What Is Reverse Osmosis and How Does an RO Water Filter Work?
What Is Reverse Osmosis? A Quick Answer
Reverse osmosis (RO) is a water treatment process that uses pressure to push water through a semipermeable membrane, reducing dissolved salts and other substances. The system sends treated water to your drinking-water faucet and carries rejected material away in a separate drain-water stream. Home RO water filters also use sediment and carbon stages, each with a different job.
EPA WaterSense explains the two water streams: permeate and concentrate.
Shopping for your kitchen?
Already own an RO system?
Find Replacement Filters for Your RO System
Select your system’s brand and model to find compatible filters, membranes and parts. Not sure of your model? Identify your RO system first.
FOR CURRENT RO OWNERS
Find Parts for Your RO System
Select your brand...
Select your model...
Not sure which model you own?
How Does Reverse Osmosis Work?
In a typical tank-based kitchen system, incoming water passes through prefilters before reaching the RO membrane. The membrane produces two streams: treated water for the faucet and concentrate for the drain. The prefilters and final carbon filter have different jobs from the membrane.
From the Cold-Water Line to Your Glass
01
Prefilter the incoming water
A sediment prefilter captures particles such as sand, silt and rust. A carbon prefilter reduces chlorine taste and odor and helps protect a chlorine-sensitive membrane.
02
Separate water at the RO membrane
Pressure moves water through the membrane. Many dissolved salts and other substances are rejected into the concentrate stream. Performance depends on the complete system and operating conditions.
03
Store treated water; send concentrate to the drain
A tank-based system stores permeate for later use. A separate drain line carries concentrate away during production. The storage tank holds water; it is not another filtration stage.
04
Finish at the drinking-water faucet
Water passes through a final carbon polishing filter on its way to the dedicated faucet. Some systems can also serve a refrigerator or ice maker when the installation and system specifications allow it.
The Science: Osmosis vs. Reverse Osmosis
How the RO Membrane Separates Water
Osmosis: water naturally moves across a semipermeable membrane toward the solution with a higher concentration of dissolved material.
Reverse osmosis: applied pressure moves water in the opposite direction. The membrane allows water to pass while rejecting many dissolved ions and other substances.
A home RO system needs feed-water pressure within its specified range. Temperature, source-water quality and membrane condition also affect production. Check your model’s manual rather than assuming every RO system has the same pressure requirement.
RO separation differs from a sediment cartridge that holds captured particles in its media. During RO production, flowing concentrate carries rejected material toward the drain. The system’s flow restrictor and valves support this operation; they are working components, not extra filtration stages.
What Is RO Water?
RO water is water treated by reverse osmosis. It generally has fewer dissolved minerals and salts than the incoming water. In a home system, carbon stages also help address chlorine taste and odor.
“RO water” describes how the water was treated. It does not mean every RO system produces identical water. Your source water, the complete filter configuration and ongoing maintenance determine the result.
Is Reverse Osmosis Water Safe to Drink?
Yes—RO-treated water can be suitable for everyday drinking when the system is matched to the source water, installed correctly and maintained. Choose a model documented to address your water concerns. A low TDS number alone does not establish that every chemical or germ has been addressed.
Does RO Remove Minerals?
RO reduces dissolved minerals, including calcium and magnesium. Health Canada says a reasonably balanced diet generally makes mineral supplements unnecessary solely because you drink RO-treated water. A remineralization stage is an option for taste preferences, rather than a requirement for every household.
Is RO Water Acidic?
RO treatment can change water’s pH; the result depends on the water and system. EPA’s pH range of 6.5–8.5 is a secondary guideline addressing aesthetic and technical concerns, including corrosion. It is not a stand-alone test of drinking-water safety.
Start with your water: review your utility report or appropriate test results, then match treatment to the identified concern. Learn how to understand your drinking water →
What Are the Filtration Stages in an RO System?
Each filter has a different job. Here are the four common treatment roles in a tank-based under-sink reverse osmosis water filter.
01
Sediment Prefilter
Captures particles
A sediment cartridge captures particles such as sand, silt and rust before later stages.
02
Carbon Prefilter
Conditions the feed water
Carbon reduces chlorine taste and odor and helps protect chlorine-sensitive RO membranes.
03
RO Membrane
Reduces dissolved substances
The membrane separates treated water from concentrate and reduces many dissolved salts and other substances.
