How Does Reverse Osmosis Work?

Reverse osmosis (RO) treats drinking water by using pressure to move water through a semipermeable membrane. Water that passes through the membrane, called permeate, is sent to a storage tank or faucet. Water carrying the rejected dissolved contaminants, called concentrate, is sent to the drain. This guide focuses on under-sink RO systems that supply a dedicated drinking-water faucet.

Most under-sink RO systems also use a sediment prefilter, a carbon prefilter, and a post-carbon filter. These stages protect the RO membrane and help improve the taste and odor of the treated water.

How Reverse Osmosis Works in 4 Steps

A sediment filter traps dirt, rust and other particles.
A carbon filter reduces chlorine and helps protect the RO membrane.
Water pressure pushes water through the semipermeable RO membrane.
Treated water moves to the storage tank, while concentrate flows to the drain.

What Is Reverse Osmosis?

Reverse osmosis is a water treatment method that separates water from many dissolved substances by forcing it through a semipermeable membrane. In a typical under-sink drinking-water system, the membrane is combined with sediment and carbon filtration to protect the membrane and improve the taste and odor of the treated water.

Most residential RO systems are installed under a kitchen sink and supply a dedicated drinking-water faucet. Some systems can also be connected to a refrigerator or ice maker when the system specifications and installation conditions allow it.

The Science Behind Reverse Osmosis

RO filtration works from pushing water through membrane During ordinary osmosis, water naturally moves across a semipermeable membrane toward the solution with a higher concentration of dissolved material. Reverse osmosis applies pressure to move water in the opposite direction.

The membrane allows water to pass while rejecting many dissolved ions and other substances. Actual performance depends on the system, membrane, incoming water quality, pressure, temperature and maintenance.

Most RO drinking water systems are installed under the kitchen sink and incorporate carbon block, sediment and granulated activated carbon (GAC) filters. Although these types of filters are incorporated into a reverse osmosis filtartion system, they are also used in a variety of water treatment processes and applications.

The real "heart" of a reverse osmosis system is the RO membrane. The membrane works by utilizing household water pressure to force tap water through a semi-permeable membrane.

As the water is pushed through the membrane, the clean water is sent to a holding tank through one tube, and the contaminants are separated out flushed away down another tube.

RO filtration works from pushing water through membrane

Water Pressure and RO Performance

Reverse osmosis depends on adequate feed-water pressure. Low pressure can reduce water production, slow storage-tank filling and affect system efficiency. Always follow the operating-pressure range listed in the system owner’s manual.

If a home’s water pressure is below the system’s required range, a properly sized booster pump may improve production. Do not assume that one minimum pressure applies to every RO system.

How an Under-Sink Reverse Osmosis System Works Step by Step

A typical under-sink RO system treats water through several components. The exact number and order of stages vary by model, but the following four-stage arrangement is common.

Stage	Component	What It Does
1	Sediment prefilter	Traps sand, rust, silt and other particles that could clog or damage later stages.
2	Carbon prefilter	Reduces chlorine, taste and odor while helping protect chlorine-sensitive RO membranes.
3	RO membrane	Reduces many dissolved solids and separates the water into treated water and concentrate.
4	Storage tank and post-carbon filter	Stores treated water and improves taste and odor before the water reaches the faucet.
Optional	Remineralization filter	Adds selected minerals back primarily to adjust taste and pH.

Why Each Filtration Stage Matters

The prefilters and postfilter are not substitutes for the RO membrane. Each component has a different job. Sediment filtration reduces particles, carbon filtration reduces chlorine and helps protect the membrane, and the membrane addresses many dissolved substances.

More stages do not automatically mean better performance. Compare the purpose of each stage, replacement cost, efficiency and the system’s certified contaminant-reduction claims.

What Does Reverse Osmosis Reduce Contaminants?

