High TDS After Changing to New RO Membrane? Do This!
Written by: Gene Fitzgerald // Last Updated: Mar 27, 2023
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Have you just replaced your RO membrane but the TDS meter shows a high reading?
Don’t worry, it doesn’t necessarily mean that you need to shop for a new reverse osmosis membrane again. There are many reasons for why you can get high TDS even or rather especially with a brand-new membrane.
This article will help you pinpoint what’s causing high TDS after changing an RO membrane.
Contents
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Key Takeaways
- When installing a new membrane, allow for sufficient flushing, and don’t test for TDS in the first hour or so.
- A high TDS level may be a result of TDS creep which is what happens when the system’s not used regularly. Discard two cups of water and check TDS then.
- The membrane must be installed correctly. If it’s not seated properly, raw water can bypass it and come out of the faucet unfiltered.
- Check if a valve’s partially closed. It can cause low water pressure and higher TDS levels.
- Make sure that the flow connector’s size matches your RO membrane.
- Check the drain line and see if it’s maybe clogged or restricted, as that can cause impurities to flow back and through the membrane.
- Flush any new pre or post-filters.
- See if your TDS meter needs re-calibration, as the readings may have nothing to do with the membrane.
- Finally, inspect the new RO membrane and see if it’s maybe damaged or ruptured, as it can be faulted and require replacing.
High TDS After Changing RO Membrane – Here’s Why
From when you test for TDS to how you’ve installed your reverse osmosis membrane, here are the most common reasons why you might be getting high TDS.
New RO Membrane Needs Time to Settle
New RO membranes are like sponges. They take their time to get hydrated, so be patient. It is not uncommon for filtered water flow to start at a high TDS rate after changing the membrane. This should decrease once the membrane has settled, after an hour or so.
By the way, it is recommended not to consume this initially filtered water, as it may contain membrane preservatives.
TDS Creeping
TDS creep is an increased level of TDS in the product water, and it happens as a result of having concentrated water resting on one side of an RO membrane and water with significantly lower salt content on the other side.
This usually occurs when the system has been idle and there hasn’t been any active pressure on the feed water side of the membrane. This automatically causes the pressure within said membrane to equalize and allows for TDS from the concentrated water to diffuse through to the filtered side.
After re-pressurizing, this high level of TDS will run through the system (this shouldn’t take more than a minute), so keep this in mind the next time you’re checking for TDS.
As TDS creep is the initial high-salt water that comes out of the system, the system needs to be properly flushed. A dramatic change in the TDS level can happen after only 15 seconds of having the faucet run.
If you’re looking to test the TDS levels in your RO water, make sure to always discard the first 2 cups of purified water. Otherwise, your meter might read a high TDS level due to TDS creep and not because your RO membrane is not filtering properly.
Water Bypassing Improperly Installed RO Membrane
Another reason why you may be experiencing a high level of TDS with your new RO membrane may happen as a result of having the water bypassing the membrane and coming out unfiltered.
This usually happens as a result of improper installation, so your best bet is to open the housing and check. See if one of the O-rings has been pinched or maybe damaged. When inserting it back, make sure that the membrane is pushed all the way to the bottom.
A Closed Valve Reducing Water Pressure
When the feed water’s pressure is low, the membrane also doesn’t receive enough pressure. This allows TDS to creep across the membrane and eventually get out from its product side.
If you already have a booster pump, the culprit may be a partially closed valve. Check the valve and see whether that’s the problem.
Flow Restrictor Does Not Match New RO Membrane
The flow restrictor is in charge of maintaining high pressure inside the membrane, providing the desired rate of the reject flow, and also controlling the system’s recovery ratio. Without a decent flow restrictor, the membrane may start performing poorly, even allowing high levels of TDS.
The flow restrictor needs to correspond with your RO membrane. If its size doesn’t fit the membrane’s production size, the overall filtration will be negatively affected. If you’re using an incorrect size, determine the ideal type of flow restrictor for your membrane and replace it.
Clogged or Restricted Drain Line
To provide high-quality drinking water, the RO system has to be able to properly rinse the impurities. That means that the rejected contaminants need to be flushed down the drain.
If the drain line is clogged or restricted, these solids may end up being forced through the membrane and into your drinking water.
Bottom line: It might be a good idea to see if your drain line is obstructed or not.
Further Reading:
New Pre or Post-Filters
Have you just installed new pre or post-filters? This is often overlooked, but if the filters are not flushed properly before being connected to the RO system, it can result in increased carbon dust and may interfere with the water’s TDS level.
Make sure to always flush new filters to discard any carbon residue.
TDS Meter Requires Recalibration
Just like with any unit, the TDS meter also depends on the occasional tuning. These meters come factory-calibrated, but to be able to read with precision and consistency, recalibration from time to time is indeed required.
If that’s something you haven’t done in a while, your new RO membrane may not even be to blame for the high TDS readings. With the help of a commercial standard solution, you can easily recalibrate the meter following the manufacturer’s instructions.
Faulty or Damaged RO Membrane
Finally, after going through all the possible problems and not finding the culprit, your new RO membrane may be possibly faulted.
Although rare, it can happen for a customer to end up with a ruptured or damaged membrane that may be allowing for larger solids to get through. Share your complaints with the manufacturer if you’re dealing with factory damage. If you’ve somehow ruptured the membrane yourself, replace it with a new one to allow proper filtration.
How High Should My RO Water’s TDS Read?
When adequate conditions are met and the ideal temperature and water pressure are achieved, a new RO membrane should reduce the TDS levels by at least 95% and up to 99%. In other words, it depends on how high the TDS content of the feed water is.
- For instance, if the TDS in your feed water is 500 ppm, at a 95% reduction rate, the TDS in the purified water will be 25.
- For a TDS of 300 ppm in the feed water, the same reduction rate will result in a TDS level of 15 ppm in the purified water.
If you have any questions about a new RO membrane causing high TDS please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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