Semipermeable membrane filtration
How a Reverse Osmosis works
Pushes water through a fine membrane that removes the dissolved solids most filters miss.
Reverse osmosis (RO) is a water-treatment method that forces water under pressure through a semipermeable membrane with openings so fine that dissolved salts, metals, and many chemicals can't pass. Clean water collects on the other side while the rejected contaminants rinse to the drain. Most home RO units sit under the kitchen sink for drinking and cooking water (NSF/ANSI 58).
Typical cost: $300–$1,000 installed
How it works
Reverse osmosis (RO) forces water under pressure through a semipermeable membrane with openings so small that dissolved salts, metals, and many chemicals can't pass through.
Clean water collects on the other side and fills a small storage tank; the rejected contaminants are rinsed down the drain. Most home RO units are installed under the kitchen sink for drinking and cooking water.
Pre-filters protect the membrane from sediment and chlorine, and a final carbon polish improves taste right before the water reaches your faucet (NSF/ANSI 58).
The components inside
What each part does, in the order water moves through the system.
- 1Sediment pre-filterCatches grit and rust so they don't clog the membrane.
- 2Carbon pre-filterStrips chlorine, which would otherwise damage the membrane.
- 3RO membraneThe heart of the system — removes dissolved solids and metals.
- 4Storage tankHolds purified water so you get flow on demand.
- 5Post-carbon filterFinal polish that perfects taste at the faucet.
Configurations & options
One reverse osmosis is not like another. These are the real choices that change cost, maintenance, and how well it fits your home — worth understanding before you get quotes.
- Under-sink (POU) vs whole-home RO
- Most homes use a point-of-use unit under the kitchen sink to purify drinking and cooking water at one tap. A whole-home RO treats every tap but is far larger, slower, and pricier, sends more water to drain, and usually needs a pump and atmospheric storage tank — reserved for cases where every fixture truly needs RO-grade water.
- 4-stage vs 5-stage
- A 4-stage unit is sediment pre-filter, carbon pre-filter, RO membrane, and post-carbon polish. A 5-stage adds an extra filtration step — typically a second carbon or a remineralization stage — for a bit more polishing or mineral balance.
- Tank vs tankless
- Tank-style RO stores purified water in a small pressurized tank so you get instant flow, but flow pauses while the tank refills. Tankless RO uses a higher-output membrane and electric pump to make water on demand with no storage tank — faster fill, smaller footprint, but it needs power.
- With vs without remineralization
- RO strips beneficial minerals along with contaminants, which some people find leaves the water flat. A remineralization stage adds back a small amount of calcium and magnesium for taste and a balanced pH — optional, and a matter of preference.
What it addresses
- Dissolved solids, lead, arsenic, chromium-6, nitrate, fluoride
- PFAS, copper, and many other dissolved contaminants
- Off-tastes and odors from the drinking-water tap
Learn about these contaminants
Common questions
- How does reverse osmosis work?
- Reverse osmosis forces water under pressure through a semipermeable membrane whose openings are too small for dissolved salts, metals, and many chemicals to pass. Pre-filters remove sediment and chlorine first to protect the membrane; purified water fills a storage tank and a final carbon stage polishes the taste before the faucet (NSF/ANSI 58).
- What does reverse osmosis remove?
- RO removes dissolved solids and a broad range of contaminants — lead, arsenic, chromium-6, nitrate, fluoride, PFAS, copper — plus off-tastes and odors at the drinking tap. It's the broadest at-the-tap removal available. It also strips beneficial minerals, which is why some units add a remineralization stage.
- How much does a reverse osmosis system cost?
- Typical installed range for an under-sink unit is $300–$1,000, depending on stages, output, and whether it's tankless. Whole-home RO costs far more. Your real price depends on your water and setup — get an estimate rather than a fixed sticker number.
- What's the difference between a 4-stage and 5-stage RO system?
- A 4-stage system is a sediment pre-filter, carbon pre-filter, RO membrane, and post-carbon polish. A 5-stage adds one more step — typically a second carbon stage or a remineralization stage that adds back a little calcium and magnesium for taste and pH balance. More stages mean more polishing, not necessarily more contaminant removal.
Pros & cons
Pros
- The broadest at-the-tap removal of dissolved contaminants
- Dramatically improves taste of drinking and cooking water
- Compact, under-sink — no whole-home plumbing changes
Cons
- Sends several gallons to the drain for each gallon produced
- Slower flow; relies on a small storage tank
- Removes beneficial minerals too — some units re-mineralize
Best for
Anyone who wants the cleanest possible drinking and cooking water at one tap.
Sizing basics
- Rated by gallons-per-day membrane output (50–100 GPD covers most homes).
- Storage tank size sets how much purified water is ready at once.
- Filters are swapped every 6–12 months; the membrane lasts 2–5 years.
Solves these water problems
Next steps
Know the tech and the options — now get a real price for your water, or find a vetted local pro to size and install it.
Sources
Explore other system types
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