Reverse osmosis systems reduce total dissolved solids and a wide range of dissolved contaminants. Under typical household conditions, an RO membrane reduces fluoride by 85–92%, lead by about 98%, and Arsenic V by 92–96%. Our contaminant chart lists typical reduction rates for more than 20 substances.

These figures describe typical membrane performance, not a guarantee for every system. Actual results vary with membrane type, system design, incoming water quality, water pressure, and maintenance. For a specific contaminant concern, confirm the reduction claims certified for the exact system you are considering.

Water-Quality Concern	Component Involved	What to Verify
Total dissolved solids and dissolved salts	RO membrane	TDS-reduction performance for the complete system.
Lead, Arsenic V, nitrate	RO membrane	Certified reduction claims for the exact system.
Chlorine, taste and odor	Carbon prefilter and postfilter	Carbon capacity, stated claims and replacement schedule.
PFAS and VOCs	Membrane and/or carbon stages	Test data or certification for the exact model.
Sediment, rust and silt	Sediment prefilter	Micron rating and replacement frequency.
Fluoride	RO membrane	Typical reduction of 85–92%; confirm the certified claim for the exact system.

NSF/ANSI 58 covers point-of-use reverse osmosis systems and includes required testing for total dissolved solids reduction. Other contaminant-reduction claims are verified separately. Choose a system based on the specific claims documented for that model.

See what reverse osmosis can reduce for the full contaminant chart and typical reduction rates.

Benefits and Tradeoffs of Under-Sink Reverse Osmosis

An under-sink reverse osmosis system can be a practical point-of-use option when a household wants to reduce dissolved substances in drinking and cooking water. It also requires sufficient pressure, replacement filters, cabinet space and a drain connection. Consider both the advantages and operating requirements before choosing a system.

Potential Benefits

Reduces total dissolved solids and many dissolved contaminants, depending on the system’s documented claims.

Carbon stages can improve taste and odor by reducing chlorine and related compounds.

Provides RO-filtered water at the kitchen faucet and may reduce reliance on bottled water.

Treats drinking and cooking water at one point of use instead of treating every gallon used in the home.

Important Tradeoffs

Produces concentrate that is sent to the drain during treatment.

Reduces dissolved mineral content; optional remineralization may be added primarily for taste and pH preferences.

Requires scheduled prefilter, postfilter and membrane replacement.

Needs adequate cabinet space, water pressure and a drain connection.

Performance claims differ by system and should be checked before purchase.

RO Water Use and Efficiency

Reverse osmosis produces a stream of treated water and a stream of concentrate, which carries rejected material to the drain. The ratio varies by system, pressure, temperature, membrane condition, and design — and today's water-efficient systems have made real progress.

Environmentally friendly RO systems, such as the PuroTwist 4000, now achieve a 1:1 waste-water ratio (50% recovery) — one gallon to the drain for every gallon of treated water. For reference, a WaterSense-labeled point-of-use RO system is required to use no more than 2.3 gallons of reject water per gallon of treated water, so a 1:1 system performs well beyond that baseline. Compare the documented efficiency or recovery information for the exact model you're considering.

Source: EPA WaterSense: Point-of-Use Reverse Osmosis Systems

Viqua VIQUA IHS22-D4 Integrated 12 GPM UV System with Sediment and Carbon with Lead Reduction Filters IHS22-D4-

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PuROTwist 4000 RO System

PuROTwist 4000 RO water filter system offers the highest of quality and performance standards. In addition, the system conserves water with a 1:1 recovery rate--a big improvement from the typical 4:1 waste water ratio of other RO systems.

The high-capacity (75 GPD) reverse osmosis system features quick-connect filters that make it easy to replace filters. The system's automatic shut-off feature signals the RO unit to stop making water until more is needed.

Is an Under-Sink Reverse Osmosis System Right for Your Home?

The best treatment method depends on what is in the incoming water and what the household wants to address. Start with a current water-quality report or appropriate water test, then compare treatment methods and certified claims.

