RO, UF or UV: Which Water Purifier Is Right For You?

An older version of this article has been published on Rediff.com. Click here to read it.

Most people assume that RO water purifiers are, by default, a better class of water purifiers than UV ones. While this may be true in technological terms, it does not mean that RO water purifiers should be the automatic choice for every home. And these days, a third class of purifiers, namely UF, has gained prominence and added to the confusion of buyers. This article lists the factors to evaluate to choose the right water purifier for your needs.

A RO purifier offers a higher level of filtration than UF or UV, but whether this enhanced filtration capability is really necessary depends on the quality of drinking water supplied to your home, and that alone must be the criteria to decide the type of purifier to go for.

Before we discuss the types of water purifiers available and what they can (and cannot) filter, let’s discuss the common types of impurities found in drinking water.

Common Types of Impurities in Drinking Water

There are various sources of drinking water supply, such as lakes and rivers, bore wells, harvested rain water, etc. These days, even seawater can be purified for drinking purposes by using industrial scale RO plants. Depending on the source of water, the quality of water supplied may differ in terms of hardness and presence of pollutants, chemicals and harmful micro-organisms such as viruses and bacteria.

Some of the common types of impurities in water are:

Type of impuritySource of contaminationCommonly found inEffect on drinking water
Undissolved solids such as sand and mudLoose soil or sand mixing with flowing waterRiver water or piped water where pipes are damagedMuddy or turbid appearance
Dissolved inorganic salts like Sodium & PotassiumSalt deposits from surfaceBore wells and seawaterSalty or brackish taste
Dissolved inorganic compounds like Calcium & MagnesiumWater flowing over rocky terrainBore wells and river waterHardness and scale formation
Organic compoundsAbsorbed from plant life growing inside water bodyLakes and pondsFoul smell or odour
Decontaminants like ChlorineAdded by municipal water suppliers to kill microorganismsMunicipal piped water supplyBitter taste
Microorganisms such as viruses & bacteriaBiological contamination of water sourcePiped water where pipes are damagedBiological contamination

Generally, water supplied from lakes and rivers as well as harvested rain water will have a lower concentration of dissolved solids, while water drawn from bore wells is likely to have a higher concentration of dissolved solids and other harmful chemicals such as lead, arsenic, etc. Water supplied through pipes or stored in tanks may be prone to biological contamination if the pipes are old and corroded or the tanks are not cleaned regularly.



Hard Water vs Soft Water

Depending on the concentration of dissolved solids in water, it is classified as soft or hard water. The degree of hardness is expressed in terms of Total Dissolved Solids (TDS) and is measured in units of Parts Per Million (PPM) or Milligrams per Litre (mg/L).

1 ppm = 1 mg/L

Soft water (for example, mineral water) will have a low TDS value (typically under 150-300 ppm), while hard or polluted water will have a high TDS value (over 500 ppm). You can check the TDS level of your water supply using a TDS meter as shown below.

TDS meter

Types of Water Purifiers

1. Reverse Osmosis (RO) Purifiers

To understand how RO purifiers work, let us 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. The solute concentration here refers to the TDS level of the water. The semi-permeable membrane has tiny holes (about 0.0001 microns wide) that are large enough for water molecules to pass through but too small for the dissolved impurities to move across. As a result, water moves across the membrane from the region of lower TDS level to the region of higher TDS level as shown below.

Osmosis principle

RO attempts to do the opposite of Osmosis, that is, push water molecules from the region of higher TDS level to the region of lower TDS level (hence the name Reverse Osmosis). It does this by pumping water into the RO chamber at high pressure, causing water molecules to be pushed across the membrane to the other side while leaving the dissolved solids and other impurities behind. These impurities along with a part of the inlet water are then discharged through another outlet as shown below.

Reverse Osmosis principle

As a result of this filtration, the drinking water generated by RO purifiers will have a much lower TDS content compared to the inlet water. RO purifiers are therefore best suited for use where the tap water TDS is very high, i.e. it is hard.

The downside of this technology is that it needs a pump to generate pressure to push water across the membrane, and therefore a RO purifier cannot operate without electricity. Also, since part of the inlet water is discharged along with the dissolved solids, this is a wasteful method of filtering water. Often a RO purifier will discharge 2-3 litres of water per litre of pure water generated.

RO water purifier

2. Ultrafiltration (UF)

Ultrafiltration also uses a semi-permeable membrane to purify water, similar to RO. However, here the membrane has much larger holes (about 0.01 microns wide) as compared to a RO membrane. As a result, while UF can remove undissolved solids and even microorganisms such as viruses and bacteria from the water, it cannot remove dissolved solids and reduce the TDS level. UF purifiers are therefore suitable for use where the TDS of the tap water is low i.e. it is not hard.

