Reverse Osmosis Water Filtration: How It Works and What It Removes

Reverse osmosis (RO) is a pressure-driven membrane filtration process used to reduce many dissolved substances in drinking water. It can be particularly useful where testing identifies salts or specific inorganic contaminants that simpler sediment filtration does not address. But RO is not a universal safeguard: performance depends on source-water chemistry, the complete system design, certified reduction claims, membrane condition and maintenance.

A useful reverse osmosis water filtration overview therefore goes beyond generic claims that RO “purifies” water. A membrane’s theoretical separation capability is not the same as the verified performance of an installed appliance. The practical question is whether a particular complete system is designed and independently certified for the contaminants present in the water.

This guide focuses on how household point-of-use RO works, what it can reduce, its water-use tradeoffs, pretreatment, mineral removal, certification and maintenance.

How reverse osmosis works

Pressure, membrane and separation

Reverse osmosis is a pressure-driven process that uses a semipermeable membrane to separate water from many dissolved substances. In natural osmosis, water moves across a membrane in response to a difference in solute concentration. In RO, applied pressure drives water through the membrane. The water that passes through is called permeate or product water; substances retained by the process leave in a separate concentrate or reject stream.

The membrane is the defining treatment stage. The CDC describes household RO membranes as having an approximate pore size of 0.0001 micron and lists a range of microorganisms and dissolved chemicals that RO technology can reduce. Separation is not uniform across all substances, however. Molecular characteristics, charge, water chemistry, pressure and the condition of the system all affect performance. For that reason, membrane capability should not be converted into a blanket claim about what a particular appliance removes.

Typical point-of-use RO components

Household systems vary, but a conventional under-sink unit commonly combines a sediment prefilter, one or more carbon stages, the RO membrane, a storage tank and sometimes a postfilter. Some designs add other treatment stages. The sediment and carbon stages can protect the membrane and address substances for which the membrane alone is not the appropriate treatment; a postfilter may further condition water before dispensing.

RO systems are commonly used at the point of use because membrane production is relatively slow and the process generates concentrate. EPA describes point-of-use RO primarily as treatment for water used for drinking and cooking rather than for toilets, bathing or clothes washing. Larger systems exist for other applications, but their engineering and operating considerations differ substantially from a typical household unit.

What RO can reduce

RO is especially useful for many dissolved salts and inorganic contaminants that are not removed simply by trapping suspended particles. The CDC says RO removes some chemicals including lead, copper, chromium, chloride and sodium, and may reduce arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium and nitrate. These are descriptions of RO capability, not promises about every product.

Dissolved salts and inorganic contaminants

Total dissolved solids (TDS) reduction is a fundamental RO performance measure. Individual dissolved constituents behave differently, however, so a TDS result cannot establish the reduction of lead, arsenic, nitrate or another specific contaminant. Where one of those substances is the treatment target, the complete device’s applicable certified reduction claim is more meaningful than a generic membrane specification.

Microorganisms and selected organic contaminants

The CDC lists parasites, bacteria and viruses among microorganisms that RO technology can remove. That capability should not be interpreted to mean that an ordinary household RO installation guarantees microbiologically safe drinking water. System integrity, feed-water assumptions, certification and maintenance remain important.

RO systems can also be configured and certified for some organic contaminants. EPA notes potential applications for substances including PFAS and volatile organic compounds, but these categories contain many different chemicals. A claim about “RO” generally is therefore not enough to establish reduction of a particular PFAS or VOC.

How to interpret reduction claims

Contaminant/parameter	RO performance context	Caveat	Verification
TDS	For WaterSense-labeled point-of-use RO systems, applicable NSF/ANSI 58 testing requires at least 75% TDS reduction under specified test conditions.	This does not mean 75% reduction of every dissolved constituent or prove that water is safe.	EPA WaterSense specification
Lead	RO can reduce lead, and lead reduction can be a contaminant-specific certified claim.	Verify the complete system’s lead-reduction claim rather than inferring it from the presence of an RO membrane.	CDC; NSF/ANSI 58
Arsenic	The CDC says RO may reduce arsenic; arsenic reduction can be evaluated under NSF/ANSI 58.	Performance is system- and claim-specific; do not assume generic RO performance covers the arsenic conditions in a particular water supply.	CDC; NSF/ANSI 58
Nitrate/nitrite	The CDC says RO may reduce nitrate, and NSF/ANSI 58 provides for nitrate/nitrite reduction claims.	Use certified system-specific performance where nitrate or nitrite is the health concern.	CDC; NSF/ANSI 58
PFAS	EPA identifies point-of-use RO as a technology that can potentially reduce PFAS.	Do not generalize that capability to every PFAS compound or every RO device; look for the applicable verified claim.	EPA WaterSense
Bacteria, viruses and parasites	The CDC lists these among microorganisms RO technology can remove.	This is not a guarantee of microbiological safety for an installed household system.	CDC
Calcium and magnesium	RO may reduce these dissolved minerals along with unwanted dissolved substances.	Mineral reduction does not establish that remineralization is medically necessary.	CDC; EPA
VOCs	RO systems can potentially reduce VOCs, and NSF/ANSI 58 provides for an optional VOC reduction claim.	Attribute VOC reduction only to a complete system with an applicable verified claim.	EPA; NSF/ANSI 58

What RO does not guarantee

No universal contaminant removal

No filter removes every possible contaminant under all conditions. RO can reduce many dissolved substances very effectively, but performance varies by substance, system configuration and operating conditions. Some water problems instead require, or are better addressed by, activated carbon, ion exchange, oxidation, disinfection or another treatment process.