An under-sink RO system may be a good fit when dissolved solids or specific dissolved contaminants are a concern, and the household wants treated water primarily for drinking and cooking. A simpler filter may be sufficient when the main goal is only sediment reduction or improved chlorine taste and odor.

Treatment Method	Often Used For	Important Limitation
Under-sink reverse osmosis	Reducing total dissolved solids and specific dissolved contaminants covered by the system’s claims.	Produces concentrate and requires regular filter and membrane replacement.
Carbon filtration	Reducing chlorine taste and odor and certain compounds, depending on the filter’s claims.	Generally does not reduce total dissolved solids.
Sediment filtration	Trapping sand, silt, rust and other suspended particles.	Does not address dissolved contaminants.
Ultrafiltration	Reducing fine suspended material while typically retaining dissolved minerals.	Generally does not reduce total dissolved solids the way RO does.

Need help comparing systems? Read the Reverse Osmosis Buyer’s Guide or browse our under-sink reverse osmosis systems.

Higher-Capacity RO Option: AXEON HYDRO 750

View the AXEON HYDRO 750

Important fit note: This is not a standard under-sink household RO system. It requires electrical power, a suitable wall-mounting location, an unrestricted drain connection and a storage configuration appropriate for the application. ESP recommends using the system with a pressure tank. Review the product specifications and owner's manual before purchase.

For most households, a conventional under-sink RO system remains the practical choice for drinking and cooking water. Offices, food-service operations, retail locations and other light-commercial applications may need substantially more water production.

ESP Water now carries the AXEON HYDRO 750, a five-stage, wall-mounted reverse osmosis system built for higher daily demand. Under the manufacturer’s specified conditions, it can produce up to 750 gallons of treated water per day and provides 96% or greater TDS rejection. Actual production and efficiency vary with feed pressure, temperature, and incoming water quality.

AXEON HYDRO 750 at a Glance

Rated production: Up to 750 GPD at 80 psi and 77°F

Configuration: Five-stage, wall-mounted light-commercial RO system

TDS rejection: 96% or greater under manufacturer test conditions

Power: 100–120V, 50/60 Hz

Dimensions: 16 x 9 x 27 inches

Features: Incoming and outgoing water-quality monitoring, automatic flushing, leak detector with automatic shutoff, color-coded twist cartridges and a float valve

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AXEON SOLO RO

Pros

✅ Space-Saving Design: Compact design with 50 percent smaller footprint than most RO systems.

✅ Advanced Technology: Incorporates highly advanced membrane technology for supreme water quality.

✅ Higher Flow Rates: 30% higher flow to the faucet, and higher-flow RO membrane fills the tank faster.

✅ Customizable: Optional remineralization cartridge available to balance water's pH.

✅ Trusted Brand: Axeon Water Technologies has a reputation for producing reliable and high-quality water treatment products.

Cons

❌ Fewer Filtration Stages: The system includes four stages of filtration instead of five stages.

❌ Replacement price: Filter replacements are slightly higher priced than other RO systems.

❌ Like all RO systems, regular maintenance (e.g., filter replacement) is necessary to ensure optimal performance.

Reverse Osmosis Maintenance at a Glance

Maintenance intervals vary by system, water quality and household usage. Always follow the owner’s manual for the exact model. The schedule below is a general starting point for many traditional under-sink systems.

Component or Task	General Timing	Why It Matters
Sediment prefilter	Every 6–12 months	Helps maintain flow and protects later stages from particles.
Carbon prefilter	Every 6–12 months	Helps protect the membrane and maintain stated taste-and-odor performance.
Post-carbon filter	About every 12 months	Helps maintain taste and odor at the faucet.
RO membrane	Often about every 24 months	Replacement timing depends on the model, usage and incoming water conditions.
sanitation and inspection	Follow the owner's manual; often performed during annual service	Provides an opportunity to inspect housings, tubing, fittings, tank pressure and leaks.

Signs That an RO System May Need Attention

The faucet flow has become noticeably slower.

The storage tank is not filling as expected.

Taste or odor has changed.

