RO TDS Removal Efficiency Guide

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Reverse osmosis systems are great at reducing TDS (total dissolved solids) levels in water.

Sometimes all the terminology and ratings can get a bit confusing though.

If you’re wondering how to interpret all of that, this article is for you.

So, here is our guide on RO TDS removal efficiency.

Key Takeaways

  • Measuring the TDS removal efficiency of a reverse osmosis system is important in monitoring its performance.
  • RO TDS Removal Efficiency can be affected by a number of factors, including the TDS of the input water, the pressure or temperature of the input water, as well as the state of the reverse osmosis membrane.

RO TDS Removal Efficiency

What is RO TDS removal efficiency?

First of all, you may have also heard of reverse osmosis TDS removal efficiency as “TDS rejection rate” – don’t get confused, the two terms are essentially the same.

In any case, reverse osmosis systems are excellent in this regard. They can easily reduce the TDS levels of input water by as much as 95-99%, leaving you with almost completely pure, uncontaminated water.

By the way, the reverse osmosis membrane does by far the bulk of TDS removal. In fact, some RO membranes are rate by their salt rejection rate.

Factors Affecting RO TDS Removal Efficiency (+ How to Optimize)

There are several factors that may affect the TDS removal efficiency of your reverse osmosis system. Some of those can be addressed directly, while others are trickier.

Input Water TDS

The TDS level of water filtered by reverse osmosis is directly proportional to that of the input water. This means that if your input water has particularly high TDS levels, your filtered water will be higher in TDS as well.

The only way to address this is through additional treatment. Adding pre or post-filtration or an extra reverse osmosis stage are commonly used approaches.

Input Water Pressure

You need to maintain sufficient pressure on the input stream to ensure that your reverse osmosis system will function correctly. Generally, this should be at least 40 psi, ideally 60.

A drop in the input pressure will result in decreased performance of the reverse osmosis system.

Input Water Temperature

Temperature is another important factor. If your input water is too warm, this can affect the operation of your reverse osmosis membrane, as it will cause it to expand and allow more dissolved solids through.


Perhaps it’s time to replace your membrane? If it’s been a while since you last did that, it’s a very likely culprit. You should generally aim to replace your reverse osmosis membrane every 2-5 years or so. This might need to be adjusted according to the condition of your water.

It’s also possible that you’ve just installed a new membrane and it still isn’t working optimally. It can take a while for a reverse osmosis membrane to get properly hydrated and start filtering water at its full potential.

There are also different types of membranes used in reverse osmosis. Most common for home use are CTA and TFC membranes. Generally speaking, TFC membranes have higher TDS removal rates than CTA membranes.

blue reverse osmosis membrane

TDS Creep

You should ideally use your reverse osmosis system on a regular basis. If you leave it unused for too long, this can cause the pressure inside the membrane to equalize, allowing some solids to pass through. This phenomenon is called TDS creep. Good news is, it’s only temporary.

Drain Line or Flow Restrictor Problems

If none of the above help, you should also check your drain line or flow restrictor. Problems with those components can cause the whole reverse osmosis system to stop functioning properly, reducing its performance.

How to Calculate Reverse Osmosis Salt Rejection Rate

To calculate the salt rejection rate of your reverse osmosis system, all you need to do is take two samples of water – one from your unfiltered source water, and one from the filtered water. Then, measure the TDS levels of each sample, and use the below formula to calculate your salt rejection rate:

Rejection rate in % = (TDS of unfiltered water – TDS of filtered water) / TDS of unfiltered water X 100

How to Measure TDS in Water

Wondering how to measure the TDS levels of your water in the first place?

There’s a simple device for that! TDS meters resemble thermometers and they’re similarly easy to use. You just need to take a sample of your water, swirl it around a bit to remove any bubbles, then submerge the TDS meter. Make sure to avoid submerging it too deeply, as this can cause the device to malfunction. After a while, the screen will show you the TDS level of the water.

If you have any questions about RO TDS removal efficiency please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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