RO vs. UV: Which Water Purifier Is Better For You?

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Many people assume that RO-based water purifiers are, generally speaking, a better class of water purifiers than UV-based ones. While this may be true in technological terms, it does not mean that a RO-based water purifier should be an automatic choice for every home.

A RO purifier offers a higher level of purification, but whether this enhanced purification capability is required depends on the quality of drinking water supplied to your home.

This article explains what factors to evaluate before taking a decision on whether to go in for a RO-based water purifier or stick to a UV-based one.

Let’s start with the basics. RO stands for Reverse Osmosis. But to understand how RO works, it is essential to understand Osmosis first.


Osmosis, as we learnt in school, is the flow of water molecules across a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration (see below image).

Osmosis principle

The semi-permeable membrane above has holes, or gaps, that are large enough for water molecules to pass through, but too small for the solid molecules to move across.

In case of drinking water, solute refers to the dissolved salts and minerals such as Calcium and Magnesium (referred to as Total Dissolved Salts or TDS) that are present in unfiltered water.

Depending on the concentration of these dissolved salts (calculated in units of Parts Per Million or PPM, or Milligrams per Litre or mg/L), water is classified as soft water or hard water. Soft water (for example, mineral water) has a lower TDS value, while hard or polluted water will have a higher TDS value.

1 ppm = 1 mg/L

Reverse Osmosis

RO attempts to do the opposite of Osmosis, that is, move water molecules from a region of higher solute concentration to a region of lower solute concentration (hence the name Reverse Osmosis). It does this by using the pressure of the incoming water to push it across the membrane, while leaving the dissolved solids and other impurities behind. The remaining water and solids are discharged through another outlet (see below image).

Reverse Osmosis principleCommon Sources of Water Contamination

There are various sources of drinking water supply, such as lakes and rivers, bore wells, water tankers, harvested rain water, etc. Depending on the source of water, the quality of water supplied may differ in terms of TDS levels, pollutants, chemicals as well as presence of harmful micro-organisms such as viruses and bacteria.

Generally, water supplied from lakes and rivers, as well as harvested rain water, will have a lower concentration of TDS, whereas water drawn from bore wells or supplied by tankers is likely to have a higher concentration of TDS and other harmful chemicals such as lead, arsenic, etc. Tanks constructed to store water can also cause contamination if not cleaned regularly and properly.

The concentration of chemicals and micro-organisms in water can also vary depending on the quality and cleanliness of the water delivery infrastructure, such as treatment plants, pipelines, intermediate storage tanks, etc. Old and/or corroded pipes are a major source of water contamination. And if a water pipe is placed close to a sewage pipe and leakage takes place, the contamination can be far more severe.

Components of a RO-based Water Purifier

A RO-based water purifier uses the Reverse Osmosis technique explained earlier to remove dissolved salts, impurities and chemicals from the water. The basic component of such a purifier is the RO membrane which performs the RO process.

Apart from this, a commercial RO water purifier may have some additional components for improved filtration and performance. These include:

  1. Ultrafiltration (UF) membrane: Before the RO stage, a UF membrane can be used to remove larger solids from the water, thereby reducing the load on the RO membrane and enhancing its life.
  2. Pressure Enhancing Pump: If the incoming water flow from the tap does not have an adequately high water pressure, the RO filter will not work optimally. In such cases, a pump is added before the RO filtration unit to boost the inlet water pressure.
  3. UV Filter: Some RO water purifiers add a UV-based purification stage after the initial RO-based filtration to further purify the water by controlled exposure to UV radiation. This is because while RO is good at removing impurities from water, it may not be capable of removing micro-organisms completely. The UV stage kills any living organisms left over after the RO stage, thereby delivering completely pure water.
  4. Sediment Pre-filter: This is added to remove sediments from water, as well as chemicals like Chlorine added by water suppliers. Using this can enhance the life of the RO/UF membranes. If not built into the purifier, this can be added externally as shown below.
  5. Carbon Post-filter: This is added to remove unpleasant odors from water, and enhance its taste.

RO water purifier

When to use RO Water Purifiers

It should be obvious by now that a RO-based water purifier is really useful only if the water supply in your home has high levels of TDS and impurities. Therefore, before choosing which water purifier to buy, you should check the TDS level of your water supply using a TDS meter, as shown below.

TDS meter

Based on the TDS level of your water supply, you can decide to go for a UV or RO purifier as explained:

  • If the TDS is less than 150 ppm, a UV purifier is enough to ensure good quality water.
  • If the TDS is between 150-300 ppm, a UV purifier is still quite adequate, although a RO purifier will enhance the taste of water.
  • If the TDS exceeds 300 ppm, then it is recommended to install a RO-based water purifier although not mandatory.
  • If the TDS exceeds 500 ppm, which is the upper limit set by the Bureau of Indian Standards (BIS) for safe drinking water, then it is strongly recommended to get a RO-based purifier.

