RO Membrane Rejection Rate: What It Means & How to Improve It

If your reverse osmosis system’s water still tastes salty, or your TDS meter reads higher than expected, the problem usually traces back to one number: the RO membrane rejection rate. This single figure tells you how effectively your membrane is separating dissolved contaminants from clean water.

In this guide, you’ll learn exactly what rejection rate means, how to calculate it, what causes it to drop, and what you can actually do to improve or restore it. Whether you run a residential under-sink RO system or manage a commercial treatment plant, the same core principles apply — only the scale changes.

What Is RO Membrane Rejection Rate?

RO membrane rejection rate is the percentage of dissolved solids, salts, and other contaminants that a membrane blocks from passing through into the permeate (treated) water. A higher percentage means more contaminants stay behind in the reject (concentrate) stream instead of ending up in your drinking water.

Rejection rate is not the same as flow rate or production capacity. A membrane can produce plenty of water while still doing a poor job of rejecting salts, and vice versa. If you’re still getting familiar with how the membrane physically works, our beginner’s guide to RO membranes is a good starting point before diving into performance metrics.

Most residential thin-film composite membranes are rated between 95% and 99% rejection under standard test conditions. Commercial and industrial membranes can fall in a similar range, though the exact figure always depends on the specific membrane model, feed water, and operating pressure.

The Rejection Rate Formula

Direct answer: Rejection rate is calculated by comparing the TDS (or conductivity) of the feed water to the TDS of the permeate water, expressed as a percentage.

The standard formula is:

Rejection Rate (%) = [(Feed TDS − Permeate TDS) / Feed TDS] × 100

Where:

Feed TDS = total dissolved solids in the water entering the membrane
Permeate TDS = total dissolved solids in the treated water leaving the membrane

This formula works whether you’re measuring in parts per million (ppm), milligrams per liter (mg/L), or using conductivity readings in microsiemens (µS/cm), as long as you use the same unit on both sides of the equation.

How to Calculate RO Membrane Rejection Percentage

Calculating rejection rate is straightforward once you have two TDS readings. Here’s the process step by step:

Measure the feed water TDS using a TDS meter before the water enters the membrane.
Measure the permeate TDS at the membrane’s clean water outlet.
Subtract permeate TDS from feed TDS.
Divide that result by the feed TDS.
Multiply by 100 to get the rejection percentage.

Worked Example

Suppose your feed water reads 300 ppm TDS, and your permeate reads 9 ppm TDS.

Rejection Rate = [(300 − 9) / 300] × 100 = 97%

A 97% rejection rate means the membrane blocked 97% of the dissolved solids from passing through, letting only about 3% into the permeate stream.

Quick Reference Table

Feed TDS (ppm)	Permeate TDS (ppm)	Rejection Rate
200	4	98%
300	9	97%
400	20	95%
500	50	90%
600	90	85%

Notice how a rising permeate TDS relative to feed TDS quickly drags the rejection percentage down, even when the raw numbers still look “low” in isolation.

What Happens During Contaminant Rejection

Inside the membrane, water molecules pass through the semi-permeable barrier while larger dissolved ions, salts, and organic molecules are physically and electrochemically blocked. This happens because the membrane’s active layer has extremely small pores, combined with a charge that repels many dissolved ions.

Rejection isn’t an all-or-nothing gate. A small fraction of dissolved solids always passes through, which is why even a “perfect” membrane never reaches 100% rejection in practice. Manufacturers publish rejection specifications based on standardized test conditions, not guarantees for every real-world scenario.

For a deeper look at the mechanics of separation, our step-by-step explanation of how an RO membrane works breaks down the filtration process layer by layer.

Factors That Affect Rejection Performance

Rejection rate isn’t fixed. It shifts with feed water conditions, system settings, and the physical state of the membrane. Understanding these variables helps you interpret readings correctly instead of assuming every fluctuation means a failing membrane.

Feed-Water TDS

Higher feed TDS increases the concentration gradient across the membrane, which can slightly reduce the percentage of rejection even when the membrane itself is healthy. This is a normal physical effect, not necessarily a sign of membrane damage.

Very high-TDS feed water — like brackish well water or seawater — will naturally produce a lower rejection percentage than treating typical municipal tap water, simply because there’s more dissolved material trying to pass through.

Feed Pressure

RO systems rely on pressure to push water through the membrane against natural osmotic pressure. Low feed pressure reduces both flow rate and rejection efficiency, since the driving force needed to separate water from dissolved solids weakens.

