You probably know that a reverse osmosis system should reduce your water’s total dissolved solids, or TDS. But what’s the TDS level you should be aiming for after installing an RO system?
We’ve answered this question in the below guide.
📌 Key Takeaways:
- A good TDS level from RO water filtration systems is 10 to 50 parts per million (PPM).
- While the RO purification process can remove most dissolved ions, it’s not usually possible or desirable to achieve a TDS of zero from this method of water treatment.
- You can reduce the TDS level of your RO drinking water by filtering the water twice, replacing filters (if necessary) and using follow-up filtration.
Table of Contents
🤔 What Is TDS?
Total dissolved solids (TDS for short) refers to the combined concentration of all organic and inorganic substances, including minerals, salts, metals, chemicals, and other dissolved particles, that are present in a water source.
TDS is usually measured in parts per million (PPM) or milligrams per liter (mg/L) of water. RO systems remove total dissolved solids, which is why a TDS test is often used to determine the effectiveness of the RO purification process.
📉 What TDS Level Should RO Water Be?
The TDS level of RO water should be in the range of 10 to 50 parts per million (PPM). This is the acceptable level – it tells you that each stage of the RO filtration process is working perfectly to reduce TDS in drinking water.
The desirable level of TDS in water from an RO system is 20 to 30 PPM. You should be able to achieve this TDS range with a high-quality RO filtration system. But if your water’s TDS levels are slightly higher, don’t assume that your RO system must be broken. It’s normal for TDS to creep up a bit over time, as your RO filters and semipermeable membrane become worn with regular use.
📏 How Can You Measure TDS From A Reverse Osmosis?
The best way to measure total dissolved solids from an RO unit is to use a digital TDS meter.
A TDS meter is a handheld device that provides a quick and easy way to determine the concentration of dissolved solids in water.
To use a TDS meter to measure the dissolved solids in your drinking water, follow these steps:
- Gather Your Materials. You’ll need a TDS meter, a glass of RO water that you want to test, and access to the RO unit.
- Calibrate the Meter (If Required). Some TDS meters need to be calibrated with a reference solution before use. Check the user manual for calibration instructions and follow these to calibrate the meter.
- Prepare the RO Water. Collect a sample of RO water in a clean container.
- Dip the TDS Meter. Switch the TDS meter on and wait for it to stabilize (it should show a reading of zero or close to it). Submerge the TDS meter’s probe into the water sample, making sure that it’s fully immersed.
- Record the Reading. The TDS meter will display a number on the screen, which represents the TDS level of the water in parts per million (PPM) or milligrams per liter (mg/L). Make a note of the reading and compare it to your expectations or desired range.
It’s a good idea to regularly test the TDS of your RO water supply to monitor the system’s performance and check that the filters and membrane are working correctly.
🔎 Should RO Water Have Zero TDS?
No, it’s highly uncommon for RO water to have zero TDS. Although reverse osmosis systems offer one of the most thorough filtration processes available, achieving a TDS reading of zero is not typical or necessarily desirable for drinking water.
Most RO systems can greatly reduce the TDS level in water, but your goal shouldn’t be to hit zero. For one, you’ll end up disappointed, because no RO filter we’ve ever reviewed can do this. For another, some TDS is actually beneficial, so you don’t necessarily want to remove everything.
Keep in mind that not all dissolved particles in water are harmful and need to be removed. For instance, calcium and magnesium, two minerals commonly found in water, are beneficial to human health.
So, if your RO filtered water has slightly higher TDS levels because of the presence of these minerals (e.g. if they’ve been added in with a remineralization filter), that’s not a bad thing.
📋 Factors Affecting The TDS Level Of RO Water
There are several factors that may affect the TDS level of your filtered reverse osmosis water:
TDS In Feed Water
Your incoming tap water quality and its TDS levels will affect the TDS reading of your filtered water.
If the TDS levels are higher in the feed water, they’ll also be higher in the RO water. This is especially likely if your water contains dissolved solids that are difficult to remove with reverse osmosis, like dissolved gases (e.g. hydrogen sulfide and many volatile organic compounds).