On the positive side, since the UF membrane has much larger holes, water can pass through it on its own using the force of gravity. Therefore a UF filter can work even without electricity. And additionally, there is zero wastage of water since there is no water held back by the membrane.

3. Ultraviolet (UV) Purification

A UV purifier subjects water to ultraviolet radiation which causes microorganisms such as viruses and bacteria to be killed or “inactivated”. It however cannot remove any dissolved or undissolved solids or chemicals. As a result, commercially sold UV purifier units come with external sediment pre-filters to remove undissolved solids as well as an activated carbon unit to remove Chlorine and some dissolved solids from the water. UV purifiers, like UF, are suitable for use where the TDS of the tap water is low. But if the water is too muddy, a UF purifier is preferable.

If the biological contamination of water is high, a combination of UF+UV may be considered.

4. Tap/Faucet Mounted or Storage Filters

These are simple sediment or sediment-cum-activated carbon filters which can remove undissolved solids such as sand, mud, etc., some chemicals and some microorganisms. The smallest models are the size of your fist and can be directly fitted on taps and can provide thousands of litres of purified drinking water before they need to be replaced. Slightly larger models come with built-in storage tanks where water can be filled and stored for use. These are suitable for use where the water TDS level is low and there is nil or minimal biological and sediment contamination of water.

Tap mounted water purifier

Which Water Purifier Should I Choose?

As stated earlier, a RO purifier is necessary only if the water supply in your home has a high level of TDS. As per the Bureau of Indian Standards (BIS) Drinking Water Specification (IS 10500) the maximum TDS limit for safe drinking water is 500 ppm. But realistically speaking, 500 ppm is on the higher side and it is recommended to go for a RO purifier if the TDS level exceeds 300 ppm.

But below 300 ppm, a RO purifier is not necessary at all and can in fact be detrimental to your health as it will reduce the TDS to abnormally low and unhealthy levels. In this case, your choice of purifier should be determined by other factors such as turbidity of water and/or presence of biological contamination.

The below table explains clearly which water purifier is suitable for different types of contaminations:

TDS contentIs water muddy?Is water biologically contaminated?Which purifier to use?
Low (below 300 ppm)NoNoTap/faucet or storage filter
Low (below 300 ppm)NoYesUV
Low (below 300 ppm)YesNoUF
Low (below 300 ppm)YesYesUF or UF+UV
High (over 300 ppm)NoNoRO
High (over 300 ppm)NoYesRO+UV
High (over 300 ppm)YesNoRO+UF or RO+UF+UV
High (over 300 ppm)YesYesRO+UF+UV

Components of a Commercial RO Water Purifier

A commercial RO water purifier has several additional components apart from the RO membrane, for improved filtration and performance. These include:

  1. Sediment Pre-filter: This is added at the inlet of the system to remove sediments from water as well as chemicals like Chlorine added by municipality into the water supply. It also helps to enhance the life of the RO/UF membranes. If the purifier doesn’t have this, it can be added externally as well.
  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 to boost the inlet water pressure before the RO stage.
  3. Activated Carbon Pre-filter: This removes some dissolved solids and chemicals from the water.
  4. RO/UF membranes: Commercial RO purifiers have a UF membrane in parallel to the RO membrane. The UF membrane delivers the same degree of filtration as RO except that it does not remove the dissolved solids. The inlet water is distributed between the two and the final mixture is adjusted using a TDS controller to control the output TDS of the system.
  5. TDS controller: As stated above, this mechanism allows adjusting the output TDS of the system. It does this by adjusting the ratio of RO-purified water (which has reduced TDS) and UF-purified water (which has normal TDS) in the final mix. A practical benefit of this is that waste water can be reduced by increasing the output TDS if the inlet water TDS is already low.
  6. UV Filter: Some RO water purifiers add a UV purification stage after the RO/UF stage to eliminate any leftover microogranisms.
  7. Carbon Post-filter: This is added to remove unpleasant odors from water, and enhance its taste.

Reducing and Reusing RO Waste Water

As mentioned before, RO purifiers generate 2-3 litres of waste water for every litre of purified water. For a resource-starved country like India, this level of wastage is almost criminal! However, the good news is that this waste water generation can be reduced and the rest reused reduced by simply following the below steps:



  • Ensure that the output TDS is at least 150 ppm. If lower than that, increase it using the TDS controller knob. An excessively low TDS value not only generates excessive waste water, it’s also bad for health! Increasing the TDS will make the drinking water healthier and also reduce wastage.
  • Store the waste water and reuse it. The waste water generated by RO purifiers is good enough to be safely reused for many domestic purposes such as watering plants, mopping floors and even for washing clothes as shown below (picture courtesy Apartment Adda). Waste water can be stored in containers and reused during the day.

