UPDATED: January 22, 2023
You have a reverse osmosis drinking water system to provide your family with healthy, low-sodium drinking water. You want to make sure that it is working properly, so you test it with a TDS meter. But you find that the TDS is much higher than you expected.
High TDS from a reverse osmosis system can be caused by the membrane malfunctioning, poor water pressure, a restricted drain line, non-reverse osmosis water contamination, higher TDS going into the reverse osmosis system, or re-mineralization of the reverse osmosis water.
You may notice that your reverse osmosis TDS is high because your reverse osmosis drinking water tastes salty or less crisp than usual. Your ice cubes may look cloudy, or your coffee or tea may taste different than usual. There can be several reasons why the TDS in your reverse osmosis water is high. Let’s find out why.
First, A Quick Lesson On TDS?
Many people misunderstand just what TDS stands for. TDS stands for Total Dissolved Solids, which refers to any solid particles that may be in the water being tested.
Even though in most circumstances the majority of the dissolved solids in water is salt, the S in TDS stands for solids, and not just salt.
A reverse osmosis system uses a semi-permeable membrane to reject dissolved solids and other impurities from water.
Watch this quick video to better understand how a reverse osmosis system reduces the TDS in your water.
How can you know how high your TDS is?
By using a TDS meter, we are able to get an approximation of the number of dissolved solids in water based on the resistance of electricity through a sample of water.
A TDS meter does not actually tell us how many dissolved solids are in our water, but rather, how much electrical resistance is in a sample of water.
Since dissolved solids in water cause the water sample to have more resistance to electricity going through it, the more dissolved solids in a sample of the water will create more electrical resistance in that sample.
A high level of resistance in water equals a high level of TDS.
And that is how a TDS meter gives us a number that we can use to approximate the number of dissolved solids in water.
If you are using a TDS meter, test a sample of water that you know the approximate TDS of to make sure that you are getting an accurate reading. You can do this by getting a bottle of TDS standard solution like the one below, that has a known number of total dissolved solids in it.
Let’s check a few things to confirm that the TDS is high from your reverse osmosis system.
Before getting into the possible reasons why your reverse osmosis system is producing water with a high TDS, let’s try a few things to make sure that you are really getting high TDS in your reverse osmosis water.
Flush out your reverse osmosis tank and check your TDS again.
To make sure that the water that you are testing is fresh, turn on your reverse osmosis faucet and let it run until all of the water that has been stored in the reverse osmosis storage tank has been emptied.
Then allow 2 – 3 hours for your reverse osmosis system to refill the tank before testing the water again. Run the water from the newly filled tank for about 10 seconds to ensure that the water being tested is fresh from the storage tank, and test the TDS again.
What to check for when you find that the TDS from your reverse osmosis system is high.
TDS CREEP!
No, I am not talking about some guy that is always hanging out by a Trendy Department Store.
I’m referring to something that happens for a brief time when a reverse osmosis system starts to make filtered water and when it stops making filtered water.
TDS creep is the change in TDS that is produced by a reverse osmosis membrane as the water pressure increases or decreases on the surface of the membrane.
TDS creep is usually not a problem when the reverse osmosis system is used regularly, and more than a cup or so of water is used each time.
If you tend to use only small amounts of water from your reverse osmosis system and your reverse osmosis system has a permeated pump on it, the pump may be causing TDS to creep by starting and stopping each time any water is used.
To help stop the high TDS created by TDS creep, you can try filling a glass pitcher with reverse osmosis water and storing it in your fridge. This will make your reverse osmosis system create filtered water in larger amounts, and the system will not be turning on and off as often.
The reverse osmosis membrane may no longer be effective.
The surface of a reverse osmosis membrane has very tiny openings that allow for water to flow through but not the dissolved solids that the membrane is supposed to reject.
Over time, these tiny openings will become larger due to water pressure compressing and then decompressing the membrane, and by natural wear and tear.
As these openings become larger, more dissolved solids will be allowed to pass through the membrane and therefore the TDS of the product water will rise.
An average reverse osmosis membrane will have an effective life of up to two years depending on the manufacturer. Some reverse osmosis manufacturers recommend that you change your reverse osmosis membrane as often as every six months.
But the only way to truly know if your reverse osmosis membrane needs to be replaced is by testing your reverse osmosis water with a TDS meter and comparing it to the non-reverse osmosis water that is going into the reverse osmosis system.
Reverse osmosis membrane inserted incorrectly.
Changing your reverse osmosis membrane may seem a little confusing the first time, I have even found that people had forgotten to remove the membrane from the plastic bag that it came in.
If your membrane is not installed correctly, it simply will not be able to filter your water correctly.
In some cases, a membrane may be only held in place by one fitting that presses into one end of the membrane, if this fitting is not secure, the membrane fitting may not seal correctly and the membrane will not function correctly.