04
Post-Carbon Filter
Polishes taste and odor
A final carbon stage provides taste-and-odor polishing before the water reaches the drinking-water faucet.
Representative cartridge designs—not a replacement set for every RO system. Your model may use different housings, combine functions or place additional filters in the treatment path.
The Parts of a Tank-Based RO System
Follow the main water path through the sediment prefilter, carbon prefilter, RO membrane, storage tank and carbon postfilter. The four numbers identify treatment stages. Supporting valves and connections are not all shown; use your model’s manual for installation and service.
See What the System Components Do
| Component | Purpose |
|---|---|
| Cold-water feed valve | Connects the cold-water supply to the RO inlet. |
| Prefilters | Sediment and carbon stages condition water before the membrane. |
| RO membrane | Separates permeate from concentrate. |
| Postfilter | Final carbon stage before dispensing. |
| Automatic shut-off valve | Stops incoming flow when the system’s tank-full conditions are reached. |
| Check valve | Helps prevent treated water from flowing backward. |
| Flow restrictor | Regulates concentrate flow and supports the membrane’s operating conditions. |
| Storage tank | Stores treated water for use between production cycles. |
| Drinking-water faucet | Dispenses treated water at the sink. |
| Drain line | Carries concentrate to the drain connection. |
Tank-Based vs. Tankless RO Systems
Tank-Based RO
The membrane makes water over time, and a separate pressure tank stores it for use at the faucet. Many conventional under-sink systems run on household water pressure without electricity. Allow room for both the filter assembly and tank.
Tankless RO
A tankless design makes water on demand without the separate storage tank used by conventional systems. Many models use an electric pump. Compare dispensing flow, power requirements and service access—not just the cabinet footprint.
What Does Reverse Osmosis Remove from Water?
A complete RO system combines different treatment methods. Understanding which component does each job helps you read product claims more confidently.
THE MEMBRANE
Dissolved Solids and Specified Contaminants
THE CARBON STAGES
Chlorine Taste and Odor
Carbon prefilters and postfilters contribute taste-and-odor treatment. Compare the stated claim, capacity and replacement schedule. Explore chlorine reduction →
THE SEDIMENT PREFILTER
Sand, Silt and Rust
Particle filtration captures suspended material. It is a different job from reducing dissolved chemicals. Understand sediment filtration →
For PFAS and other specified chemicals, check testing or certification for the particular substance and complete membrane/carbon configuration. See the full RO contaminant guide for more detail.
How to Read an NSF/ANSI 58 Certification Listing
NSF explains that NSF/ANSI 58 requires TDS-reduction testing; additional claims, such as lead, fluoride or cyst reduction, are optional and evaluated separately.
- Match the exact manufacturer and model shown in the listing.
- Read the specific reduction claims rather than assuming the standard covers every contaminant.
- Check what is certified: a membrane or component certification does not automatically cover the assembled system.
Use the current listing and performance data sheet when comparing systems.
What Does Reverse Osmosis Not Remove?
No single treatment method addresses everything. Nebraska Extension identifies dissolved gases such as hydrogen sulfide and some pesticides, solvents and VOCs as limitations of RO membrane treatment. Carbon stages can add treatment for certain organic chemicals, but the complete system must have the relevant claim. A TDS meter does not test for these substances.
Does RO Remove Bacteria and Viruses?
CDC describes RO as capable of removing bacteria, viruses and parasites. That technology-level capability is different from a claim that every household RO unit is a disinfectant. Nebraska Extension recommends feeding household RO microbiologically safe water. For an unsafe well or source, use testing and an appropriate disinfection plan before relying on a kitchen RO system.
Plan treatment for well water → · Understand microbial treatment options →
Benefits and Tradeoffs of Reverse Osmosis
Why People Choose RO
- Reduces dissolved solids and specific contaminants covered by the system’s claims.
- Provides treated water at the kitchen tap for drinking and cooking.
- Can reduce reliance on purchased bottled water when it replaces what you normally buy.
What to Plan For
- A reject-water stream during production.
- Space for the assembly, any storage tank and service access.
- Ongoing filter changes, membrane replacement and model-specific sanitizing.
How Much Water Does RO Send to the Drain?
