Reverse Osmosis Membrane Efficiency Calculator
The Reverse Osmosis Membrane Efficiency Calculator estimates how much total dissolved solids (TDS) your reverse osmosis system rejects by comparing the water entering the membrane with the treated water leaving it. Enter two TDS readings in parts per million (ppm) to get a TDS Rejection percentage.
This measurement can help you follow changes in membrane performance over time. It is an approximation of dissolved-solids rejection, however, and does not tell you the removal rate for every contaminant in your water.
What the Reverse Osmmosis Membrane Efficiency Calculator does
The calculator compares feed water TDS with permeate water TDS. Feed water is the water supplied to the reverse osmosis membrane. Permeate is the treated water that has passed through the membrane.
The result shows the percentage reduction in measured TDS between those two points. For example, a result of 95.00% means the permeate reading is 95% lower than the feed reading. It does not mean that the membrane removes exactly 95% of every individual substance.
TDS readings generally reflect dissolved substances that contribute to a meter’s conductivity measurement. The composition of those substances can vary, so two water samples with the same TDS reading may contain different dissolved materials. A TDS meter also cannot identify every possible contaminant.
The calculator uses illustrative default readings of 500 ppm feed water TDS and 25 ppm permeate water TDS. Replace them with your own measurements to estimate the rejection percentage for your system.
How to use the Reverse Osmmosis Membrane Efficiency Calculator
Take your readings under comparable operating conditions, using the same units for both measurements. Then enter them into the two available fields:
- Measure feed water TDS. Take a reading from the water supplied to the membrane and enter the value in ppm. The calculator accepts whole-number values from 1 to 10,000.
- Measure permeate water TDS. Take a reading from the treated water leaving the membrane and enter the value in ppm. The calculator accepts whole-number values from 0 to 10,000.
- Read TDS Rejection. The calculator displays the estimated percentage to two decimal places.
For a useful comparison, collect the samples close together in time and follow your TDS meter manufacturer’s instructions. Use clean sample containers and make sure the meter is suitable for the readings you are taking. If you are tracking performance over several months, use a consistent sampling method each time.
The permeate reading should represent water leaving the membrane. Water collected after a remineralization stage, for example, may have a higher TDS reading because minerals have been added back. That reading would not isolate the membrane’s rejection performance.
How the Reverse Osmosis Membrane Efficiency Calculator formula works
The calculator uses the standard TDS rejection equation:
TDS Rejection (%) = (Feed water TDS − Permeate water TDS) ÷ Feed water TDS × 100
Subtracting the permeate reading from the feed reading gives the reduction in measured TDS. Dividing that difference by the feed reading expresses the reduction as a fraction of the starting value. Multiplying by 100 converts it to a percentage.
For the calculator’s default example, assume the following readings:
- Feed water TDS: 500 ppm
- Permeate water TDS: 25 ppm
The calculation is:
(500 − 25) ÷ 500 × 100 = 95.00%
In this example, the permeate TDS is 95% lower than the feed TDS. The numbers are illustrative; your result depends on your own readings. As another example, assumed readings of 750 ppm for feed water and 15 ppm for permeate produce a TDS rejection result of 98.00%.
The feed value must be greater than zero because it is the divisor in the formula. If the permeate reading is zero, the formula returns 100.00%. A meter reading of zero should still be understood within the meter’s resolution and limitations; it does not establish that every contaminant has been removed.
Use cases for the Reverse Osmosis Membrane Efficiency Calculator
A rejection percentage can be more informative than looking at the treated-water TDS reading alone because it also accounts for the TDS level entering the membrane. Homeowners and system operators can use it in several practical ways:
- Establish a baseline. Record feed and permeate readings when your system is operating as expected. Keep the date and relevant operating conditions with the result.
- Follow changes over time. Compare later results with your baseline to spot a sustained change that may warrant further investigation.
- Understand changing source water. Feed TDS can vary. Calculating rejection alongside both readings provides context when the treated-water TDS changes.
- Support maintenance decisions. A declining rejection percentage can prompt you to check measurements and review the system and membrane manufacturer’s maintenance guidance.
- Compare readings after service. Consistent measurements taken before and after maintenance can help you observe whether the result changed.
A single calculation is a snapshot, not a complete diagnosis. If a result is unexpected, repeat the readings and review the sampling method before drawing a conclusion about the membrane. Manufacturer specifications and the conditions under which they apply also matter when evaluating a particular system.
Other factors to consider when calculating TDS Rejection
Operating conditions: Pressure, water temperature, feed-water composition and the condition of the system can affect membrane performance. Compare readings taken under similar conditions whenever possible, and record meaningful changes alongside your results.
Sampling location: The two readings need to represent the water entering and leaving the membrane. Taking the treated-water sample after additional treatment can change the comparison. Consult your system documentation to identify suitable sampling points.
Meter accuracy: TDS meters estimate dissolved solids from electrical conductivity. Calibration, temperature compensation, sample handling and meter resolution can affect the displayed values. Small changes may be less meaningful when they are close to the meter’s measurement uncertainty.
Time and consistency: The reading can vary with how the system has been operating. Follow a consistent procedure and compare measurements under similar conditions rather than relying on a single isolated result.
Negative results: If permeate TDS is higher than feed TDS, the formula produces a negative percentage. Check the readings, units, sampling locations and meter operation. A negative result is a reason to investigate the measurements and system conditions, not a normal rejection percentage to interpret on its own.
Contaminant-specific information: TDS rejection does not establish the removal rate for an individual contaminant. If you need to know whether a specific substance is present or being reduced, use an appropriate water test and consider the applicable system documentation.
Water recovery: This calculator does not measure how much feed water becomes permeate or how much leaves as concentrate. Water recovery requires different measurements and a different calculation.
Reverse Osmosis Membrane Efficiency Calculator FAQ
What is a good TDS rejection percentage for a reverse osmosis membrane?
There is no single percentage that describes every membrane and operating condition. Compare your result with the membrane manufacturer’s specifications and your own consistently measured baseline. An unexpected or sustained decline is a useful reason to check the readings and review the system’s maintenance guidance.
Can I use the TDS reading from my drinking-water faucet?
It depends on where that faucet sits in the treatment process. If the water passes through a remineralization stage or another component that changes its TDS after the membrane, the faucet reading may not represent membrane permeate alone. Use a suitable sampling point identified in your system documentation.
Does a high TDS rejection result mean my water is safe to drink?
No. TDS rejection is an estimate based on a broad measurement of dissolved solids. It does not identify all substances in the water or verify removal of individual contaminants. Appropriate testing is needed to answer questions about specific water-quality concerns.
Why is my calculated TDS rejection negative?
A negative result means the entered permeate TDS is higher than the entered feed TDS. Recheck both readings, confirm that they use the same units, and verify where the samples were taken. Meter calibration, sample handling and treatment stages after the membrane can also affect the comparison.
Is TDS rejection the same as reverse osmosis water recovery?
No. TDS rejection compares the dissolved-solids readings of feed and permeate water. Water recovery concerns the amount of feed water that becomes permeate. The two measurements describe different aspects of system performance.