Quality Of RO Membrane
The quality of the reverse osmosis membrane is a factor that will determine how thoroughly your water is filtered, and therefore the TDS level of the RO water.
Different reverse osmosis membranes differ in quality and pore size. The higher-quality the membrane and the smaller the pores, the more contaminants the membrane can block. That means purer water with a lower TDS will make it to the other side of the system.
Membrane Age & Condition
The age and condition of the RO membrane will also determine the TDS level of the purified water.
The older a reverse osmosis membrane, the more worn and damaged the membrane surface. This is normal, and the reason why you have to replace the membrane periodically (usually once every 2 years).
You may notice that the TDS level of your RO water is slightly higher towards the end of the RO membrane’s lifespan because it’s less capable of rejecting contaminants than it was when newly installed.
The presence of a remineralization filter – or lack of – is another factor that affects the TDS levels of RO filtered water.
Remineralization filters are used in many modern reverse osmosis systems to reintroduce healthy minerals that we removed during the RO filtration process. The purpose of adding these minerals back into water is to improve its taste and prevent quality issues associated with water that’s mildly acidic.
These essential minerals are classed as dissolved solids, so they’ll increase the TDS of the reverse osmosis water. And that’s not a problem – not all TDS are bad!
📖 How To Reduce RO Water’s TDS Level
Not happy with the TDS level of your RO drinking water? There may be a few things you can do to reduce your water’s TDS further.
Replace Your Filters
If you’ve noticed your water’s TDS levels creeping up over time, you’re probably due a filter change.
The average lifespan of the filters in an RO unit are:
- Sediment filter: 12 months
- Carbon filter: 6-9 months
- RO membrane: 12-24 months
- Post-filter: 12 months
Even if your filters haven’t reached the end of their lifespans, if your RO water’s TDS is increasing quickly, one of the filters may have become degraded and will need to be replaced sooner than anticipated.
Certain dissolved solids present in tap water may damage the RO membrane and may not fully be removed. These include iron and water hardness minerals.
If you want to preserve the quality of your RO filters, install a pre-treatment system that will remove these contaminants from your water upstream of the RO unit. A water softener is the best solution for removing hardness minerals. For well water containing iron, use an iron filtration system.
Filter The Water Twice
If you have a countertop RO filter system, you can filter your drinking water twice to further reduce its TDS value.
Dispense a batch of filtered tap water into a large jug or clean container. Continue until the system has processed all the water, then refill the tank with the purified water and wait for it to be filtered again. If any contaminants were missed the first time, they should be caught this time around.
Adjust Your Water Pressure
Check that your incoming water pressure is optimal for your RO unit. Most POU systems require a water pressure of 60 PSI. If your water pressure is too high or too low, it could affect the system’s ability to reduce TDS in water.
You can fix low water pressure by installing a pressure booster pump upstream of the system.
Use Follow-Up Filtration
If there are specific total dissolved solids that your RO system can’t remove, try using a follow-up filtration process that can remove these contaminants.
For instance, RO systems struggle to remove chlorine byproducts (produced as a result of chlorination at water treatment plants) and some VOCs, but some of the best water filter pitchers can remove many of these contaminants.
So, you could filter your water with RO first, then put it through a water pitcher filter to further remove additional impurities.
📑 Final Word
Thanks for visiting WaterFilterGuru to learn about the desirable TDS level of RO water. We hope you were able to find what you were looking for.
While we’ve offered advice in this article on how to get your RO water’s TDS level as low as possible, we don’t think you should get too caught up on TDS.
The reality is that TDS doesn’t matter as much as some manufacturers want you to assume. There’s a big difference between dissolved solids (which could include anything from minerals and salts to carbon dioxide and other harmless gases) and contaminants (harmful substances that you don’t want in your water).
That’s why most water filters don’t significantly reduce TDS – they retain the healthy dissolved solids, but they still improve water quality by filtering out the bad stuff. Make sure to keep that in mind when taking TDS readings of your RO water.