RO uses a reject-water stream to carry material away from the membrane. Compare the reject-water-to-treated-water ratio, not just the membrane’s GPD production rating.
PUROTWIST 4000
1:1
Listed reject-water-to-treated-water ratio: about one gallon to the drain per gallon of treated water under rated conditions.
WATERSENSE BENCHMARK
2.3:1 or less
EPA requires labeled point-of-use RO systems to send no more than 2.3 gallons to the drain per gallon of treated water.
A 1:1 ratio corresponds to 50% recovery under those conditions. Actual water use includes startup and service flushing. PuROTwist’s published ratio is not a claim of WaterSense certification.
What Affects RO Water Production and Performance?
The GPD number on a product page is a production rating under specified conditions. It is not the same as the amount of water you can dispense at one time. Compare three different measurements:
Production
Gallons per day
How much treated water the membrane can make under its rating conditions.
Storage
Usable tank water
How much treated water is available between refill cycles; tank volume and drawdown differ.
Dispensing
Faucet flow
How quickly water comes out when you open the tap.
Check Feed-Water Conditions
Pressure and temperature: low feed pressure and cold water can reduce production and lengthen tank-refill time. A compatible booster pump may help when pressure is below the model’s required range.
Water chemistry: TDS, hardness, iron, sediment and other source-water conditions can affect suitability and maintenance. Carbon pretreatment helps protect chlorine-sensitive membranes; additional treatment may be needed for other conditions.
Read the exact model’s operating limits and test conditions. Pressure, temperature and water-chemistry limits are specific to the membrane and system. Use the owner’s manual and current performance sheet for your unit.
RO Performance Terms in Plain Language
- Permeate: water that passes through the RO membrane.
- Concentrate: the reject-water stream carrying material rejected by the membrane.
- TDS: total dissolved solids; a useful performance measure, not a test for every contaminant.
- GPD: gallons per day, used for rated production.
- Rejection or reduction: how much of a specified substance is reduced under the stated conditions.
- Recovery: the share of incoming water that becomes treated water; it describes water use, not the contaminant-removal percentage.
- PSI: pounds per square inch, the pressure unit used in system specifications.
Is RO the Right Treatment for Your Home?
Choose the treatment that matches your water concern. RO is useful when reducing dissolved solids or a specific documented contaminant is the goal; other methods serve different needs.
Test Your Water Before You Choose RO
Identify the concern before choosing the equipment. Nebraska Extension recommends testing first and reviewing your utility’s Consumer Confidence Report. For a private well, choose laboratory tests suited to local risks. Then compare the exact system’s reduction claims, feed-water limits and upkeep with your results.
| Treatment | Common reason to compare it | What matters for your choice |
|---|---|---|
| Under-sink RO | Dissolved salts and specified dissolved contaminants. | Reduces dissolved minerals and produces concentrate. Check model-specific reduction claims. |
| Activated carbon | Chlorine taste and odor; specified chemical concerns. | Lead or other health-related reduction requires the relevant model-specific claim; carbon is not a general TDS treatment. |
| Sediment filtration | Sand, silt and rust particles. | Choose the cartridge rating and fit. Capturing particles is different from reducing dissolved substances. |
| Ultrafiltration | Fine suspended material while retaining dissolved minerals. | Germ-reduction claims vary by membrane and complete system. Do not assume universal virus reduction. |
| UV disinfection | Microbial concerns in a suitable treatment plan. | Inactivates microorganisms; does not remove dissolved salts. Water clarity, flow and maintenance matter. |
| Distillation | Producing water with reduced dissolved minerals by boiling and collecting vapor. | Uses heat and energy. Some volatile chemicals can carry over; compare the complete unit’s claims. |
Further reading: CDC’s treatment-method overview and NSF’s guide to treatment standards. These are technology comparisons, not certification claims for every product.
For a practical shopping checklist, see what to look for when buying an RO system. Our RO buyer’s guide explains the broader options.
Compare Under-Sink RO Systems
These are complete kitchen drinking-water systems—not retrofit assemblies. Compare the priorities below, then check current dimensions, water requirements, included parts and pricing on the product page.
EVERYDAY VALUE
Microline TFC-400-ESP
A dependable, value-focused kitchen RO workhorse with four-stage filtration and a membrane rated up to 50 GPD. Includes the tank, chrome air-gap faucet and installation kit; runs without electricity.
