RO can significantly reduce TDS. UV does not remove TDS, and conventional UF does not significantly reduce TDS either.
| Technology | Removes TDS? | Main Purpose |
|---|---|---|
| RO | Yes | Reduces many dissolved salts and other dissolved substances |
| UV | No | Inactivates microorganisms |
| UF | Generally no | Removes suspended particles and larger microorganisms |
What Is TDS?
TDS stands for Total Dissolved Solids. It refers to the combined concentration of dissolved substances in water.
These can include substances such as:
- Calcium
- Magnesium
- Sodium
- Potassium
- Chlorides
- Sulfates
- Bicarbonates
- Other dissolved substances
TDS is commonly expressed in ppm or mg/L.
Importantly, TDS is not a complete measure of water safety. A TDS meter does not tell you exactly which substances are responsible for the reading.
Does RO Remove TDS?
Yes.
Reverse osmosis (RO) is the household water-purification technology among these three that is specifically capable of substantially reducing many dissolved substances.
An RO system uses pressure to force water through a semi-permeable membrane. The membrane allows water molecules to pass while rejecting a large proportion of many dissolved substances.
As a result, RO-treated water generally has a much lower TDS than the incoming water.
How Much TDS Can RO Remove?
There is no single percentage that applies to every RO purifier.
Actual TDS reduction depends on factors such as:
- Input-water TDS
- RO membrane condition
- Water pressure
- Water temperature
- Feed-water chemistry
- Membrane specifications
- Purifier design
A properly functioning RO membrane can provide substantial reduction of dissolved solids, but the actual performance should be compared with the manufacturer’s specifications and measured under consistent conditions.
Example of RO TDS Reduction
Suppose the incoming water has a TDS of 800 ppm and the RO output measures 80 ppm.
The approximate reduction is:
[(800 − 80) ÷ 800] × 100 = 90%
Therefore, the measured TDS reduction is approximately 90% in this example.
This does not mean every RO purifier will produce the same result.
Does UV Remove TDS?
No.
Ultraviolet (UV) treatment does not remove dissolved salts or minerals from water.
Instead, UV light is used to inactivate microorganisms when the system provides an adequate UV dose.
UV therefore does not significantly change the TDS reading.
For example, if water entering a UV purifier has a TDS of 400 ppm, the treated water may still have approximately the same TDS.
The exact reading can vary slightly because of measurement conditions, but UV itself is not a TDS-removal process.
What Does UV Remove?
UV treatment is primarily intended for microbial disinfection.
Depending on the system and operating conditions, UV can inactivate microorganisms such as:
- Bacteria
- Viruses
- Some parasites
UV does not remove:
- Dissolved salts
- TDS
- Hardness
- Fluoride
- Arsenic
- Lead
- Nitrate
Therefore, UV and RO solve different water-treatment problems.
Does UF Remove TDS?
Generally, no.
Ultrafiltration (UF) uses a membrane with very small pores to physically retain suspended particles and certain microorganisms.
However, dissolved salts and other small dissolved substances generally pass through a conventional UF membrane.
Consequently, UF does not normally produce a significant reduction in TDS.
What Does UF Remove?
Depending on the membrane and system design, UF can help remove or retain:
- Suspended particles
- Fine particulate matter
- Some bacteria
- Some protozoa and cysts
UF does not normally remove dissolved salts, so it should not be selected simply because you want to reduce TDS.
RO vs UV vs UF for TDS Removal
| Feature | RO | UV | UF |
|---|---|---|---|
| Reduces TDS | Yes | No | Generally no |
| Reduces dissolved salts | Yes | No | No |
| Reduces hardness | Yes, substantially in appropriate systems | No | No |
| Inactivates microorganisms | Can remove many microorganisms | Yes | No; physically retains some microorganisms |
| Removes suspended particles | Often with pre-filters | Not its primary function | Yes |
| Uses electricity | Usually | Yes | Not necessarily |
| Produces RO reject water | Yes | No | No |
Why Does RO Remove TDS but UV Does Not?
