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.

Click here to see the BIS Drinking Water Specification (IS 10500).

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 →



    Thanks for the lot of useful info and patience!

  2. Pravin ChaudhariPravin Chaudhari11-18-2015

    Hi Vijay,
    Thanks for such an informative article.
    Please let me know is there any apparatus available to check biological contamination of water or is there any unit for it like that of TDS which we can measure by TDS meter in PPM.

    • Vijay PadiyarVijay Padiyar11-20-2015

      No you will need to get it tested in a water testing lab for presence of bacteria. There are many such labs in all cities.

  3. seetharamanseetharaman11-14-2015

    Great Post….Simple for a layman like me…..

  4. P GuptaP Gupta11-08-2015

    I recently have shifted to Pune 4 months back. In Pune, We are getting Municipalwater with TDS 70-90. I have a UV water purifier for last 7-8 years. But I am getting infection again & again after shifting to Pune. Doctor advised me to change purifier. Pl advice which purifier to be used.

    • Vijay PadiyarVijay Padiyar11-08-2015

      First try boiling water from the UV purifier for a month or so, as an additional safety measure. If that helps in reducing your infections, then you can think of replacing the water purifier. RO is not really required for such low TDS.

  5. KrithiKrithi11-08-2015

    Hi, I stay in a place where cauvery river flow. We are using borewell to take underground water for usage. My friend suggest me to go only with RO. Others suggest to go with RO+UV+Minerals.TDS will be less in my area.

    Can you please suggest should i need RO+UV+Minerals or only RO sufficient?

  6. RAVIRAVI11-06-2015

    Hi vijay
    I must say one of the most helpful blog I have ever read. Thanks in advance
    Coming to my query I’m using kent ro+uv+uf. Do I need to install sediment filter separately?
    I’m using borewell water having tds more than 500.
    Ravi rathi

    • Vijay PadiyarVijay Padiyar11-06-2015

      If the water is muddy or slightly dirty in appearance, a sediment prefilter is recommended as it can enhance the life of the RO membrane.

  7. NeerajNeeraj10-30-2015

    Wich company is best in RO .

  8. AmarAmar10-30-2015

    Hi sir,
    Can i use the RO+UV purified water to wash my hairs. TDS level of water in my home is 650 without purification and i think it is also effecting my hairs, and what are the advantage of using RO+UV?

    • Vijay PadiyarVijay Padiyar10-30-2015

      Using purified drinking water for bathing is an overkill. If you think hard water is affecting your hair, try installing a water softener for the domestic usage water which will reduce the hardness.

  9. TruptiTrupti10-29-2015

    Just wanted to know what is the difference between RO and Water purifiers.. Also which one is better to use and which one is cost effective??

    • Vijay PadiyarVijay Padiyar10-30-2015

      Seems you haven’t read my blog and first asked the question :)

      • sameersameer11-13-2015

        hi vijay
        i am sameer and from nagpur
        which make and which type of water purifier is best for
        well water for drinking purpose

  10. Alpa desaiAlpa desai10-25-2015

    Hello vijayji
    Am tooo a barodian and lives in baroda only. Sama area
    Well i have installed one assembled RO at home, and everytime when i call any person for filter cartridge change ( i change its cartridge three months) the service person tell that ur membrance is not working?
    The ppm level is 250 at my place
    Plz guide me . Should the membrance be changed every six months?
    Awaiting n thanking in adv…

    • Vijay PadiyarVijay Padiyar10-26-2015

      A ppm of 250 should not cause membrane to wear out in 3-6 months. It should last at least a year.

  11. KiranKiran10-24-2015

    Dear Sir,

    I am really confusing, I Had a RO + UV and was using for quiet some time
    recently I have moved to some other location, there we get 24 hours corporation water
    when we check the PPM with TDS meter it was 50 PPM before RO and when I check the water what filtered through RO the PPM is appearing as 120

    I was really surprised and stopped RO and now using only UV + UF

    but still I am not satisfied with the technology

    now before filter (UV+UF) the ppm is appearing 59 and after UV + UF the PPM is 72

    please help me whether I should continue use UV + UF,?
    Otherwise I just use normal corporation water with a dust filter.

