Reverse Osmosis System: How It Works and What It Removes

A reverse osmosis system forces water through a membrane with a pore size of about 0.0001 micron, according to CDC guidance, compared with about 0.1 micron for a microfiltration filter. That fine membrane comes at a cost in water, because a typical point-of-use RO system will generate five gallons or more of reject water for every gallon of treated water produced, according to EPA WaterSense data.

EPA estimates that approximately one million POU RO systems are sold annually in the United States, most of them under kitchen sinks. The sections below follow the water through each stage, then set what the membrane reduces against what it lets through, how much it sends to the drain, how to keep it working, and whether the water it makes is healthy to drink.

Key Takeaways

• A reverse osmosis filter has a pore size of about 0.0001 micron, and per CDC guidance, reverse osmosis removes parasites, bacteria, and viruses.
• Typical POU RO systems operate at efficiencies between 10% and 20%, meaning that they send approximately four to nine gallons of water down the drain for every gallon of treated water produced.
• For 1,000 gallons of permeate a year, a 10% efficient system wastes 9,000 gallons, while a WaterSense-labeled RO system at 30% wastes 2,333, by EPA’s calculation.
• Point-of-use RO systems that earn the WaterSense label must send just 2.3 gallons of water or less down the drain for every gallon of treated water they produce.
• Cold water slows the membrane: there is a 1 to 2% decrease in treated water produced for every degree below the standard 77°F, according to the University of Nebraska-Lincoln Extension.
• EPA research shows these types of membranes are typically more than 90% effective at removing a wide range of PFAS, including shorter-chain PFAS.
• As a certification baseline, NSF/ANSI 58 requires all systems to reduce TDS by at least 75%, and WQA-certified products carry the Water Quality Association Gold Seal.

What is a reverse osmosis system?

A point-of-use reverse osmosis (RO) system is a water filtration device that is connected to a single fixture (e.g., under the kitchen sink), according to the EPA. RO is the process by which 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.” EPA estimates that approximately one million POU RO systems are sold annually in the United States.

These systems can potentially remove water contaminants such as lead, volatile organic compounds (VOCs), PFAS, arsenic, bacteria, and viruses. Point-of-use RO systems are typically installed in residential settings but can also be found in commercial office spaces or kitchens. Units that earn the WaterSense label must send just 2.3 gallons of water or less down the drain for every gallon of treated water they produce.

In a home, most RO systems are POU systems placed under the sink or on the countertop, and a separate faucet is generally installed at the sink to allow the option of using treated water only for drinking and cooking. EPA does not have any data on the number of RO systems that are currently in use, so the sales estimate is the best national yardstick available.

How does a reverse osmosis system work to purify water?

A reverse osmosis filter has a pore size of about 0.0001 micron, per CDC guidance, and water only gets through it under pressure. In reverse osmosis, pressure is applied to the concentrated side of the membrane (the contaminated side), University of Nebraska-Lincoln Extension explains, so pure water is forced from the concentrated (contaminated) side to the dilute (treated) side.

In osmosis, a membrane separates two solutions containing different amounts of dissolved chemicals. The natural tendency is for water to move through the membrane from the dilute to the concentrated solution until chemicals reach equal concentrations on both sides of the membrane. Treated water is collected in a storage container, and the rejected contaminants on the concentrated side of the membrane are washed away as wastewater.

Source: CDC 2024

The water may also go through additional filters before and after it goes through the reverse osmosis filter. That is why a working unit is a chain of cartridges rather than a single membrane, and why the stages below matter as much as the membrane itself.

Recovery rate vs rejection rate: Recovery rate refers to the fact that only part of the water that flows into an RO system comes out as treated water. The rejection rate is the percentage of contaminant that is not allowed to move through the membrane. A high rejection rate cleans better; a high recovery rate wastes less.

Pressure drives both numbers. The incoming feed line pressure must be adequate to overcome the osmotic pressure and any back pressure generated from the storage tank, and generally, the higher the pressure difference across the membrane, the better the rejection of contaminants and recovery rate. Auxiliary pumps can be added to increase incoming water pressure as necessary, while EPA describes a permeate pump as a non-electric device that can be used to retrofit a POU RO system to reduce the back pressure from the storage tank.

