What Should My TDS Reading Be After the RO Membrane?
Written by: Gene Fitzgerald // Last Updated: Mar 21, 2023
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A reverse osmosis system uses a semi-permeable membrane to remove water impurities, including total dissolved solids (TDS).
Naturally, your TDS reading should reduce after the water goes through an RO membrane. But to what extent?
In this article, we’ll explain what your TDS reading should be after your RO membrane and highlight some factors that could affect your TDS reading.
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Key Takeaways
- Ideally, your TDS reading after the RO membrane should be at least 95% less than your feed water TDS. So, the reading to expect primarily depends on your feed water TDS.
- Other factors can affect your TDS reading, like feed water pressure and temperature, age, condition of membrane and pre-filters, TDS creep, and CO2 content in the water.
What Should My TDS Reading Be After the RO Membrane?
An effective reverse osmosis system/membrane will reduce your water’s TDS level by at least 95%. That means there’s no “standard” reading for water TDS after an RO membrane. The reading to expect largely depends on the amount of TDS in your feed water.
For example, if you have a TDS reading of 500 before the RO membrane, your TDS reading after filtration will be around 25 (95% less than 500).
And there are other factors that affect output water TDS, too. We’ll see them below.
Factors Affecting TDS of RO Water
These factors influence TDS reading after an RO membrane.
Feed Water TDS
Your feed water TDS is the most crucial factor that affects your final TDS reading. The higher your feed water TDS, the higher the TDS of your RO water.
Feed Water Pressure
Reverse osmosis works by forcing water under high pressure through tiny membrane pores. At a pressure between 40-80 psi, the RO membrane will block and remove most dissolved solids from water.
But when the water pressure is low (less than 40 psi), the membrane can only remove fewer contaminants. Most dissolved solids will escape through the membrane, and the TDS of the permeate water will be higher.
Further Reading:
Feed Water Temperature
A high water temperature will expand RO membrane pores, allowing more dissolved solids to pass through. So if warm water runs through your RO membrane, you can expect a higher output TDS reading.
Membrane Age and Condition
New RO membranes are naturally more effective and will remove more TDS. Not so with an old membrane. You’ll get a higher TDS reading. Only thing you can do is replace.
Condition/Age of Pre-Filters
Pre-filters also become less effective with time. The older your RO system pre-filters, the more your chances of getting a high TDS reading.
TDS Creep
If you don’t use your RO system for some time, the pressure in the membrane will equalize allowing dissolved solid levels to find balance.
When you eventually open your reverse osmosis faucet, the first cup or so of water that flows out of the membrane will contain a high TDS level (called TDS creep).
To prevent scaringly high readings caused by TDS creep, discard the first two cups of water if you’ve not used the reverse osmosis filter system in a while. If you use an RO tank, open the faucet and drain out all the water. After draining the tank, check the TDS of the freshly filtered RO water to get the latest reading.
Brand New RO Membrane
Brand new RO membranes need some time to hydrate before they can block impurities properly. So, the first batch of water a brand-new membrane purifies usually has high TDS content. You should dispose of the water it produces within the first couple of hours.
Brand New Pre-Filters
When you install pre-filters and the membrane simultaneously, you must flush certain pre-filters before connecting them to the RO membrane. Otherwise, carbon dust from the filters may transfer to the membrane, increasing the TDS content.
CO2 Content in the Water
RO membranes cannot remove CO2 from water. So if your feed water contains a lot of CO2 and you test it immediately after filtration, you’ll have high readings. But if you wait a few hours after filtration, the CO2 will gas off naturally, giving you a more accurate TDS reading.
TDS Meter Calibration
Your TDS meter calibration will also affect its reading. You’ll get the most accurate readings when you calibrate your TDS meter at a value close to your water’s temperature and conductivity.
For drinking water at home, that’s usually 25°C (77°F), and a solution range of 90 ppm to 700 ppm.
Remineralization Filter
After reverse osmosis water filtration, a remineralization filter may add minerals (mostly dissolved solids) into your water. So if you use one and you’re testing the final water from your RO faucet, you’ll definitely have a high TDS.
You can check your actual TDS level by testing water in the pipe after the RO membrane but before the remineralizer.
Issues With Flow Restrictor or Drain Line
An RO unit’s drain line or flow restrictor discharges dissolved solids and wastewater, while filtered water enters your faucet.
If this drain line gets clogged (because of overtime use, for example), dissolved solids will find their way through the RO membrane and into your faucet, so you’ll have a high TDS reading. To prevent this, replace the flow restrictor often, especially if you have hard water.
Also, a wrong-sized flow restrictor will do a poor job and might even damage your RO membrane (automatically, you’ll have more TDS in your water). So, always use a suitable flow restrictor in your RO unit. For example, a 300 ml/min restrictor is acceptable for a unit that filters 50 gallons daily.
Installation Issues
If your RO system is installed incorrectly, it will fail to work as intended, possibly resulting in a high TDS reading.
For example, if the O-rings or RO membrane do not fit right or are faulty, the RO unit will not work effectively (and this gives room for unfiltered water to escape through the membrane and into your drinking water).
What Is TDS in Water and How to Measure It Correctly?
TDS stands for “total dissolved solids”, which refers to the number of dissolved particles in your water. These dissolved particles include minerals, metals, salts, and organic matter. Because water is soluble, these particles can easily dissolve entirely. You won’t see them, but they are right there in your water unless you filter them out.
Most people measure TDS before and after RO filtration to confirm that their RO unit is working effectively. The best way to measure TDS in water is by using a TDS meter. TDS meters are easy to use: dip the meter into your water sample and check the readings.
Tip: Always run the RO unit for 10-15 minutes before testing with a TDS meter to prevent TDS creep from affecting your readings. If you use a tank RO system, flush it out entirely before checking the TDS.
Help, My TDS Is Higher Than Normal!
So you’ve checked your TDS with a meter, but the reading is consistently high? No worries, it could be because of any of the factors we mentioned above. Once you confirm which apply to you, you can take steps to correct your TDS.
How to Calculate TDS Rejection Rates
TDS rejection rate measures how much dissolved solids your RO unit is removing. If it stays consistent, it’s a good sign that your reverse osmosis membrane is still effective. But if the value has reduced significantly, the RO membrane is no longer effective.
Now, here’s how you calculate TDS rejection rates:
- Measure the TDS of unfiltered water.
- Measure the TDS after the water goes through the RO membrane.
- Use this formula to get the rejection rate: Rejection rate in % = (TDS of untreated water – TDS of purified RO water) / TDS of untreated water × 100
When to Change a Reverse Osmosis Membrane
You should change your RO membrane if the rejection rate goes below 80%. Other signs that you may need to replace your RO membrane are:
- The treated water smells and tastes terrible.
- The RO unit produces less water.
- The system drains water constantly.
If you don’t notice any of these signs, but the manufacturer’s instructions say that your RO membrane is due for replacement, you should still change it.
Water Quality Recommendations
Normal TDS levels in drinking water range between 50-1000 ppm, depending on the type of water.
- For unfiltered drinking water, TDS is usually between 100-500 ppm.
- For RO-filtered water, TDS should be 25 ppm or less (remember, depending on the TDS of feed water).
- For bottled mineral water, TDS is often between 400-650 ppm.
- According to the WHO, a TDS level in drinking water above 1200 is unacceptable.
If you have any thoughts about the question, what should TDS be after the RO membrane, please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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