Most water purifier manufacturers (e.g. Kent, Eureka Forbes, Zero B, etc.) offer free check-up of the water TDS level, so you can avail of that. Also remember to check the water pressure at your house. If the inlet water pressure is below the stipulated level for the purifier, you may need to install a Pressure Enhancing Pump as explained above.

Important Specifications of a RO Water Purifier

There are some important specifications to keep in mind when purchasing a RO water purifier. These are:

  1. Number of purifying stages: Based on your needs, you can either go for a RO-only purifier, a RO+UV purifier, or a RO+UF+UV purifier, with additional pre- or post-filtration stages as necessary.
  2. Storage Tank capacity: RO filtration is a slow process, which is why most RO purifiers provide a built-in storage tank. A tank size of 7 to 9 litres is common and should be adequate.
  3. Flow Rate and Duty Cycle: If heavy usage is likely, it is necessary to check the flow rate, i.e. the rate at which the purifier can deliver filtered water. It is also important to know the maximum amount of water that can be purified in a day, called the duty cycle. These are indicated in units of liters per hour/day.

Lastly, as with any product, please remember to check the product reviews, the quality of after-sales service and availability of spares before narrowing down on any specific model.

Pros and Cons of RO Technology


  1. Clean & effective: Delivers far more effective purification for water with high levels of TDS and impurities, and does so without the use of chemicals.
  2. Removes all impurities: Apart from dissolved solids, it can also remove pesticides, metals and chemicals which contaminate water.
  3. Multi-stage purification: Multiple stages can be chosen based on need and can effectively filter out both living and non-living impurities, remove odors, enhance taste, etc.
  4. Energy efficient: RO filtration does not require electricity (although other components such as the UV lamp or pump require it).


  1. Excessive water wastage: For every liter of water that is purified, about 2-3 liters of water are discharged as “waste”. The good news is that this water can be used for mopping the floor, watering the plants, washing utensils and the car, etc.
  2. Can remove essential minerals: Natural drinking water contains some essential minerals like Calcium, Magnesium and Potassium which are good for the body. When using a RO purifier, do ensure that the output TDS level does not dip below 30-40 ppm so that some minerals are retained. This can be done using a TDS controller which comes built into most models.
  3. Expensive: Being a relatively new technology, and because of the multiple filtration stages, RO water purifiers are currently quite expensive. A good RO water purifier can cost 2 to 4 times as much as a good UV water purifier.

About the Author

Vijay PadiyarI'm a friendly guy with a witty sense of humor. I was born in Baroda, Gujarat and am currently settled in Bangalore, Karnataka. I'm a Leo by birth and certainly by character! So pick up any good book on zodiac signs, flip to the Leo section, and you'll know me rather well!View all posts by Vijay Padiyar →


  1. A.KhambekarA.Khambekar10-18-2014

    Hello Vijay, Really informative article. We have very hard water so soap doesn’t wash off easily, leavins hair sticky and white scaling on bathroom fixtures and surfaces. I am thinking of installing a water purification system in the bathroom to address this problem. Could you please advise me which filtration process can address this. Thanks

    • Vijay PadiyarVijay Padiyar10-18-2014

      You can look at installing a water softener for your bathroom.

      • A.KhambekarA.Khambekar10-20-2014

        We considered that option, but were told by the company that the softner will treat only the hot water and we have to keep adding salt very often so maintaining is a problem. Hence I would like to know if installing a RO filter take care of this problem. Thanks

        • Vijay PadiyarVijay Padiyar10-20-2014

          You can install a single large water softener for the entire house, at the main water inlet. So it will soften the entire water supply to the house. Yes you need to add salt but it is not very frequent. It is more cost efficient than a RO purifier for the entire house, as normally such low TDS is not required for non-cooking/drinking purposes.

  2. ManishManish10-17-2014

    Any Details about ph Balance,,, RO or UV Or TDS effect on it ?

    • Vijay PadiyarVijay Padiyar10-18-2014

      RO purification may reduce the pH level by a small amount depending on the chemical composition of the inlet water and presence of gases like Carbon Dioxide in it.

  3. paragparag10-16-2014

    Where can i get the data for my apartment water TDS? is there any Government data base or i need to get it checked at a water testing lab?
    I get borewell water. Is there any home method to check if the quantity of dissolved salts is tolerable?


    • Vijay PadiyarVijay Padiyar10-17-2014

      You can call one of the purifier companies to send someone for a product demo. They will analyse the water using a TDS meter. Or you can buy a TDS meter for around Rs. 500.