Temperature matters too. There is a 1 to 2% decrease in treated water produced for every degree below the standard 77°F, and well water at 45°F (a typical temperature for Nebraska groundwater) would produce about half the amount of treated water that would be produced at 77°F. If your home runs on a private well, judge a unit’s daily rating against winter water, not the test conditions on the box.

How much water does a reverse osmosis system make per day?

The amount of treated water that an RO membrane typically used in the home can produce per day is in the range of 10 to 35 gallons per day, according to the University of Nebraska-Lincoln Extension. Most household RO systems are designed with a 20% to 30% recovery rate, so a system with 100 gallons/day of untreated water fed to it and a 20% recovery rate would yield 20 gallons/day of treated water and dispose of 80 gallons/day in the waste stream.

What contaminants does reverse osmosis remove?

Reverse osmosis systems will remove some types of chemicals from water, including lead, copper, chromium, chloride, and sodium (salt), according to the CDC. They may also reduce levels of arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate, and phosphorus. As a baseline, NSF/ANSI 58 requires all systems to reduce TDS by at least 75%.

University of Nebraska-Lincoln Extension lists the contaminants for which RO can be a practical treatment method for household drinking water in four groups.

Source: University of Nebraska-Lincoln Extension 2014

Check the system’s label for the specific chemicals it removes. Two units with the same membrane can carry different certified claims, and the label, not the product photo, is the promise.

Key finding: In EPA’s research on high-pressure membranes, approximately 80% of the feed water passes through the membrane to the effluent (treated water), while approximately 20% of the feedwater is retained as a high-strength concentrated waste. EPA suggests this technology may be best suited as a point of use technology for a homeowner, since the volume of water being treated is much smaller.

Does reverse osmosis remove PFAS?

High-pressure membranes, such as nanofiltration or reverse osmosis, have been extremely effective at removing PFAS, EPA researchers report, and reverse osmosis membranes are tighter than nanofiltration membranes. Research shows that these types of membranes are typically more than 90% effective at removing a wide range of PFAS, including shorter-chain PFAS.

What Reverse Osmosis Does Not Remove

A membrane reduces a contaminant by a percentage, so what comes out depends on what goes in. If the water supply contains 80 mg/L nitrate, an 85% rejection rate would reduce the nitrate concentration to 12 mg/L in the treated water, the University of Nebraska-Lincoln Extension calculates, which is above the maximum contaminant level (MCL) of 10 mg/L nitrate set by the EPA.

With nitrate at 40 mg/L, an RO membrane with 85% rejection would reject 34 mg/L nitrate, leaving 6 mg/L in the treated water. The RO membrane’s efficiency in reducing the amount of contaminant in the water depends on the contaminant concentration, chemical properties of the contaminant, the membrane type and condition, and operating conditions.

Some contaminants pass the membrane outright. Contaminants not removed from water by RO filters include dissolved gases such as hydrogen sulfide, a common nuisance contaminant with a characteristic rotten egg odor, and some pesticides, solvents, and volatile organic chemicals (VOCs) that are not removed by RO.

That is the job of the carbon stages. AC filters remove chlorine and certain pesticides and organic solvents that the RO membrane is not as effective at removing, which is why activated carbon (AC) filtration and/or sediment filtration is commonly used in conjunction with RO filters. No one piece of treatment equipment manages all contaminants.

Not a substitute for disinfection: Reverse osmosis can remove microorganisms. However, it is not recommended for that use (i.e., only coliform-free water should be fed to the system) because membrane deterioration can occur due to the bacteria, and contamination may occur through pinhole leaks. If a well test shows coliform bacteria, treat that problem before relying on the membrane.

How much water does a reverse osmosis system waste?

A typical point-of-use RO system will generate five gallons or more of reject water for every gallon of treated water produced, according to the EPA, and some inefficient units will generate up to 10 gallons of reject water for every gallon of treated water produced. Typical POU RO systems operate at efficiencies between 10% and 20%.

