How Reverse Osmosis Water Filtration Works

Run the name backwards and you have the whole idea. Osmosis is a natural pull: when a thin membrane separates clean water from salty water, water drifts on its own toward the saltier side, trying to even out the concentration. Reverse osmosis turns that pull around. It uses pressure to push water the other way, from the dirty side through the membrane to the clean side, leaving dissolved salts and contaminants behind. That single reversal, driven by pressure against a membrane, is the engine inside every RO system under a kitchen sink.

This guide explains the machine itself: the membrane, the stages around it, the wastewater it produces, the storage tank and slow faucet, and the trade-off that it strips good minerals along with bad ones. It does not test your water to tell you whether you need RO in the first place (see our guide on how to find out what is in your water (146)), argue whole-house versus point-of-use placement (see our guide on whole-house versus point-of-use filters (147)), or walk you through the under-sink installation. RO ties into your cold supply line and a drain, so for the plumbing connection, follow the unit’s manual or use a licensed plumber.

The Core Principle: Pushing Water Through a Semipermeable Membrane

Reverse osmosis works by forcing water through a film with holes so small that water molecules pass but most dissolved contaminants cannot.

The EPA describes the mechanism plainly: in an RO system, “pressure forces water through a semi-permeable membrane, creating a stream of treated water, called ‘permeate,’ and a stream of reject water called ‘concentrate’ or ‘brine.'” The clean stream is the water you drink. The reject stream carries away what the membrane blocked.

The membrane is the part that does the real separating, and its openings are extraordinarily fine. The CDC puts the pore size of a reverse osmosis filter at “about 0.0001 micron,” far smaller than the pores in an ordinary cartridge filter. At that scale the membrane stops not just particles but dissolved ions, which is why RO can lower the level of substances that a sediment or carbon filter would let straight through.

This is also why RO needs pressure to run. Left alone, osmosis would move water in the opposite direction. Reversing it means working against that natural tendency, and your home’s water pressure supplies the push. Lower household pressure means slower production, which is one reason RO output is a trickle rather than a gush.

The Stages of a Typical RO System, Pre-Filter to Post-Carbon

A home RO unit is not a single membrane. It is a short train of filters, with the membrane in the middle and protective stages on either side.

Most systems run water through stages in roughly this order. A sediment pre-filter goes first, catching sand, rust, and grit so larger particles never reach the membrane. A carbon pre-filter usually comes next, reducing chlorine and similar compounds that can damage the membrane material over time. Then comes the RO membrane itself, the stage that removes dissolved contaminants. After the membrane, a post-filter, often carbon, gives the water a final polish before it reaches the faucet, cleaning up any taste the storage tank may have left.

The order matters because each pre-filter exists to protect the one expensive part. The membrane is the slowest to clog and the costliest to replace, so the sediment and carbon stages take the abuse first. That is the logic behind the multi-stage design, rather than stages being added simply to sound thorough. NSF/ANSI 58, the standard that certifies point-of-use RO systems, is written around exactly this kind of complete multi-stage unit with a membrane, pre-filters and post-filters, and a tank.

Each stage has its own replacement rhythm, and the cartridges do not all last the same length of time. This guide stays on why the stages exist; the schedule for changing each one is a maintenance topic in its own right and depends on your water and your specific unit, so follow the manufacturer’s cartridge guidance.

Why RO Makes Wastewater (the Reject Stream)

Reverse osmosis sends part of your water down the drain on purpose. The reject stream is how the system carries away the contaminants the membrane held back.

Think about what happens at the membrane surface. Clean water is being pushed through, which means everything that does not pass keeps concentrating on the other side. If that concentrated water just sat there, it would foul the membrane. So the system continuously flushes it away as the reject stream, also called concentrate or brine, taking the rejected contaminants with it.

That flushing is not a small amount, and it is worth knowing before you buy. According to the EPA, “a typical point-of-use RO system will generate five gallons or more of reject water for every gallon of treated water produced. Some inefficient units will generate up to 10 gallons of reject water for every gallon of treated water produced.” More efficient designs do far better. The EPA notes that WaterSense-labeled RO systems send “just 2.3 gallons of water or less down the drain for every gallon of treated water they produce.” If reject ratio matters to you, that label is the number to look for, because the spread between a wasteful unit and an efficient one is large.

The Storage Tank and Dedicated Faucet: Why RO Is Slow

RO makes clean water slowly, so it stores up a supply in a small tank and serves it through its own faucet. That is the reason you do not get instant high flow from an RO tap.