  4. ManishManish10-15-2014

    Our TDS Test report is 220 PPM,,
    Right now we are using Aquaguard Gold UV, Still water taest is very bad & water very hard to Drink & smell too, What you suggest What i will use,,
    I am looking for Kent Supreme or aquaguard Enhance Green Ro,,
    If your not telling brand so tell me what to use UV only or RO+UV+UF

    Thanks in advance


  5. Sibram NayakSibram Nayak10-13-2014

    I have tested TDS level & found 200PPM. Which Purifier will be suitable UV, RO or UV+RO.

    • Vijay PadiyarVijay Padiyar10-14-2014

      You can go for a RO purifier. RO + UV is required if you suspect there may be biological contaminants in the water.

      • Sibram NayakSibram Nayak10-14-2014

        Thanks Sir. Like TDS value we could check through TDS meter but for biological contaminants are available in water or not how to check. please advise.

  6. Amit soniAmit soni10-13-2014

    Hi Vijay,
    My apartment Borewell water is contaminated with sewage water and it smells but appears crystal clean. Is it safe to make this water drinkable through R O water purifier. Please suggest what to do?

    • Vijay PadiyarVijay Padiyar10-14-2014

      RO cannot remove bacteria and other microorganisms from the water. Go for a RO + UV purifier so that the UV stage will kill off any living organisms in the water.

  7. anil chopraanil chopra09-28-2014

    hello mr. vijay,
    The kaveri water in our house appears to be dirty. The tds is 122ppm. should i change
    our old filter ( kenstar U V). kindly suggest .

    • Vijay PadiyarVijay Padiyar09-28-2014

      122 ppm is not very bad. You don’t need a RO purifier. If the water appears dirty, add a sediment pre-filter before the existing UV filter to remove the physical impurities from the water.

  8. Nishit patelNishit patel09-18-2014

    I m Nisht patel.
    I m living in a bad in Ghodasar.
    I have RO purifier last 5 years but i done AMC from local person and I found water test was change and I called and found TDs below 25 its 16 so they add some chemical filter additives and then found TDs 36 to 40 pls suggest me it is ok to increase TDs this filter used and no any harm full to us in future pls suggest me what I have to do.
    Thanks & Regards
    Nishit patel

    • Vijay PadiyarVijay Padiyar09-18-2014

      You can just adjust the output TDS using the TDS controller in your RO purifier, if it has one.

  9. AnupAnup09-02-2014

    I have forebs uv Vijay model ,and I got my water tested yesterday and tds was 257 of the
    corporation water and 248 of the uv filtered water , and he had placed some electrodes in
    the water and water color changed to orangewith deposits at the bottom. Salesmen says we need a RO+UV which he is offering for 8000 (Aquafresh Grand + model) , money is not the problem but water quality is important. what should I do ?

  10. lakshmilakshmi09-01-2014

    nice article on RO and UV thanks

  11. vibhorvibhor09-01-2014

    How to test the bacteria and viruses in drinking backeria.Is it possible tds meter is giving good tds level but water is contaminated fm bacteria.Which purifier is best for bacteria

    • Vijay PadiyarVijay Padiyar09-01-2014

      The UV stage in a purifier kills bacteria and viruses. If TDS level is low, go for a UV purifier else go for a RO+UV purifier.

  12. Syed ImranSyed Imran08-31-2014

    Thanks for the detailed analysis on RO and UV water purifiers.
    Wondering what exactly is Quartz UV technology and how different it is from normal UV technology…
    As a visually challenged guy, I’m unable to determine the TDS level of the water being supplied here by the municipality (on my own) but a salesman from Eureka Forbes visited me for a demo and he says it is 118 PPM. May be there’s a talking TDS meter available somewhere in the market but I’m yet to come across one.

    • Vijay PadiyarVijay Padiyar08-31-2014

      Yes TDS meters are available in the market and are very cheap. A TDS of 118 ppm is quite low and a UV purifier will be adequate for your purpose (assuming the salesman reported the value correctly).

      Quartz is not a new technology. Most UV purifiers already have it. Basically the UV lamp is placed inside a Quartz sleeve which looks like a glass test tube. This sleeve isolates the lamp from the water and ensures that most of the UV radiation (almost 100%) reaches the water without being absorbed by the sleeve. See an image of a Quartz sleeve here:

  13. pradeeppradeep08-27-2014

    iam planning to buy a water purifier..which is good RO or RO+UV?? some of them r showing adds on kent purifiers that will retain minerals and others dont?? iam bit confused..can u give some info abt that and suggestions..

  14. KiranKiran08-02-2014

    Hi sir,
    I bought a kent uv water filter. Can i use the filter to sweeten and purify the tap water? Or i need a Ro specifically for this purpose?