Efficiency here means the share of intake water that reaches your glass. WaterSense criteria require an RO system to achieve 30% efficiency to earn the label. ASSE 1086 sets a stricter minimum efficiency of 40%, meaning that the system sends 1.5 gallons of water down the drain or less for every one gallon of treated water produced, yet EPA was only able to identify one system that had been certified to the standard.

EPA turned those percentages into household gallons. Assuming a 15% average efficiency rating and production of approximately 1,000 gallons of permeate per year, a system would generate 5,667 gallons of reject water per year. At the WaterSense threshold, the amount of reject water generated would be reduced to 2,333 gallons per year, representing an average reduction in reject water of 59%. A system at 10% efficiency would send 9,000 gallons down the drain for the same output.

By the numbers: Selecting a WaterSense labeled model will reduce water use on average by more than 3,100 gallons of water per year compared to the water usage of a typical point-of-use RO system, EPA estimates, which translates to a savings of 47,000 gallons over the lifetime of the system. If all point-of-use RO systems sold in the United States were WaterSense labeled, the country could save more than 3.1 billion gallons of water annually.

The drain water has a second cost on rural lots. The wastewater, carrying rejected contaminants, typically is connected to a household drain, and this wastewater increases the load on the septic system. A WaterSense label, or a permeate pump on an older unit, is the simplest lever alongside the ways to conserve water at home.

Stages of a Reverse Osmosis System

Typical RO systems consist of a pretreatment filter, the RO membrane, flow regulator, post-treatment filter, storage tank, and dispensing faucet, according to the University of Nebraska-Lincoln Extension, and the storage tank generally has a capacity of 2 to 5 gallons. AC or sediment filters before the RO membrane and AC filters after the RO membrane are commonly used.

Source: University of Nebraska-Lincoln Extension 2014

RO membranes are generally made of thin film composite (TFC), cellulose acetate (CA), or cellulose triacetate (CTA). TFC membranes have higher total dissolved solids rejection rates, are more resistant to microbial attack, and are more tolerant of high pH, but TFC membranes deteriorate in chlorinated water.

If the feed water is chlorinated and a TFC membrane is used, an AC prefilter is needed to remove chlorine from the water. If the feed water is not chlorinated, AC filters should not be used for pre-filtration because they can encourage microbial growth on the membrane surface. In this case, only a sediment pre-filter is recommended.

Microorganisms (alive or dead) can clog RO membranes. This is called biofouling. Disinfect RO systems regularly with products provided by the manufacturer. Damaged RO membranes are not easily detected, so many systems are equipped with a monitor that indicates high total dissolved solids content or inadequate TDS rejection.

For relatively hard water, pretreatment of the water by a softener can increase the life of the membrane. If your fixtures scale up quickly, compare water softener options before replacing membranes on a short cycle.

How often should you change reverse osmosis filters?

Pre-filters and post-filters require regular replacement, and the length of time before prefilter replacement depends upon water volume, quality, and contaminant concentration, according to the University of Nebraska-Lincoln Extension. On labeled systems, WaterSense criteria ensure the RO membrane will last at least one year before requiring replacement. Manufacturers and dealers can assist in determining replacement intervals.

Reverse Osmosis Certification: NSF/ANSI 58, ASSE 1086 and WaterSense

There is no federal standard that prescribes water efficiency or performance requirements for RO systems sold in the United States, the EPA notes, but the International Plumbing Code and the Uniform Plumbing Code require POU RO systems to be certified to NSF/ANSI 58. NSF/ANSI 58 requires all systems to reduce TDS by at least 75%.

The National Sanitation Foundation and the Water Quality Association evaluate performance, construction, advertising, and operation manual information. WQA-certified products carry the Water Quality Association Gold Seal. The CDC lists Standard 58 (reverse osmosis) alongside Standard 42 (taste and odor) and Standard 53 (cyst reduction) among the NSF standards for water treatment.

WaterSense adds water efficiency on top. Its performance criteria cover membrane life, total dissolved solids reduction, and contaminant reduction for any additional contaminants the manufacturer claims it can remove. Those three marks are the first filter when comparing reverse osmosis systems for the kitchen.