The fundamental difference is how the technologies treat water.
RO is a separation process. Its membrane can reject a large proportion of dissolved ions and other dissolved substances.
UV is a disinfection process. UV light damages microorganisms so that they cannot reproduce or cause infection, but the dissolved salts remain in the water.
Therefore, RO changes the dissolved-solids concentration while UV generally does not.
Why Does UF Not Remove TDS?
UF membranes have pores designed primarily for retaining particles and larger microorganisms.
Dissolved ions and salts are much smaller than the particles that conventional UF is designed to retain.
They therefore pass through the membrane, meaning that UF normally does not substantially reduce TDS.
Can RO Remove All TDS?
No.
RO does not normally remove 100% of every dissolved substance.
Its rejection performance depends on the membrane, water chemistry and operating conditions.
A small amount of dissolved material can remain in the RO output.
Can UV and UF Be Used With RO?
Yes.
Many household purifiers combine several technologies because each performs a different function.
A purifier might use:
RO + UV + UF
In such a system:
- RO handles substantial dissolved-solids reduction.
- UV provides additional microbial disinfection.
- UF provides physical membrane filtration.
The exact configuration and purpose of each stage varies between manufacturers and models.
Does RO + UV Reduce TDS?
Yes, primarily because of the RO stage.
Adding UV does not make the water’s TDS lower. The RO membrane is responsible for the dissolved-solids reduction.
Does RO + UF Reduce TDS?
Yes, primarily because of RO.
UF does not normally contribute significant TDS reduction. Its purpose is generally physical filtration rather than dissolved-salt removal.
Does UV + UF Reduce TDS?
Generally no.
A UV+UF purifier can provide microbial treatment and physical filtration, but neither technology is designed to provide the broad dissolved-solids reduction associated with RO.
This type of purifier can therefore be suitable for certain low-TDS water sources where dissolved-solids removal is not required.
Does a Gravity Purifier Reduce TDS?
Most conventional gravity-based purifiers do not significantly reduce TDS.
Depending on the design, gravity purifiers may use sediment, activated carbon, ceramic or UF filtration.
These technologies can address different types of impurities, but they generally do not provide the dissolved-solids reduction of RO.
Does Activated Carbon Reduce TDS?
Generally no.
Activated carbon is useful for reducing chlorine, odors, unpleasant taste and certain organic compounds.
It is not a general-purpose dissolved-salts removal technology.
Therefore, water can taste significantly better after carbon filtration while its TDS remains relatively unchanged.
Does Boiling Reduce TDS?
No.
Boiling can inactivate many microorganisms, but it does not remove dissolved salts and minerals.
If water evaporates during boiling, the concentration of dissolved substances in the remaining water can actually increase.
Can TDS Be Reduced Without RO?
Yes, but the appropriate technology depends on the particular dissolved substance.
Specialized processes such as ion exchange or distillation can reduce certain dissolved substances.
However, for broad dissolved-solids reduction in a typical household drinking-water application, RO is one of the most common technologies.
Should You Buy RO Just Because TDS Is High?
Not necessarily.
TDS is useful information, but it does not tell you what is causing the TDS reading.
Before choosing a purifier, consider:
- Water source
- TDS
- Hardness
- Known contaminants
- Microbial quality
- Fluoride
- Arsenic
- Nitrate
- Iron
- Lead
This is especially important for borewell, tanker and other variable water sources.
When Is RO a Better Choice?
RO may be appropriate when water contains elevated dissolved solids or when testing identifies dissolved contaminants that an RO system is designed to reduce.
RO is particularly useful for water sources such as some:
- Borewell supplies
- Tanker supplies
- Brackish-water sources
- High-TDS groundwater sources
However, the purifier should be selected according to the actual water quality and the manufacturer’s specifications.
When Might UV or UF Be Better Than RO?