This distinction matters because a product described simply as an “RO system” does not thereby have every possible contaminant-reduction claim. The contaminants of concern should be identified first and then matched to documented performance for the complete device.

Microbiological safety is not automatic

RO membranes can provide a barrier to microorganisms, but household systems should not automatically be assumed to make microbiologically unsafe source water safe. System integrity, feed-water assumptions, certification and maintenance matter.

RO’s ability to remove microorganisms should not be interpreted as a microbiological-safety guarantee for an installed household system.

TDS and taste are not safety tests

Many contaminants cannot be reliably detected by taste, smell or appearance. Likewise, a TDS meter measures dissolved material collectively; it does not identify which ions or chemicals are present. A low product-water TDS does not prove that a particular health-related contaminant is below a desired concentration.

This is why water testing remains important when treatment is intended to address lead, arsenic, nitrate, PFAS or another specific concern.

Pretreatment and membrane protection

Why prefiltration matters

Reverse osmosis is best understood as part of a treatment train rather than an isolated membrane. Prefiltration can remove suspended matter and help protect membrane performance and service life. EPA notes that accumulation of soluble and particulate material on an RO membrane can reduce efficiency and contaminant removal.

Upstream filtration and other pretreatment should be selected for the actual source-water conditions and the RO manufacturer’s requirements.

Feed-water conditions and additional treatment

Additional pretreatment needs are source-water and manufacturer specific. Remineralization, when used, is generally a downstream water-conditioning step rather than membrane protection.

There is no single hardness, iron, manganese, chlorine or sediment threshold that can appropriately be applied to every household RO system. Pretreatment requirements depend on source-water chemistry and the manufacturer’s operating limits. Testing and system-specific specifications should therefore drive the design.

Recovery, reject water and efficiency

Permeate and concentrate

RO necessarily divides its incoming flow into permeate and concentrate. The concentrate carries substances retained by the membrane and is normally discharged from a household point-of-use system. This water-use consequence is important when comparing RO with treatment methods that do not continuously produce a reject stream.

EPA reports that a typical point-of-use RO system can send five gallons or more to drain for each gallon of treated water produced, and that inefficient units can reach 10 gallons of reject water per gallon of treated water. Newer, more efficient designs can use substantially less.

Efficiency versus membrane recovery

“Efficiency” and “recovery” describe related but different measurements. EPA defines system efficiency around the proportion of incoming water ultimately delivered to the user under conditions approximating normal operation. Membrane recovery describes how much membrane influent becomes RO-treated water under its specified test conditions.

Those numbers can differ because a storage tank develops backpressure as it fills, which can reduce real-use efficiency, and some systems consume additional water during automatic flushing. A membrane recovery specification should therefore not automatically be read as the household appliance’s actual water efficiency.

What WaterSense changes for point-of-use RO

EPA’s WaterSense specification applies to qualifying point-of-use RO systems, not point-of-entry or whole-house RO. A WaterSense-labeled system must have an efficiency rating of at least 30%, equivalent under the specification to sending no more than about 2.3 gallons to drain per gallon of treated water. The specification also requires an automatic shutoff device to prevent reject-water discharge when treatment is not needed.

Continuous flow to the drain when the system is not producing water can indicate an automatic-shutoff problem. EPA’s WaterSense RO information is particularly useful when water efficiency is a selection criterion.

Minerals, TDS and remineralization

Mineral and TDS reduction

RO reduces calcium, magnesium and other dissolved minerals along with many unwanted dissolved substances. This is one reason product water generally has lower TDS than its feed water. It can also change taste because mineral composition contributes to the sensory character of drinking water.

The fact that RO removes minerals does not by itself establish that RO water is unsafe or that remineralization is medically required. The authoritative guidance reviewed for this article documents mineral reduction but does not establish a universal health requirement to add those minerals back.

What a TDS reading means

TDS is a broad operational measurement, not a direct contamination or health metric. Naturally occurring calcium and magnesium contribute to TDS, but so can sodium and many other dissolved substances. Conversely, a health-relevant contaminant may be present at a concentration too small to dominate the overall TDS reading.

If treatment is being used for a specific health-related substance, contaminant-specific testing is the appropriate verification tool.

Remineralization as an optional downstream choice

Some RO systems include a downstream remineralization stage that adds minerals after membrane treatment. Such cartridges may be selected for taste or a particular water-chemistry objective. Their presence should not be interpreted as evidence that all RO water requires remineralization for health.