Conversely, operating within the membrane’s designed pressure range typically improves rejection, up to a point. Excessive pressure beyond manufacturer specifications can damage the membrane surface rather than improve performance.

Temperature

Warmer feed water reduces viscosity and can slightly increase permeate flow, but it often causes a small drop in rejection rate because dissolved ions move more freely at higher temperatures. Most membrane specifications are rated at 25°C (77°F) for this reason.

Cold water, on the other hand, tends to reduce flow and can make rejection appear artificially stable while overall production drops. Always compare rejection readings taken at similar temperatures.

Membrane Age and Condition

Membranes degrade gradually through normal use. Chemical exposure, mechanical wear, and biological fouling all reduce the integrity of the membrane’s active layer over time, which shows up as declining rejection rate.

A membrane that rejected 98% when new might settle into the mid-90s after a year or more of service, depending on water quality and maintenance. This is expected aging, but a sharp or sudden drop usually points to a specific problem rather than normal wear.

Membrane Type and Quality

Different membrane technologies and grades are engineered for different rejection targets. If you’re comparing membrane options for a new system or replacement, our guide to RO membrane types explains how construction and material differences affect performance, including rejection capability.

Higher-quality membranes generally maintain more consistent rejection over a longer service life, but even premium membranes vary in real-world performance depending on water chemistry and system design.

Concentration Polarization

Concentration polarization occurs when dissolved solids build up in a thin layer right at the membrane surface, faster than the flow can sweep them away. This local buildup increases the effective concentration the membrane “sees,” which can lower rejection and increase fouling risk.

Proper cross-flow velocity, correct system design, and adequate flush cycles all help minimize this effect. It’s a normal part of membrane operation, but poor system design can make it worse than it needs to be.

Recovery Rate

Recovery rate — the percentage of feed water converted into permeate — also interacts with rejection. Running a system at a higher recovery rate than it was designed for concentrates dissolved solids more aggressively on the reject side, which can pull down rejection performance and accelerate fouling.

How Permeate TDS Relates to Rejection Rate

Permeate TDS and rejection rate are two sides of the same measurement, but they tell you slightly different things. Permeate TDS tells you the absolute quality of your treated water. Rejection rate tells you how well the membrane is performing relative to what it’s being asked to filter.

This distinction matters. A membrane treating low-TDS municipal water might show a lower rejection percentage than one treating high-TDS well water, even though the first system produces cleaner permeate overall. Always look at both numbers together, not one in isolation.

If your GPD-rated system is producing water that doesn’t match expected output alongside a shifting rejection number, it’s worth reviewing your membrane’s rated capacity. Our guide to understanding RO membrane GPD ratings explains how production capacity and performance connect.

How to Test Rejection Rate in Practice

Testing rejection rate doesn’t require lab equipment. A basic handheld TDS meter is sufficient for most residential and light commercial applications.

Steps for accurate testing:

Let the system run for several minutes to stabilize flow before testing.
Take the feed water sample directly before the membrane inlet, not from the tap.
Take the permeate sample directly from the membrane’s clean water line.
Test both samples with the same meter, back to back, to avoid calibration drift between readings.
Record the temperature at the time of testing, since it affects the result.
Apply the rejection rate formula to calculate the percentage.

For commercial systems, testing periodically and logging results over time is more useful than a single reading. A gradual downward trend tells you far more than any single data point.

What a Good Rejection Rate Looks Like

Direct answer: Most healthy residential RO membranes maintain 95–99% rejection, while commercial systems often target 95–99.5% depending on membrane grade and application.

These figures are general guidance, not universal guarantees. The right benchmark for your system depends on:

The specific membrane model and its manufacturer specification
Feed water chemistry and starting TDS
System pressure and recovery settings
How long the membrane has been in service

A membrane performing at 96% rejection isn’t automatically a problem — it may simply be operating normally for its age, water source, and design. Context always matters more than chasing a single “ideal” number.

Causes of Declining Rejection Rate

When rejection rate drops noticeably from its baseline, it’s usually one of these underlying issues:

Membrane fouling from scale, organic matter, or biological growth coating the membrane surface
Chemical damage from chlorine or oxidizer exposure degrading the active layer
Physical damage such as a torn or compromised membrane, often from pressure spikes or manufacturing defects
O-ring or seal failure, letting unfiltered feed water bypass the membrane entirely
Scaling from hard water minerals building up faster than the system flushes them away
Normal aging, which produces a slow, gradual decline rather than a sharp drop

A sudden, sharp drop in rejection usually points to physical damage or a seal bypass rather than gradual fouling. Distinguishing between these causes helps you choose the right fix instead of guessing.