COMPACT FILTER ASSEMBLY
AXEON Solo I
A space-saving all-in-one filter assembly with four stages and a membrane rated up to 50 GPD. Its separate storage tank still needs cabinet space.
QUICK CHANGE + LOWER WASTE
PuROTwist 4000
Quick-change filters and lower listed water use: four stages, up to 75 GPD, and a listed 1:1 wastewater ratio under rated conditions.
Identify Your RO System and Replacement Parts
Use your model number and specified part numbers when ordering. Open the replacement-parts finder above, or use the housing photos below as a starting point.
For membrane replacement, match membrane fit, GPD and flow control. Similar capacity ratings do not make different cartridges interchangeable.
Recognize Your Filter Housing Layout?
These photos can help you identify a starting category. Confirm the brand, model and specified part number before ordering; matching the number of housings alone is not enough.
Keep Suitable Parts When You Upgrade
If proprietary filters are costly or difficult to find, a compatible RO conversion kit may let you replace the treatment assembly while retaining a suitable tank and faucet. Check the named system match and tubing requirements for the exact kit.
RO Maintenance at a Glance
A simple service schedule for the filters and membrane in a traditional under-sink RO system.
6–12
MONTHS
Sediment + Carbon Prefilters
Replace the specified cartridges to support flow and membrane protection.
6–12
MONTHS
Post-Carbon Filter
Replace the final polishing filter to maintain its taste-and-odor treatment.
~24
MONTHS
RO Membrane
Follow the model’s service interval and performance checks. Some conditions can require earlier replacement.
These are general guidelines. Follow the replacement schedule in your specific RO system’s owner’s manual and cartridge instructions. Water quality and use may require earlier service.
Include Sanitizing and Inspection
Use the procedure approved for your system and sanitizer. Service is also a time to inspect housings, fittings, tubing, tank operation and leaks. Choose the correct sanitizing procedure →
Signs Your RO System May Need Attention
- Faucet flow becomes noticeably slower.
- The storage tank is not filling as expected.
- Taste or odor changes.
- A TDS comparison shows declining membrane performance.
- Water keeps flowing to the drain after the tank should be full.
- A housing, fitting or tube shows moisture or leakage.
Diagnose the cause before ordering parts. Use the RO troubleshooting guide or full maintenance guide for next steps.
Where Is Reverse Osmosis Used?
One drinking-water tap, an entire property or a business: the application determines the system design.
AT HOME
ONE TAP
At the Kitchen Sink: Point of Use
A point-of-use (POU) RO system treats water at a specific drinking-water outlet. Most home units sit under the kitchen sink and supply a dedicated faucet. Some can also connect to a refrigerator dispenser or ice maker when available pressure and system instructions permit it.
WHOLE PROPERTY
POINT OF ENTRY
Throughout a Property: Point of Entry
A whole-house, or point-of-entry (POE), RO installation treats water for the wider property. This requires a different design for demand, storage, pretreatment and any supporting post-treatment. A household that mainly wants treated drinking and cooking water may be better served by point-of-use RO. Compare whole-house and under-sink RO needs →
FOR BUSINESS
PROCESS + DEMAND
Commercial and Industrial Applications
RO technology is also used in food and beverage operations, agriculture, boiler-feed treatment, hotels, pharmaceutical processes, marine applications and desalination. Requirements vary widely: these are applications of RO technology, not claims that a household unit suits each one.
Higher-demand installations can require different pretreatment, pumping, storage and controls. For example, the AXEON HYDRO 750 is a wall-mounted light-commercial option rated for up to 750 GPD under its specified conditions and requires electrical power. Explore commercial RO systems, or call our team to discuss the application.
Reverse Osmosis Frequently Asked Questions
What is the difference between an RO membrane and an RO system?
The membrane performs the separation of water from many dissolved substances. A complete RO system includes supporting filters, valves, tubing and dispensing components. A tank-based system also includes storage. Choose by the complete configuration’s documented performance.
Does RO water still contain minerals?
RO reduces many dissolved minerals and salts; the amount remaining depends on the incoming water and system. Some systems add a remineralization filter to adjust taste and pH. RO water does not have one universal mineral content.
Does a reverse osmosis system need electricity?
Conventional tank-based under-sink systems such as the examples above use household water pressure and do not require electricity. Models with electric pumps or powered features require a suitable power connection.