    • Vijay PadiyarVijay Padiyar08-04-2014

      UV water purifiers cannot sweeten the water. You will need a RO purifier for this.


    thank you. very informative and usefull information. may i know the source of your information provided.
    thank you

  16. Niranjan PadhyNiranjan Padhy07-08-2014

    Hi Vijay,

    Right now TDS lever of my water is 155 ppm. I want to install only UV filteration. But if it needed to upgrade into RO later incase then it is possible to upgrade with only RO cost.


    • Vijay PadiyarVijay Padiyar07-08-2014

      Yes you can, but I doubt there will be much difference between RO-only and RO+UV models of any brand.

  17. Hemant GurbaniHemant Gurbani07-07-2014

    which brand i will prefer for buy ro ? My budget is around 20000 rs

    • Vijay PadiyarVijay Padiyar07-08-2014

      Sorry I do not like to recommend specific brands. Do your research and pick the best one for you!

  18. DipDip06-27-2014

    Dear Vijay,
    First of all, thank you for this excellent article which provides information about this technology to novices like us. I have a Kent Supreme water purifier, which rejects about 75% of the water. Is it recommended to use this rejected water for cleaning utensils, clothes, or toilet purposes ?
    Thank You.

  19. venkateshvenkatesh06-26-2014

    Sir, our bore water level tds is 270. Is UV sufficient ?

    • Vijay PadiyarVijay Padiyar06-27-2014

      UV is acceptable, but RO will give better taste at this TDS level (though not strictly necessary to install).

  20. Ranjeet KumarRanjeet Kumar06-24-2014

    Dear Vijay,

    Thank you for a brief information regarding RO and UV. I am planning to buy one RO+UV system. You have mentioned that some amount of hard water contains calcium & magnesium which is good for our health and can be consumed. Drinking water through RO+UV may lessen required minerals in our body. What should be the solution?

    Ranjeet Kumar

    • Vijay PadiyarVijay Padiyar06-24-2014

      You should ensure output water TDS doesn’t go below 30-40 ppm. Use the TDS controller on the RO+UV unit to adjust the output TDS.

  21. atulatul06-07-2014

    Hi Vijay,

    Let’s say that the TDS is less than 300 ppm, which does not really require an RO as you mentioned. However, how would one get rid of metals like lead in the water. By having a tds of less than 300 ppm, does that mean the lead is not harmful if it is indeed present in the water.
    I have used Aquaguard UV purifier but after I boil the water for sometime that comes out of it (even after installing a new filter), I can see white powder at the bottom of the pot – which I think is chlorine. This means the carbon filter is not filtering the chlorine. Will RO remove it better? Sorry, I’m confused and have just started to learn about water purifiers.
    Also, which RO + UV brand would you recommend? Thank you.

    • Vijay PadiyarVijay Padiyar06-08-2014

      Hi Atul

      Certain UV water purifiers do remove chemicals such as lead and arsenic. Since you already have a UV purifier, a cheap solution I would suggest (as compared to buying a new RO unit) would be to purchase something like PureIt Advanced or some such water storage device, where you can store water after purification from UV water purifier and which performs lead/rust removal.

      Regarding Aquaguard UV purifier, it has a activated carbon filter which should be removing chlorine. Are you servicing it and replacing the carbon filter and sediment pre-filter regularly?

      And sorry, I would prefer not to recommend any particular brands. I would suggest you to do your own research by asking your friends and colleagues.



  22. SonitSonit05-25-2014

    Hi Vijay,

    Thanks, very useful post. If possible can you please post, connection diagram for RO and RO +UV systems in details and how to troubleshoot RO systems and some links.

  23. AnupamAnupam04-16-2014


    I have Kaveri water that needs to be filetered for drinking purposes in Bangalore. Which filter do you suggest? RO or UV?

    • Vijay PadiyarVijay Padiyar04-16-2014

      UV is enough in most cases. However, get the drinking water in your house tested and then you can decide based on the TDS amount.

  24. udaiudai03-31-2014

    Hi Vijay
    I have an Aquaguard Enhance UV. When the water level goes below a certain point it starts to fill automatically but the waste water seems to be at least 10 times the filtered water. Is something wrong? How many litres of water is wasted for 1 litre of filtered water in this kind of UV (no RO) filter?

    • Vijay PadiyarVijay Padiyar03-31-2014

      RO purifiers generally waste a few litres of water for every litre of purified water. The exact ratio of wasted water to purified water varies from model to model. The waste water can however be used for other purposes like mopping floors, watering plants, etc.

      Newer RO purifiers have improved on the technology so that the amount of wastage is reduced, however the wastage cannot be completely avoided due to the nature of the technology.

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