Point-of-Use vs Whole-House Reverse Osmosis Systems

Point-of-use filters usually filter water in batches and deliver it to a single tap, such as your kitchen sink, the CDC explains, while whole-home filters, also called point-of-entry filters, filter water as it comes into your home. EPA chose to exclude point-of-entry (POE) systems from the scope of the WaterSense specification because they treat, and subsequently waste, much larger quantities of water than POU systems.

Not all end uses of water require or even benefit from the quality of water produced by an RO system (e.g., water used for toilet flushes, clothes washing, or bathing). In most cases, it is more practical to install a POU system that treats water at just one fixture in a house than it is to install a POE system to treat the entire household water supply.

How much is a whole house reverse osmosis system?

None of the federal or extension sources behind this explainer publish a price range for whole-house RO, so no figure is given here. POE reverse osmosis units are more costly to purchase, install, and operate than POU systems, although POE RO systems are generally more efficient due to their tendency to include electric booster pumps and/or recirculate some of the concentrate water.

Do I need reverse osmosis if I have a whole house filter?

It depends on what the whole-house filter is certified to remove. The CDC advises checking the system’s label for the specific chemicals it removes, and RO at the drinking tap adds reduction of lead, copper, chromium, chloride, and sodium (salt). Compare the label of your whole-house water filter system against a current water test before adding a second unit.

Is reverse osmosis water healthy to drink?

Germany’s Federal Institute for Risk Assessment (BfR) found that mineral, spring, and table water with very low mineral content does not have any long-term adverse effects on health, assuming a balanced diet and normal drinking quantities. Humans absorb minerals as part of all foods consumed on a daily basis, with water or mineral water being only one of several day-to-day sources of mineral intake.

The BfR assessed low-mineral bottled water rather than RO water, but it answers the question RO owners ask about drinking water stripped of minerals. It also names exceptions: in the case of individuals engaged in prolonged exercise or strenuous physical activity (longer than 1.5 hours), the use of mineral water with very low mineral content is not recommended as the sole or primary type of beverage.

Worth noting: The BfR says people with very unbalanced diets, people who consciously avoid certain foods, as well as pregnant or breastfeeding women, should pay attention to their total daily intake of certain essential minerals if they use low mineral content water as their primary source of fluids over a longer period of time.

What are the downsides of reverse osmosis water?

Water treated by RO can be more corrosive than untreated water, so special plumbing, in addition to the faucet, is installed with RO systems. RO may also reduce levels of fluoride, calcium, and magnesium, per the CDC, and every gallon carries the drain-water cost covered above. The benefits of reverse osmosis systems weigh against these trade-offs.

Is there anything better than reverse osmosis?

No one piece of treatment equipment manages all contaminants. Distillation systems boil water and then collect the water as it cools, and the CDC notes that distillation generally uses more energy and takes longer than other water treatment systems. The water distiller options suit small daily volumes.

On water use, EPA points the other way. Due to the water-intensive nature of RO, WaterSense does not intend to promote the installation of RO systems for all applications, and in some cases, other treatment methods that use little or no water, such as filtration, are sufficient to meet consumer treatment needs.

Is RO water better than tap water?

If your tap water is already free from harmful germs and chemicals, you may not want to filter it, the CDC advises, because filters remove both good and bad substances from your water. RO water is better than tap water where a test finds a contaminant the unit is certified to reduce; a drinking water test kit or your utility’s annual report is the place to start.

Conclusion

A reverse osmosis filter has a pore size of about 0.0001 micron, and a typical point-of-use RO system will generate five gallons or more of reject water for every gallon of treated water produced. Those two facts frame the purchase: RO earns its place where a water test finds lead, nitrate, arsenic, or PFAS, and it costs water everywhere else.

Private well owners facing nitrate, and households whose water report shows lead or PFAS, gain the most from a certified under-sink unit. When replacing an older system, look for NSF/ANSI 58 certification and the WaterSense label, which cut reject water by more than half at the same output.