A TDS comparison shows declining membrane performance.

Water continues flowing to the drain after the storage tank should be full.

A housing, fitting or tube shows moisture or leakage.

For detailed instructions, see how to maintain a reverse osmosis system or use the RO troubleshooting guide.

Reverse Osmosis Frequently Asked Questions

How long does an under-sink reverse osmosis system last?

The housings, fittings, faucet and storage tank can remain in service for many years when the system is installed and maintained correctly. Service life varies by model, water conditions and usage. Filters and the RO membrane are consumable components and must be replaced according to the owner’s manual.

Does reverse osmosis remove minerals from water?

Reverse osmosis reduces many dissolved minerals along with other dissolved substances. Some households prefer the taste of standard RO-filtered water, while others choose an optional remineralization stage to adjust taste and pH.

How much water does an under-sink reverse osmosis system send to the drain?

The amount varies widely. EPA reports that a typical point-of-use RO system may send five gallons or more to the drain for each gallon of treated water. Enviro-friendly systems like the PuROTwist 4000 send one gallon of reject water per gallon of treated water to the drain. Check the efficiency or recovery information for the exact model.

How much water pressure does an under-sink RO system need?

Pressure requirements vary by system. Low pressure can slow water production and storage-tank filling. Check the operating range in the system owner’s manual. A properly sized booster pump may help when incoming pressure is below the required range.

What is the difference between 3-stage, 4-stage and 5-stage under-sink RO systems?

The number describes how many treatment components the manufacturer counts. A common four-stage system includes sediment filtration, carbon prefiltration, the RO membrane, and a post-carbon filter. A fifth stage may be an additional carbon filter, remineralization filter or another component. Compare what each stage does rather than selecting a system only by stage count.

Does reverse osmosis remove fluoride from water?

Yes. Under typical household conditions, a reverse osmosis membrane reduces fluoride by 85–92%. Fluoride is a dissolved ion, so sediment and carbon filters alone will not reduce it, and boiling concentrates it rather than removing it. Actual reduction depends on membrane type, water pressure, temperature and membrane age. For more detail, see how to remove fluoride from water.

How often should RO filters and membranes be changed?

Many traditional systems use sediment and carbon prefilters that are replaced every 6–12 months, a post-carbon filter replaced about annually and an RO membrane replaced about every 24 months. These are general guidelines. Follow the schedule in the owner’s manual for the specific system.

Can I install an under-sink reverse osmosis system myself?

Many under-sink systems are designed for do-it-yourself installation. The work usually involves connecting to the cold-water supply, installing a drain connection, positioning the system and storage tank, and installing a dedicated faucet. See our step-by-step reverse osmosis installation guide for general installation instructions. Follow the model-specific manual and applicable plumbing requirements. Hire a plumber if the installation exceeds your tools or experience.

Does reverse osmosis reduce chlorine?

The carbon prefilter in a typical RO system is the primary component used to reduce chlorine taste and odor. Carbon prefiltration also helps protect chlorine-sensitive RO membranes. Check the complete system’s performance claims and replace the carbon filter on schedule.

Does an under-sink RO system require electricity?

Many traditional tank-based under-sink RO systems operate using household water pressure and do not require electricity. An under-sink system with an electric booster pump requires a power connection.

Can an under-sink RO system be used when microbiological water quality is a concern?

Do not assume that every residential RO system is validated for microbiological reduction. When the source water presents this concern, select treatment specifically designed and validated for the condition and follow applicable local guidance and system instructions.

Ready to Find the Right Under-Sink RO System?

Compare under-sink reverse osmosis systems by daily production (GPD), filtration stages, filter style, faucet connection and water-use efficiency. Already own an RO system? Use our identification guide to find compatible replacement filters and membranes.

Compare Under-Sink RO Systems

Identify My RO System

Need help choosing? Call ESP Water at 1 (877) 377-9876, Monday through Friday, 8 a.m.–5 p.m. Central Time.

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