Troubleshooting High TDS in Reverse Osmosis Water

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If your reverse osmosis system is working correctly, filtered water should have a very low TDS – no more than 50 ppm in most cases, and sometimes as low as 10.

If you perform a TDS test and notice values that are significantly higher, this could indicate a problem with your reverse osmosis system.

Time for some troubleshooting!

Key Takeaways

Here are the potential reasons that could lead to high TDS in reverse osmosis water and how to troubleshoot them:

  • TDS meter not properly calibrated – recalibrate
  • TDS creep – flush RO system properly before measuring TDS
  • Remineralizing filter – remove
  • New RO membrane – allow up to a couple of hours to settle
  • Old/faulty membrane – replace
  • RO membrane leaks unfiltered water – reinstall
  • New pre or post-filters – prime
  • High feed water TDS – add pre-treatment
  • Low feed water pressure – increase
  • High feed water temperature – lower
  • Partially closed valve – open
  • Wrong flow restrictor – replace
  • Restricted drain line – clean

Troubleshooting High TDS in Reverse Osmosis Water

So, how do you troubleshoot high TDS levels in reverse osmosis water?

High TDS in reverse osmosis water can be traced to a number of potential causes. Since it’s often difficult to guess which the most likely culprit is, you should just eliminate the different possibilities one by one. Here is a handy list that should help you in this regard.

TDS Meter Not Properly Calibrated

First things first – are you sure your readings are correct to begin with? Some people don’t realize this, but TDS meters need occasional recalibration in order to function properly, even though they are typically calibrated out of the box when they’re first bought.

If it’s been a while since you last recalibrated your TDS meter – or if you’ve even never calibrated it at all – you should follow the steps outlined in the manual.

tds meter in glass of water

TDS Creep

If your reverse osmosis system hasn’t been used in a while, you may experience something known as TDS creep. When the membrane’s input side remains without active pressure for too long, its internal pressure eventually equalizes. As a result, some TDS makes its way over to the filtered water side.

The good news is that this is only temporary. You simply need to keep your faucet running for a brief time to flush out the accumulated TDS, and then take your readings as usual.

Remineralizing Filter

A remineralizing filter will naturally increase the TDS levels of your RO water – that’s one of its main purposes, after all. If you want to conduct a proper test, make sure to disconnect the remineralizing filter first, and allow the system to flush properly.

It’s also possible that the filter is delivering too many minerals for your taste – in this case, changing to a different remineralizing filter model might be enough.

You Have a New Reverse Osmosis Membrane That Needs Time to Settle

Like many types of water filters, reverse osmosis membranes don’t work immediately, and need some time to activate. And while simple filters may only need to be soaked for 10-15 seconds or so, you need to be a bit more patient with your reverse osmosis membrane.

Give a new membrane at least an hour to properly settle – you should notice a significant reduction in your TDS levels after that point.

You Have an Old or Faulty RO Membrane

Remember to maintain and replace your reverse osmosis membrane regularly! How often this should be done depends on factors like your water quality and consumption habits, but you should generally do it every 1-2 years or so.

If you leave an old RO membrane running for too long, you will eventually start to notice your water returning to its original contaminated levels. And with that, its TDS levels will go up as well.

RO Membrane Incorrectly Installed

If none of the above points bring up anything useful, you should check if the reverse osmosis membrane is installed correctly. It’s possible that an installation mistake is allowing water to bypass the membrane partially or even completely.

Sometimes these problems take time to develop. In this case, you wouldn’t notice anything out of the ordinary when first installing the membrane and conducting the standard basic tests, but the issue will become more noticeable over time.

New Pre or Post-Filters That Require Priming

Just like your reverse osmosis membrane itself, some of your pre or post-filters may also need to be primed before they’re good to go. How this is done varies from filter to filter, and sometimes it’s as simple as allowing water to run through it and discarding the first few batches.

High Feed Water TDS

Reverse osmosis only reduces the TDS level of water by a certain percentage – it doesn’t completely lower it to zero. This means that the TDS level of the filtered water depends directly on the TDS level of the input water. If the feed water has an exceptionally high TDS level, the remaining portion could still be large enough to be noticeable in the smell and taste of water.

Feed Water Pressure Too Low

Reverse osmosis requires a high level of pressure for optimal operation. If the feed water’s pressure level is too low, the system will not work correctly.

That’s why one of the most important points to address before investing in a reverse osmosis system is to ensure that your home’s plumbing can support the pressure levels required by the filter.

Feed Water Temperature Too High

Unknown to some users, feed water temperature is another factor that affects the performance of reverse osmosis systems. If water is too warm – or even hot – this causes an expansion in the reverse osmosis membrane, reducing its efficiency.

Closed Valve Reduces Water Pressure

Check if there’s a closed valve somewhere along the system. Depending on its location, even one valve can sometimes be enough to affect the pressure of your system, impairing the operation of the reverse osmosis membrane stage.

Wrong Flow Restrictor

The flow restrictor is a crucial component of a reverse osmosis system. Without it, the membrane will be unable to maintain sufficient pressure for optimal operation, and more water will go down the drain, while some dissolved solids will make their way into the filtered water.

It’s not only important to have a flow restrictor in place, but also to ensure that it’s correctly sized for your system.

under sink reverse osmosis system

Restricted Drain Line

The wastewater generated by the reverse osmosis process must go somewhere. If the drain line gets clogged, that water will start leaking over to the filtered side as more solids will be pushed through the membrane. Make sure to keep your drain line clear of any obstructions. That’s also why it’s recommended to keep the line as straight as possible, without any bends that could lead to kinks.

What Is the Ideal TDS Concentration in Reverse Osmosis Water?

If you’re using a reverse osmosis system, your filtered Ro water should have a TDS concentration of 10 – 50 ppm. More than that could be indicative of a problem with your system.

In general though, anything below 300 is considered excellent overall, as far as the drinking quality of water is concerned.

How to Test RO Water TDS

The simplest way to test the TDS levels of your reverse osmosis water is by purchasing a digital TDS meter. These devices are small and portable, and they resemble a thermometer somewhat.

Using a digital TDS meter is straightforward – you simply insert it into a water sample, stir it around a little to ensure there are no bubbles, and then wait for the device to take its measurements.

If you have any questions about high TDS in reverse osmosis water 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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