If the source water has relatively low dissolved-solids concentration and the primary concern is microbial contamination or particulate filtration, UV, UF or a combination of the two may be more appropriate.
Using RO when dissolved-solids reduction is unnecessary can introduce additional considerations such as:
- Reject-water production
- Electricity consumption
- RO membrane replacement
- Additional maintenance
Does TDS Tell You Which Purifier You Need?
It provides useful information, but it should not be the only factor.
A simple decision framework is:
| Water Situation | Technology to Consider |
|---|---|
| High dissolved solids | RO |
| Low TDS + microbial concern | UV or UV+UF |
| Low TDS + particulate filtration required | UF or other appropriate filtration |
| High TDS + microbial concern | RO with appropriate additional treatment |
This is a starting point rather than a universal rule. Actual water testing should guide treatment when contamination is suspected.
Important: TDS Is Not the Same as Water Safety
A low TDS reading does not guarantee safe drinking water.
Likewise, high TDS does not automatically mean that water is unsafe.
TDS meters cannot identify individual contaminants such as:
- Arsenic
- Lead
- Fluoride
- Nitrate
- Bacteria
- Viruses
If you are concerned about the safety of your drinking water, appropriate laboratory testing is more informative than relying only on a TDS meter.
Final Verdict
RO, UV and UF do not perform the same job.
RO can significantly reduce TDS because its membrane rejects a substantial proportion of many dissolved substances.
UV does not remove TDS. It is primarily used to inactivate microorganisms.
UF generally does not remove TDS. It is primarily a physical filtration technology that can retain suspended particles and certain microorganisms.
Therefore, if your main objective is to reduce dissolved solids, RO is the relevant technology among the three. But if your water already has low TDS, RO may not be necessary solely because you want to purify the water.
The best purifier is the one that addresses the actual problems in your source water, rather than simply having the largest number of purification stages.
Frequently Asked Questions
Does RO remove TDS?
Yes. RO can substantially reduce many dissolved substances and therefore significantly lower TDS.
Does UV remove TDS?
No. UV is primarily a microbial-disinfection technology and does not significantly reduce dissolved salts.
Does UF remove TDS?
Generally no. Conventional UF membranes do not normally remove dissolved salts and therefore do not significantly reduce TDS.
Which is better for reducing TDS: RO, UV or UF?
RO is the appropriate choice among these three when substantial TDS reduction is required.
Does UV+UF reduce TDS?
Generally no. UV and UF can address microorganisms and particles but do not normally provide substantial dissolved-solids reduction.
Does RO + UV reduce TDS?
Yes, because of the RO stage. UV itself does not reduce TDS.
Does boiling reduce TDS?
No. Boiling does not remove dissolved salts and minerals.
Can a carbon filter reduce TDS?
Generally no. Activated carbon primarily targets substances such as chlorine, odors, tastes and certain organic compounds.
Does high TDS always mean I need RO?
No. High TDS is a useful indicator, but the source and actual composition of the water should also be considered.
Does low TDS mean water is safe?
No. Low TDS does not rule out microbial contamination or specific chemical contaminants.
About the Author
ManBearPig is a part-man, part-bear, part-pig creature and a full-time expert on global warming—mostly because he has personally experienced the consequences of being extremely, extremely, super cereal.
When he isn’t roaming the wilderness, terrorizing unsuspecting humans, or demanding that people take climate change seriously, ManBearPig studies rising temperatures, melting ice, extreme weather, carbon emissions, and other inconvenient signs that the planet may not be particularly happy with us.
His research philosophy is simple:
“I’m super cereal.”
ManBearPig believes climate change is too important to discuss without facts—and too ridiculous to discuss without a little humor. Through this website, he explores global warming, climate science, environmental changes, energy, and the strange ways humans respond when their own planet starts sending them warning signals.
He has no university degree, no laboratory, and absolutely no intention of becoming a respectable academic.
But he does have hooves, claws, a snout, and an unusually strong opinion about melting ice.