RO waste water reuseHope this helps!


Signature

379 Comments

  1. Ashok DashAshok Dash06-27-2016

    dear Sir,
    My bore well water tds is 560.i have also purchased Dr.Aquaguard HD RO+ UV .is it enough or I will add extra ironnil

    • Vijay PadiyarVijay Padiyar06-28-2016

      Check the specifications of the product. It will tell what is the maximum input TDS it can work with.

  2. MSRaoMSRao06-26-2016

    Dr. Mr. Vijay,
    Thank you very much for very useful information.
    We are getting municipal water in Hyderabad which is of bitter in taste even after UV filtering. Presently I am using aquaguard UV. Would like to go for RO + UV. Please advise.

  3. Nitesh bagriNitesh bagri06-25-2016

    Which RO would be good for 4000 TDS

  4. Omi PathakOmi Pathak06-22-2016

    Hello sir! Can I use the waste water from RO purifier to wash my hairs ? Is it good or can cause harm?

    • Vijay PadiyarVijay Padiyar06-22-2016

      Better not to use. You can use for mopping floor, watering plants, washing car, etc.

  5. sureshsuresh06-18-2016

    Thank you so much it is very useful tips

  6. sramsram06-15-2016

    hi vijay,

    Our house tds water level is around 225. bore water. should i go for RO or UV UF. Eureka man suggests medium RO.

    Thanks,

  7. AmitAmit06-14-2016

    Hi Vijay..what your views on “Electroadsorbtion Filter”, they can optimally replace RO+UV+UF water purifier system..

    • Vijay PadiyarVijay Padiyar06-14-2016

      This is a new technology and I don’t find much research material on this so I cannot comment. Also as of now only one manufacturer seems to be offering this technology.

  8. srinathsrinath06-06-2016

    In my home bore water which contain above 2000ppm tds and i buy one ro purifier from snapdeal and i installed but water dint get taste, i checked water with tds meter which showing 250ppm thats maximum taste i got, i altered lot in purifier and checkef lot of website ,i dint get solution please help me

  9. AvinashAvinash05-27-2016

    hello sir..
    I have question, as uf pore size is greater than ro then in domestic system why ro is used before UF..

    • Vijay PadiyarVijay Padiyar05-27-2016

      RO is not used before or after UF, they both are used in parallel. Depending on desired output TDS (configured via TDS controller), the ratio of RO to UF water is decided and output water is a mix of the two.

  10. Amit KrAmit Kr05-15-2016

    How effective is ceramic water filters ( with candles) for MCD supply in South Delhi. In that impurities are visible . We are using in our home for more than 25 years and are still fine but we are likely toshift from south delhi to nbcc town , khekra phase 1 where the TDS has been measured as 450. Can we use a ceramic filter there and is it sufficient . They are supplying the ground water

    • Vijay PadiyarVijay Padiyar05-15-2016

      Please refer to the table. TDS of 450 is too high. Ceramic filters cannot reduce TDS. A RO purifier is recommended.

  11. SitaramSitaram05-14-2016

    This is very useful article. Thanks a lot for sharing.

    Where can we get data of TDS levels in various cities of India to determine in which city, which kind of purifier needed ?

    • Vijay PadiyarVijay Padiyar05-14-2016

      Use a TDS meter and check the TDS in your house. There is no city-wide TDS data in India as far as I know.

  12. NamanNaman05-14-2016

    how to check tds

    • Vijay PadiyarVijay Padiyar05-14-2016

      Read the blog and find out!

    • swapnilswapnil06-19-2016

      hi
      my boring water tds 345 then which purifier suggest please confirm urgent thanks

      • Vijay PadiyarVijay Padiyar06-19-2016

        Quickest way to get your question answered is to read the article 🙂

  13. vipinvipin05-12-2016

    hello vijay
    My locality water is coming from spring source and is very soft in nature as it has low amount of calcium and magnesium ions in it. There are major health issues prevailing in my locality. can you suggest some methods to improve the mineral content in water.
    Reply ASAP.

    • Vijay PadiyarVijay Padiyar05-12-2016

      You will need to install a remineralizer to add minerals to the water, or otherwise add Himalayan pink salt to the water. This salt is rich in essential minerals and is commonly available and cheap.

      • vipinvipin05-13-2016

        what kind of remineralizer is suitable for treating 18 lakh gallons of water everyday…and the water has less calcium and magnesium. Is that heavy capacity of remineralizer available in market.

        • Vijay PadiyarVijay Padiyar05-13-2016

          You will have to enquire in the market. I am sure there would be suitable products available.

