EPA advises looking for certification under NSF/ANSI 53 or 58 that covers the specific PFAS you want to reduce.^[source]

What RO can and can't do

A low TDS meter reading after RO only shows that dissolved minerals went down. It's not evidence your original water was contaminated, and a high reading isn't evidence it was.

Things to weigh

• Water to drain. RO always sends some water to the drain. EPA notes household RO units can use about one gallon of reject water per gallon treated.^[source] WaterSense-labeled systems must be certified to NSF/ANSI 58, have an automatic shut-off and reach an efficiency rating of at least 30 percent.^[source]
• Space. Expect the filter housings and tank to take a good share of the cabinet under your sink.
• Drain and faucet. The system needs a drain connection and usually a dedicated faucet hole in the sink or countertop.
• Remineralization stages. Some systems add minerals back for taste. That's a taste preference; we don't make health claims for it.
• Flow. RO produces water slowly; a tank or a higher-output tankless unit fills that gap.

Installation

An under-sink RO ties into your cold-water line and your drain, so it's plumbing work. We help you choose and size the equipment; here's how installation is handled.

Maintenance

RO needs its cartridges and membrane changed on the manufacturer's schedule; EPA notes filters only perform if they're maintained that way.^[source] In practice there are three groups: pre-filters (sediment and carbon) changed most often, the membrane changed least often, and the post-filter in between. Chlorine reaching the membrane shortens its life, so on-time carbon changes matter. A Care Plan puts those changes on a calendar so you don't have to remember them.