How to Improve or Restore Rejection Performance

Once you’ve identified a decline, the right response depends on the cause. Not every drop in rejection means the membrane needs replacing.

When Cleaning May Help

Cleaning is the appropriate first step when fouling or scaling is the likely cause, especially if the decline has been gradual and flow rate has also dropped.

Use a cleaning solution matched to the fouling type (acidic cleaners for mineral scale, alkaline cleaners for organic fouling)
Follow manufacturer-recommended soak times and concentrations
Flush thoroughly after cleaning to remove residual cleaning chemicals
Retest rejection rate after cleaning to confirm improvement

Cleaning often restores a meaningful portion of lost performance, but it rarely brings a membrane back to its original factory rejection rate. Some permanent loss is normal after cleaning cycles, particularly on membranes that have gone through multiple cleanings over their service life.

When Membrane Replacement Is Necessary

Replacement becomes the right call when:

Rejection rate stays low even after a proper cleaning cycle
The drop was sudden, suggesting physical or chemical damage
The membrane has reached or exceeded its typical service life
Permeate TDS no longer meets your water quality requirements

If you’re unsure which membrane size or type fits your existing housing, our membrane size guide covering 1812, 2012, 2812, 3012, and 3013 formats can help you match a replacement correctly. For a broader comparison of production capacities during replacement, the comparison of 50, 75, and 100 GPD membranes is useful if you’re also reconsidering system capacity at the same time.

Preventive Measures

Preventing rejection decline is generally more cost-effective than restoring it after the fact:

Maintain proper pre-filtration to reduce sediment and chlorine reaching the membrane
Avoid running the system at pressures or recovery rates outside manufacturer specifications
Schedule periodic sanitization for commercial systems handling higher fouling loads
Monitor TDS regularly instead of waiting for noticeable taste or quality changes

Common Mistakes When Interpreting Rejection Rate

Even experienced users misread rejection numbers occasionally. Watch out for these common errors:

Comparing rejection rates across different feed water sources. A membrane treating harder water will naturally show different numbers than one treating soft municipal water.
Ignoring temperature differences between readings. A rejection rate measured in winter and summer can differ without any real membrane change.
Assuming flow rate and rejection rate move together. They’re independent measurements; a membrane can maintain flow while rejection quietly declines.
Chasing 100% rejection. No RO membrane rejects contaminants completely under real operating conditions, and treating anything below 99% as a failure sets unrealistic expectations.
Skipping baseline testing. Without a starting rejection percentage recorded when the membrane was new, it’s hard to judge how much decline has actually occurred.

Residential vs. Commercial Considerations

Residential systems typically prioritize convenience, and a slow decline in rejection rate is usually addressed through routine membrane replacement on a set schedule, often every 2–3 years depending on water quality.

Commercial and industrial systems generally require more active monitoring, since production volume is higher and water quality specifications are often tied to regulatory or process requirements. Regular TDS logging, scheduled cleaning-in-place (CIP) cycles, and documented rejection trends are standard practice in these settings.

Regardless of scale, reverse osmosis remains one of the most effective barriers available for reducing dissolved salts and ionic contaminants from water, which is why maintaining rejection performance matters for both drinking water quality and equipment longevity downstream.

If you’re still deciding on membrane specifications for a new installation or upgrade, our complete RO membrane buying guide covers types, sizing, and replacement planning in one place.

Conclusion

RO membrane rejection rate tells you how effectively your system separates dissolved contaminants from clean water, and it’s calculated by comparing feed TDS against permeate TDS. Healthy membranes typically sit between 95% and 99% rejection, but the right benchmark always depends on your specific feed water, pressure, temperature, and membrane type.

Watch for gradual decline as a sign of normal aging or fouling, and treat a sudden drop as a signal to check for physical or seal damage. Cleaning can restore some lost performance when fouling is the cause, but a membrane that stays weak after cleaning, or one that’s reached the end of its service life, is best replaced.

Testing regularly, keeping a baseline reading, and understanding the variables that influence rejection will help you catch problems early and keep your RO system delivering the water quality you expect.