What is a reverse osmosis system?

How does a reverse osmosis system work?

Pressure does the work: the home's water line pressure pushes water against the membrane, clean water passes through to the treated side, and the contaminants left behind are rinsed away to the drain. EPA calls the two streams permeate and concentrate, and Nebraska Extension explains why it is called reverse osmosis: “In reverse osmosis, pressure is applied to the concentrated side of the membrane (the contaminated side).” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “This forces the osmotic process into reverse so that, with adequate applied pressure, pure water is forced from the concentrated (contaminated) side to the dilute (treated) side.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “The rejected contaminants on the concentrated side of the membrane are washed away as wastewater.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “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.”” Source: U.S. Environmental Protection Agency (EPA), WaterSense, read 2026-10-29. “The line pressure of the water system in the home usually supplies the pressure for RO.” Source: Connecticut Department of Public Health, Private Well Program, read 2026-10-29.

How do EPA WaterSense and Nebraska Extension describe a reverse osmosis system?

A reverse osmosis (RO) system is a water treatment unit connected to one fixture, most often under the kitchen sink or on the countertop, that pushes water through a membrane to lower the dissolved solids in it. The U.S. EPA WaterSense program and University of Nebraska-Lincoln Extension describe it and its usual six parts: “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) and uses the process of RO to remove contaminants from the water supplied to that fixture.” Source: U.S. Environmental Protection Agency (EPA), WaterSense, read 2026-10-29. “Reverse osmosis (RO) membrane filters frequently are used to reduce the levels of total dissolved solids and suspended particles within water.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Most RO systems are POU systems placed under the sink or on the countertop.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Typical RO systems consist of a pretreatment filter, the RO membrane, flow regulator, post-treatment filter, storage tank and dispensing faucet as shown in Figure 3.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29.

What does a reverse osmosis system remove?

Mostly dissolved ions and metals: Nebraska Extension's Table I lists arsenic, fluoride, lead, nitrate, radium and uranium among the contaminants a household unit can practically treat, and its worked example shows an 85 percent rejection membrane taking water with 40 mg/L of nitrate down to 6 mg/L. No membrane takes out all of anything: “This table is not an exhaustive list of contaminants that RO may remove, but rather lists those for which RO can be a practical treatment method for treating household drinking water.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “RO is an effective method to reduce certain ions and metals, such as nitrate and arsenic.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “For example, if the water supply contains nitrate at 40 mg/L, an RO membrane with 85 percent rejection would reject 40 x 0.85 = 34 mg/L nitrate, leaving 6 mg/L in the treated water.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Also effective in removing some detergents, some taste, color and odor-producing chemicals, certain organic contaminants, uranium, and some pesticides.” Source: Connecticut Department of Public Health, Private Well Program, read 2026-10-29. “RO membranes cannot be certified to remove 100 percent of anything.” Source: Connecticut Department of Public Health, Private Well Program, read 2026-10-29.

What does a reverse osmosis system not remove?

Dissolved gases such as hydrogen sulfide, the rotten-egg smell, pass through the membrane, and some pesticides, solvents and volatile organic chemicals get through as well. That is why many home units pair the membrane with an activated carbon filter, which Nebraska Extension's Table II credits with chlorine and radon: “Contaminants not removed from water by RO filters include dissolved gases such as hydrogen sulfide, a common nuisance contaminant with characteristic rotten egg odor, which passes through the RO membrane.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Some pesticides, solvents and volatile organic chemicals (VOCs) are not removed by RO.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “AC filters remove chlorine and certain pesticides and organic solvents that the RO membrane is not as effective in removing (see Table II).” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “The pre-filter removes sand, silt, and sediments, while the activated carbon filter removes the organic materials and any dissolved gases not treated by the RO membrane.” Source: Connecticut Department of Public Health, Private Well Program, read 2026-10-29. “No one piece of treatment equipment manages all contaminants.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29.

Does reverse osmosis remove PFAS?

Yes, it can greatly reduce them: EPA found that the reverse osmosis point-of-use systems it studied cut PFAS levels, and the Minnesota Department of Health says RO membranes have been shown to remove PFAS, while water softeners are not likely to and boiling will not. EPA points buyers to the NSF/ANSI 58 standard for RO and notes the April 2024 limit of those standards: “EPA found that the GAC, IX, and RO point-of-use systems studied can greatly reduce PFAS levels.” Source: U.S. Environmental Protection Agency (EPA), Water Research, read 2026-10-29. “Filters containing activated carbon or reverse osmosis membranes have been shown to be effective at removing PFAS from water supplies.” Source: Minnesota Department of Health, read 2026-10-29. “Boiling water will not remove PFAS.” Source: Minnesota Department of Health, read 2026-10-29. “Look on product labels for certification under NSF/ANSI 53 or NSF/ANSI 58 for PFAS reduction.” Source: U.S. Environmental Protection Agency (EPA), Water Research, read 2026-10-29. “It's important to note that the current certification standards for PFAS filters (as of April 2024) do not yet indicate that a filter will remove PFAS down to the levels EPA has now set for a drinking water standard.” Source: U.S. Environmental Protection Agency (EPA), Water Research, read 2026-10-29.

Does reverse osmosis remove bacteria and viruses?

It can, but the sources say not to rely on it alone: EPA WaterSense lists bacteria and viruses among the contaminants RO systems can potentially remove, while the Connecticut Department of Public Health does not recommend RO units by themselves for microorganisms. Germs can also clog the membrane, which Nebraska Extension calls bio-fouling: “These systems can potentially remove water contaminants such as lead, volatile organic compounds (VOCs), PFAS, arsenic, bacteria, and viruses.” Source: U.S. Environmental Protection Agency (EPA), WaterSense, read 2026-10-29. “Reverse osmosis can remove microorganisms.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Alone, reverse osmosis (RO) units are not recommended for treatment of bacteria and other microscopic organisms.” Source: Connecticut Department of Public Health, Private Well Program, read 2026-10-29. “Microorganisms (alive or dead) can clog RO membranes.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Disinfect RO systems regularly with products provided by the manufacturer.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29.