Restricted/Clogged drain line.
In order for a reverse osmosis system to deliver high-quality filtered water, it needs to be able to get rid of the unwanted impurities in the water.
The semi-permeable membrane in a reverse osmosis system does this by rejecting the dissolved solids and rinsing them away to waste.
If the reverse osmosis system can not discharge the dissolved solids out of the drain line, they will get forced through the membrane of the system resulting in high TDS coming out of your reverse osmosis system.
This is because reverse osmosis uses a semi-permeable membrane that allows water to move back and forth through it. If the water that washes away the impurities is restricted, the backpressure will result in the impurities being able to flow through the membrane along with the water.
A drain line can get kinked from being pressed into a very sharp angle, or compressed from two points causing it to kink in the center.
Sometimes, the alignment of the end of the drain line may be off due to the drain saddle that holds the drain line getting pushed or knocked out of position.
If the drain line is not centered over the hole in the drainpipe, the water flow will be restricted and therefore the wastewater containing the unwanted impurities can back up and end up in the water coming out of your reverse osmosis system.
Because the impurities that were intended to be discarded from the water have been backed up through the membrane, the TDS coming from the system will be high.
Poor water pressure.
In order for a reverse osmosis system to work correctly, it needs a certain amount of water pressure to force the desired water through the semi-permeable membrane and to discard the unwanted impurities that it rejects from the system.
A standard residential reverse osmosis drinking water system will work most efficiently when the incoming water pressure is 60 pounds per square inch (PSI), but will also function quite well anywhere from 40 – 80 PSI.
But if the water pressure drops below 40 PSI, the efficiency of a home reverse osmosis system will be greatly reduced.
Because the water pressure is low, the semi-permeable membrane will not compress which leaves the openings in it larger than optimum. When this happens the TDS from your reverse osmosis system may be high from impurities getting past the membrane.
Without a minimum water pressure of 40 PSI, a residential reverse osmosis system will not have sufficient pressure to properly force the water through the membrane of the system, and therefore there will be an excess of water going to waste.
Does your reverse osmosis system have a remineralization cartridge?
Because reverse osmosis greatly reduces the number of dissolved solids in your drinking water, many people choose to get a reverse osmosis drinking water system with a remineralization cartridge stage to add the beneficial mineral to their water that will also make the water taste better.
By remineralizing the reverse osmosis water you are adding dissolved solids back into the water which means that the TDS in the water will be high compared to a reverse osmosis system without a remineralization stage.
Just how high your TDS will be due to having a remineralization cartridge will depend on the TDS coming from the reverse osmosis membrane, what exactly is in your remineralization cartridge, and how much of the minerals your reverse osmosis water can absorb while going through the remineralization cartridge.
If you would like to test the TDS coming from your reverse osmosis without the high TDS that is coming from your mineral cartridge, you can shut off the water feeding your reverse osmosis system and remove the tubing that is going into the remineralization cartridge and test a sample from there with a TDS meter.
If your reverse osmosis system does not have a remineralization stage, one can be added to the water line going to the reverse osmosis faucet.
Hole in the bladder of a Water against Water storage tank.
A standard reverse osmosis storage tank uses pressurized air on the outside of the water bladder to force the reverse osmosis water out of the tank and to your faucet.
But some reverse osmosis systems use a tank that uses the water from your home to force the reverse osmosis water out of the storage tank bladder and to your faucet.
Since the household water will have a TDS that will be considerably higher than the reverse osmosis water, a hole in the bladder will allow the household water to mix with the reverse osmosis water and therefore the TDS coming from the faucet will be high.
This is not a common problem, but just like a standard reverse osmosis storage tank, the bladder of a “Water against Water” storage tank will eventually fail and allow the high TDS water to mix with the filtered water.
Your TDS coming out of your reverse osmosis may be higher because the TDS of the water going into your reverse osmosis is higher.
High or low amounts of rainfall can change the number of TDS that is in the water coming into your home. Even water runoff from hills or mountains from melting snow or heavy rain can significantly change the TDS of your water.
In areas where salt is used to melt snow and ice from roadways, the melting of salt-filled snow left on the side of the road can make its way to your water supply, making the TDS of your water going into your reverse osmosis high, to begin with, which means that the water coming out of your reverse osmosis system will also be high.
So just because you find that the TDS from your reverse osmosis system is high, doesn’t necessarily mean that there is a major problem with the system itself.
If your reverse osmosis membrane has been used longer than is recommended by the manufacturer, replace it and test your TDS with a TDS meter.
If you find that the TDS from your reverse osmosis system is still high, check the other most likely causes, and you should have your reverse osmosis system working correctly in no time.
But if you find that your reverse osmosis system has seen better days and it is finally time to upgrade to a new system, check out my recommended reverse osmosis systems to see the most popular models.