    I believe I am losing some of the minerals using these filters

    Thanks and regards

    • Vijay PadiyarVijay Padiyar10-26-2015

      Please be clear about the need of RO, UV, etc. RO is required ONLY if the input ppm is high, otherwise it has no use. UV is required ONLY if you suspect biological contamination of water, otherwise not. If neither is the case, then a simple sediment filter to remove impurities from water will do.

  12. PritikaPritika10-16-2015

    Hello Vijay,

    Thanks for valuable information.
    I have a query regarding filter to purchase. Whether I should put RO or UV+ RO as TDS level at my place is 730 and if water is kept in a bucket white sedimentation is there after 2-3 hours.

    • Vijay PadiyarVijay Padiyar10-18-2015

      RO is for removing hardness, UV is for removing bacterial contamination from water. If you just want to remove hardness then UV is not required.

  13. MahavirMahavir10-13-2015

    Dear Vijay, Many many thanks for your valuable informations regarding water purifications.

    I want to know from where I can buy a TDS checking machine, to know the TDS level in our bore well water.

    Is it enough to believe on the agents machines who comes to install RO/UV.

    Thanks again. Regards……Mahavir

  14. GR PatelGR Patel10-04-2015

    Thanx for brushing up knowledge on RO filters.
    We want to install RO plant in kitchen. Static head of water is around 30 feet. Should we go for a pump in RO filter?
    Thanx again.

  15. shamshuddinshamshuddin10-02-2015

    the currently launched water purifier from LG does not recommend TDS controller. I saw they have crossed the TDS controller in their Product manual. Do you see any reason behind that ?

  16. BhavaniBhavani09-28-2015

    Dear Sir,
    we have bore water with slightly high TDS.Very soon we will be getting drinking water connections which will be filled into the over head tank regularly.Now we are planning to buy water purifier which can be used for both hard and soft water.Kindly suggest a model which can suit both types of water.Thank you so much…Bhavani

  17. vickyvicky09-23-2015

    Most of them need manttenance and if that is not done correctly then it can be problem. what do you think, i had water purifier and i made mistake while cleaning it the water was hard and salty.

  18. SubhashSubhash09-21-2015

    Hi Vijay,
    I live in an apartment, where we have 2 water sources. Corporation Tap water TDS is 150, but borewell TDS is 700. Both are stored in 2 separate overhead tanks. TAP water getting frequency is lesser than borewell which is available 24*7 regularly. My job is transferable anywhere in India, so there may be different water sources with different TDS levels. Kindly suggest me whether i shall go fo UV or RO or RO+UV.

    Its urgent. Pls reply.

    • Vijay PadiyarVijay Padiyar09-22-2015

      Well if your job is transferable then obviously it makes sense to go for RO+UV only.

      • SubhashSubhash09-22-2015

        Thanks for your quick reply and a
        For correcting a spelling mistake.

        Now just one thing, on which water source should i connect the device?
        Or shouldi use both water sources?

        And last
        Which brand, Aquaguard, Kent or Pureit?

        • Vijay PadiyarVijay Padiyar09-22-2015

          It’s your wish. You can connect it to only the borewell tank or to both. But note that even tap water may require a sediment filter and UV purifier if there are sediments and bacteria in the water, even though RO stage is not required since TDS is low. So even if you use RO+UV only for borewell, you may still require another UV purifier or at least a simple sediment filter for the tap water. As for brands, sorry I don’t recommend specific brands.

  19. Dhruv VaishnavDhruv Vaishnav09-20-2015

    Hello Vijay.

    I read your post and its quit useful to me for understanding RO vs. UV. I have still little bit confusion. I am living at Ghatlodiya,Ahmedabad area. And here in morning Bore water is falling in the tank and in the night Narmada water is falling in the same tank. So afterwards, we will get the mixed up water. Can you please suggest me the best water purifier? So I can easily get pure water.

    Thanks in Advance.

    Dhruv Vaishnav.

    • Vijay PadiyarVijay Padiyar09-21-2015

      The procedure to decide remains the same as explained here. Check the tap water TDS and decide accordingly.

  20. shyshy09-17-2015

    We are using borewell water, wil you recommend us which purifier we need


  21. Ayesha MohanAyesha Mohan09-15-2015

    Hi Vijay,

    Thank you for the brilliantly helpful article.

    We drink mcd water which is filtered through this small filter attached to the tap. The TDS is 83. What water purifier do you recommend?

    Many thanks.

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