How much water can an RO system produce?
GPD is a daily production rating under specified conditions. It is different from faucet flow or the amount of water available in a tank at one time. Pressure, water temperature, incoming water and membrane condition affect actual production and refill time.
Can RO supply a refrigerator or ice maker?
Many under-sink systems can when their specifications, available pressure and installation allow it. Check the manufacturer’s instructions and our RO-to-refrigerator connection guide before adding a line.
Can I install an under-sink RO system myself?
Some homeowners can install a suitable system using its instructions. Check cabinet space, faucet mounting, feed connection and drain requirements first. Plumbing changes or unfamiliar work may call for a qualified installer. See the RO installation guide.
Is RO the same as bottled or distilled water?
No. Reverse osmosis is a treatment process; bottled water is a packaged product that may use different sources and treatment methods. Distillation uses evaporation and condensation. The water’s properties depend on the source and process, so compare documented treatment rather than assuming these labels mean the same thing.
Do I need whole-house reverse osmosis?
Most household drinking-water RO systems treat water at a dedicated kitchen faucet. Whole-house RO serves a different need and requires a separate design for the source water, peak demand, storage and supporting treatment. Compare RO system types before choosing.
How long does a reverse osmosis system last?
Housings, fittings, the faucet and storage tank can remain in service for many years when installed and maintained correctly. Service life depends on the model, conditions and use. Filters and membranes are consumables; replacing them is part of ownership rather than a reason to replace the whole system.
Is a reverse osmosis system noisy?
Many pressure-driven under-sink systems operate quietly, although you may hear concentrate flowing to the drain. An air-gap faucet may gurgle while the tank refills. Persistent or changed sounds should be checked against the manual and operating conditions. Understand air-gap and non-air-gap faucets or use our troubleshooting guide.
Does reverse osmosis remove fluoride?
An RO membrane can reduce fluoride, but the actual reduction depends on the system and conditions. Choose the complete model’s documented fluoride-reduction claim for your goal, rather than a universal percentage. Read the fluoride treatment guide.
Which part of an RO system reduces chlorine?
The carbon prefilter is the main stage used for chlorine taste-and-odor treatment in a typical RO system, and it also helps protect chlorine-sensitive membranes. The membrane and carbon stages have different roles. Maintain the specified carbon cartridges on schedule.
Is an RO system worth the cost?
Compare the system and installation price, filters, membranes, service and water use with what you currently spend on drinking water. It may offer savings when it replaces purchased bottled or delivered water, but savings depend on your household. Treatment fit and convenience matter alongside cost. Use the RO buying checklist to compare ownership costs.
How often should RO filters and membranes be changed?
Many traditional systems use 6–12-month intervals for prefilters and postfilters, with an RO membrane commonly checked or replaced around 24 months. These are general planning references, not a universal schedule. Follow your exact model’s manual, water-quality limits and performance checks. See the RO maintenance guide.
Is reverse osmosis water safe to drink?
RO-treated water can be suitable for drinking when the system matches the source water and is installed and maintained correctly. Verify its documented reduction claims; taste, pH and a low TDS reading alone do not establish safety. Read the safety and mineral guidance above.
Does reverse osmosis remove PFAS?
Some complete RO systems have documented PFAS-reduction claims. Check the exact substances covered, the model’s current testing or certification, and its filter-replacement requirements. Do not treat a generic RO or NSF/ANSI 58 label as proof of every PFAS claim. Learn what to check for PFAS treatment.
Does an RO system remove chloramine?
Look for a complete system with a carbon stage specifically documented for chloramine reduction, at the stated flow and capacity. Do not assume every carbon cartridge has that claim. Compare carbon-filter treatment claims.
Shop RO Systems and Replacement Parts
Compare under-sink RO systems for your home, or find the filters, membrane and parts that fit the system you own.
Need help? Call 1 (877) 377-9876 · Monday–Friday, 8 AM–5 PM Central Time
Explore More RO Guides
Choose an RO System
Compare water needs, installation fit and ownership costs.
Understand RO Reduction Claims
Match the exact system’s claims to your water concern.
Weigh Benefits and Drawbacks
Consider water use, ongoing care and everyday convenience.
Explore Conventional Cartridge Value
Review the PureValue 5EZ50 design and replacement kits.