Certification and performance claims

NSF/ANSI 58 and contaminant-specific claims

For household treatment devices, independent certification helps distinguish demonstrated system performance from broad marketing language. NSF/ANSI 58 covers point-of-use reverse osmosis drinking-water treatment systems and addresses areas including material safety, structural integrity, TDS reduction, efficiency and recovery, consumer information and applicable contaminant-reduction claims. NSF reports NSF/ANSI 58-2025 as the current published edition as of 2026.

An important distinction is that certification to an RO standard does not mean every optional contaminant claim applies to a product. TDS reduction is a core RO requirement, while reductions for substances such as arsenic, nitrate/nitrite, lead, chromium, fluoride and VOCs are contaminant-specific claims. Buyers treating a known problem should verify that exact claim for the complete system.

WaterSense for point-of-use RO

EPA requires WaterSense-labeled point-of-use RO systems to be independently certified to applicable requirements, including NSF/ANSI 58. Under EPA’s specification, systems must meet the specified TDS-reduction and water-efficiency requirements, and advertised elective chemical or mechanical reduction claims must be appropriately verified.

EPA’s RO WaterSense page was updated August 21, 2026 and notes that EPA issued two technical clarifications in August 2026 affecting Section 5.2 packaging and documentation requirements. Because specifications can change, current EPA information and the product’s certification listing should be checked rather than relying on an old label description alone.

Reading the performance data sheet and installation requirements

Generic statements such as “purifies water” or “removes contaminants” are not enough. The product documentation should identify the substances it is certified to reduce and the conditions associated with those claims. Certification of a device for one reduction claim should not be silently extended to unrelated contaminants.

Installation matters as well. A certified product installed incorrectly or operated outside its specified feed-water conditions may not deliver the intended result.

Maintenance and when RO is appropriate

When household point-of-use RO makes sense

Accurate reverse osmosis water filtration testing begins before installation. The CDC recommends identifying harmful chemicals or germs in tap water and choosing treatment that addresses those concerns; where harmful contaminants are not present, household treatment may not be necessary. This avoids using RO simply because a TDS number appears high or because “purification” sounds preferable.

Point-of-use RO is most appropriate when drinking or cooking water has an identified treatment objective for which a suitable system has demonstrated performance. Source chemistry can also determine whether pretreatment is required. Private wells, in particular, should be evaluated according to local guidance and the risks relevant to the well rather than by a generic testing schedule.

Source protection remains important even after treatment is installed. RO should not be treated as a substitute for protecting a well, maintaining infrastructure or correcting an upstream contamination problem where that problem can be addressed directly.

Maintenance and warning signs

Long-term performance depends heavily on maintenance. Filters become loaded and membranes can foul; tanks and other wetted components require appropriate care. Owners should follow the manufacturer’s schedule, operating limits and sanitation instructions and keep records of service and relevant water-quality results.

RO component	Purpose	Failure/maintenance issue
Prefilter(s)	Provide upstream filtration and help protect membrane performance and service life.	Replace according to manufacturer instructions. EPA cites about six months as a common example, not a universal rule.
RO membrane	Provides the pressure-driven separation that produces permeate and concentrate.	Fouling can reduce production, efficiency and contaminant reduction. EPA gives roughly one to five years as a general replacement range depending on water quality and use; system instructions take precedence.
Postfilter/additional treatment	Provides model-specific treatment downstream of the membrane.	Media function and replacement frequency vary by product; follow the manufacturer’s schedule.
Storage tank	Stores permeate so treated water is available on demand.	Tank backpressure affects efficiency. Follow manufacturer instructions for sanitation.
Automatic shutoff device	Stops incoming and reject flow when treatment is not required or the storage tank is full.	A malfunction can cause continual discharge to the drain. Inspect or service according to product instructions.

EPA household guidance gives examples of prefilter replacement at about six months and membrane replacement roughly every one to five years, depending on source water and use. These are general examples rather than universal maintenance intervals. A heavily loaded source, different cartridge design or manufacturer requirement can produce a different schedule.

Testing and verification after installation

TDS measurements cannot confirm individual contaminant concentrations. Where arsenic, lead, nitrate, PFAS or another specific substance motivated the installation, appropriate contaminant-specific analysis provides more meaningful verification.

Changes in flow, unusually slow tank refill, unexpected taste or odor, or continuous discharge to the drain warrant investigation. Fouling, exhausted prefilters, membrane condition, pressure, seals and the automatic shutoff are among the possible causes; the correct diagnosis depends on the system.

Reverse osmosis can provide substantial contaminant reduction, but it does not solve every water problem by itself. Source-water chemistry, pretreatment needs, microbiological conditions, installation quality and long-term upkeep all shape real-world performance. The durable decision rule is straightforward: identify the water-quality problem first, select a complete system with the appropriate verified performance, maintain it as specified, and use contaminant-specific testing when the treatment objective calls for it.