How much water does reverse osmosis waste?

More than it makes: EPA WaterSense says a typical under-sink unit sends five gallons or more down the drain for each gallon of treated water, and units that earn the WaterSense label must stay at 2.3 gallons or less. Nebraska Extension puts most household systems at 20 to 30 percent recovery, and Connecticut's health department sets a 30 gallon a day guide for septic systems: “For example, a typical point-of-use RO system will generate five gallons or more of reject water for every gallon of treated water produced.” Source: U.S. Environmental Protection Agency (EPA), WaterSense, read 2026-10-29. “Some inefficient units will generate up to 10 gallons of reject water for every gallon of treated water produced.” Source: U.S. Environmental Protection Agency (EPA), WaterSense, read 2026-10-29. “In contrast, point-of-use RO systems that earn the WaterSense label must demonstrate that they send just 2.3 gallons of water or less down the drain for every gallon of treated water they produce.” Source: U.S. Environmental Protection Agency (EPA), WaterSense, read 2026-10-29. “Most household RO systems are designed with a 20 percent - 30 percent recovery rate.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Flows of 30-gallons per day or less of reject water to a septic system are considered to be incidental and should not damage the septic system.” Source: Connecticut Department of Public Health, Private Well Program, read 2026-10-29.

How often should you change the filters in a reverse osmosis system?

There is no single interval: Nebraska Extension says pre-filter and post-filter life depends on how much water runs through and how dirty it is, and points to the maker's schedule. For the membrane itself, the EPA WaterSense specification requires a labeled unit's membrane to last at least one year before it needs replacing: “Pre-filters and post-filters require regular replacement.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “The length of time before prefilter replacement depends upon water volume, quality and contaminant concentration.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Manufacturers and dealers can assist in determining replacement intervals.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Membrane Life: Ensures the RO membrane will last at least one year before requiring replacement.” Source: U.S. Environmental Protection Agency (EPA), WaterSense, read 2026-10-29. “You should continue to test the quality of both the untreated and treated water annually or more frequently (quarterly or semi-annually) if high levels of contaminants are present in the raw water.” Source: Connecticut Department of Public Health, Private Well Program, read 2026-10-29.

What does the pressure tank in a reverse osmosis system do?

It stores treated water so the faucet has a usable flow: the membrane makes water slowly, so a small pressurized tank of about 2 to 5 gallons, according to Nebraska Extension, holds it until the tap opens. A shut-off valve should stop the flow into the system once the tank is full: “This treated water comes out much slower than water from a regular tap, so a tank is used to store the treated water.” Source: Connecticut Department of Public Health, Private Well Program, read 2026-10-29. “The storage tank generally has a capacity of 2 to 5 gallons.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “It is pressurized to provide adequate flow when the tap is open.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Shut-off valves are important to stop water flow when the storage tank is full, so excess water is not wasted.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “The incoming feed line pressure must be adequate to overcome the osmotic pressure and any back pressure generated from the storage tank “down-line” from the membrane.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29.

What material is a reverse osmosis membrane made of?

Home membranes are made of one of three synthetic materials, thin film composite (TFC), cellulose acetate (CA) or cellulose triacetate (CTA), wound into a compact tube. Nebraska Extension says TFC removes more but breaks down in chlorinated water, so a TFC membrane on city water needs a carbon pre-filter: “RO membranes are generally made of thin film composite (TFC), cellulose acetate (CA), or cellulose triacetate (CTA).” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “TFC membranes have better durability as well as generally higher rejection rates than CA/CTA membranes.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “Cellulose type membranes are less costly and can tolerate chlorine, which is commonly used for disinfection of drinking water.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “If the feed water is chlorinated and a TFC membrane is used, an AC prefilter is needed to remove chlorine from the water.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “The membrane can be spiral wound (like a rolled-up newspaper), or individual hollow fibers can be bundled together.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29.

How long does a reverse osmosis system take to fill?

Hours rather than minutes: the Connecticut Department of Public Health says units take from 2 to 78 hours to make one gallon, and Nebraska Extension says a home membrane typically makes 10 to 35 gallons a day. Cold water slows it, with output falling 1 to 2 percent for each degree below 77°F: “Each manufacturer’s RO units differ, but the time needed to produce one gallon of water ranges from 2-78 hours.” Source: Connecticut Department of Public Health, Private Well Program, read 2026-10-29. “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.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “There is a 1 to 2 percent decrease in treated water produced for every degree below the standard 77°F.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29. “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.” Source: University of Nebraska-Lincoln Extension, read 2026-10-29.

Sources

• Drinking Water Treatment: Reverse Osmosis (NebGuide G1490) — University of Nebraska-Lincoln Extension; published 2014-09; read 2026-10-29.
• Point-of-Use Reverse Osmosis Systems — U.S. Environmental Protection Agency (EPA), WaterSense; published 2026-08-21; read 2026-10-29.
• Identifying Drinking Water Filters Certified to Reduce PFAS — U.S. Environmental Protection Agency (EPA), Water Research; published 2025-11-18; read 2026-10-29.
• PFAS and Home Treatment of Water — Minnesota Department of Health; published 2024-04-10; read 2026-10-29.
• Publication No. 21: Reverse Osmosis Treatment of Private Drinking Water Systems — Connecticut Department of Public Health, Private Well Program; published 2009-03; read 2026-10-29.