Pushing water through a membrane that fine takes time. A system cannot fill a glass on demand at full sink flow the way an open tap can, so it works in the background, producing treated water drip by drip and banking it in a pressurized storage tank. When you open the RO faucet, you are drawing from that stored reservoir, which is why the first part of a pour comes out at a reasonable pace and then tapers as the tank draws down and the membrane catches up.

Because the system runs on its own pressure logic and its output is dedicated to drinking and cooking, RO units use a separate, smaller faucet at the sink rather than tying into your main tap. That dedicated faucet keeps the treated water on its own line and lets the rest of your plumbing run at normal pressure and flow. It is also the visible sign that a home has RO at the kitchen sink.

The tank and faucet are why an RO system feels different from a whole-house filter. You are not treating every gallon in the house. You are slowly making a small batch of highly filtered water for the tap where you drink.

What RO Removes, and Why It Also Strips Beneficial Minerals

Reverse osmosis removes a very wide range of contaminants, and in doing so it also removes minerals many people consider good. The membrane does not sort “bad” from “good.” It blocks dissolved substances broadly.

On the contaminant side, the reach is real. The EPA states that point-of-use RO systems “can potentially remove water contaminants such as lead, volatile organic compounds (VOCs), PFAS, arsenic, bacteria, and viruses,” and the standard performance measure for these systems is reduction of total dissolved solids. The CDC adds that reverse osmosis removes germs including parasites, bacteria, and viruses, and reduces chemicals “including lead, copper, chromium, chloride, and sodium (salt),” and “may also reduce levels of arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate, and phosphorous.”

Two of those names point to the trade-off. Calcium and magnesium are the same minerals that make water hard, and they are also minerals some people prefer to keep for taste and dietary reasons. RO does not spare them. Because it works by blocking dissolved ions in general, it pulls beneficial minerals out alongside the contaminants. The result is very clean water that some drinkers find flat or slightly acidic in taste.

This is why some RO units add a remineralization stage after the membrane, a post-filter that puts a measured amount of mineral back into the finished water to restore taste and balance. It is an optional stage, not a requirement, and it exists precisely because the membrane removes minerals so thoroughly. Whether you want it comes down to taste preference and what you value in your drinking water.

One clarification belongs here. Because RO reduces calcium and magnesium, it lowers the hardness of the water it treats. That does not make it a water softener. A softener treats the whole house through ion exchange and is built for hardness specifically (see our guide on how a water softener works (141)), while RO treats only the water at one tap and is built for broad contaminant reduction. They solve different problems, even though both touch hardness.

Frequently Asked Questions

Does reverse osmosis remove everything from water?
No, but it removes a great deal. RO can reduce a wide range of dissolved contaminants, including lead, arsenic, PFAS, nitrate, many salts, and germs such as bacteria and viruses, and its standard measure is total dissolved solids reduction. What any specific system is proven to remove depends on the unit and its certification, so the practical rule from the CDC is to check the system’s label for the contaminants it is rated to reduce. It is also true that RO removes beneficial minerals such as calcium and magnesium along with the contaminants.

Why does an RO system waste water?
Because the membrane has to flush away what it blocks. As clean water is pushed through, contaminants concentrate on the other side, and the system rinses that concentrated reject water down the drain so it does not foul the membrane. The EPA says a typical point-of-use unit produces five gallons or more of reject water per gallon of treated water, while WaterSense-labeled systems get that down to 2.3 gallons or less. The ratio varies widely between efficient and inefficient designs.

Is RO water bad for you because it has no minerals?
RO water is very low in minerals because the membrane removes calcium, magnesium, and similar dissolved minerals broadly. Most people get the bulk of these minerals from food rather than water. If you prefer mineral content for taste or other reasons, some RO systems include a remineralization stage that adds a measured amount back after filtration.

This is general information, not professional advice. Reverse osmosis connects to your water supply and a drain, so for installation, code questions, or any work on your plumbing, follow the manufacturer’s manual or consult a licensed plumber and your local water authority.

Sources

EPA WaterSense, Point-of-Use Reverse Osmosis Systems: https://www.epa.gov/watersense/point-use-reverse-osmosis-systems
CDC, About Home Water Treatment Systems: https://www.cdc.gov/drinking-water/about/about-home-water-treatment-systems.html
NSF, NSF/ANSI 58: Reverse Osmosis Drinking Water Treatment Systems: https://www.nsf.org/knowledge-library/nsf-ansi-58-reverse-osmosis-drinking-water-treatment-systems