  14. RPSRPS05-11-2016

    Nice information..thanks

  15. JorgeJorge05-08-2016

    Hello Vijay, I have the following setup in this order: Sediment, Pre Carbon, UF, Post Carbon, TCR Filter, UV.

    As you can see the system is very complete. The problem I have is that the Post Carbon and TCR filter release carbon and the water gets a greyish thing that collects at the bottom of the reservoir. My Question is can i change the UF to the last stage (only before the UV) so it collects this greyish thing without affecting the water purification process? THANKS

    • Vijay PadiyarVijay Padiyar05-09-2016

      You need to refer to the technical specifications of the post carbon and TCR filter. If it allows installation before UF/RO module then you can do it.

  16. NileshNilesh05-07-2016

    Hi Vijay, Pretty well explained article..It means UF (high TDS) & RO (low TDS) are connected in parallel and water output of these two are mixed to achieve required TDS. This means dissolved solids /bacteria / viruses from UF output are added to RO purified water. N then UV is used to purify further. So it is as good as UF + UV. As they say ” a chain is as strong as weakest link”. N mixing of UF & RO output is weak link here.
    Logically one should go for RO (without TDS control), with minerals added externally. What do u say Vijay? Are there such products available in market?

    • Vijay PadiyarVijay Padiyar05-07-2016

      But then there will be excess water wastage as compared to adjusting TDS with UF. And UF is also adequate to filter out bacteria and viruses. Only TDS will pass through.

  17. Manish GuptaManish Gupta05-06-2016

    Hello Vijay,
    You described the RO system nicely and it removes dissolved solids but I think also removes the harmful bacteria and viruses out since its tiny holes are 0.0001 microns almost.
    So Do you think after an RO in place their is any use of UV & UF . Because in UF holes are 0.01 microns and since RO also removes bacteria and Virus so their is no need of UV system when RO is in working place. So only RO Water purifier is worth taking.
    Am I right Vijay. Can you explain me if not.
    Regards,
    Manish

    • Vijay PadiyarVijay Padiyar05-06-2016

      UF is used in parallel to RO since it removes all the things which RO removes except TDS. So by varying the ratio of UF and RO water in the final mix the TDS controller achieves the desired TDS. And UV is just an additional safety measure to kill any bacteria/viruses left over after RO/UF stages.

      • Manish GuptaManish Gupta05-07-2016

        Hello vijay, sorry I am not satisfied with your answer to my question. As per fundamentals of RO n UF I think when RO is there then no need of UF. Secondly, I am not sure after filtration by RO at 0.0001 micron level how much percent of bacteria n virus are not filtered still and hence UV is required or not.
        I want you to contradict my thinking and discuss more so that to come to best conclusion. Pls.
        Rgds…

        • Vijay PadiyarVijay Padiyar05-07-2016

          1. If only RO is there then output TDS control will not be possible. So if input TDS is already low then output TDS will be undesirably low. Hence UF is added in parallel (not sequentially) to RO so that TDS control is possible.
          2. Since output water may be mix of RO and UF, hence UV stage is desirable as a secondary safety measure since UF (and maybe RO) membrane may not fully filter out bacteria and viruses.

          • Manish GuptaManish Gupta05-08-2016

            Thanks Vijay. Now I am fully satisfied. It has enhanced my knowledge about water puriere system. Thanks again.

  18. yathish Kumaryathish Kumar05-01-2016

    How is the quality of Aqua Grand which having RO, uv with tds controler is available chief around Rs 5000/- capacity 12 ltrs. kindly suggest me

    • Vijay PadiyarVijay Padiyar05-01-2016

      Sorry I do not provide specific product recommendations or reviews.

  19. SRINIVAS AWARUSRINIVAS AWARU04-27-2016

    Hi Vijay ji , THANKS A LOT FOR VALUABLE INFORMATION. PRESENTLY, WE ARE COMPLETELY DEPENDING ON BORE WATER. WE HAVE NOT TESTED THE TDS LEVELS AT ANY TIME. I AM PLANNING TO GO FOR RO +UV+UF. PLEASE SUGGEST ME WHETHER I SHOULD GO ONLY FOR RO PURIFIER OR RO +UV+UF? PLEASE WAITING FOR YOUR VALUABLE REPLY SO THAT I CAN TAKE THE DECISION SOON. REGARDS SRINIVAS FROM HYD

    • Vijay PadiyarVijay Padiyar04-27-2016

      Please refer to the given table and make your choice based on your water type.

  20. Preety SinghPreety Singh04-06-2016

    Hello sir,

    I am searching for a ro purifier in which I have two choices one is purit marvella slim having tds remover feature and in economic cost and 2nd one is Kent grand plus having tds controller featue but in double cost. If I choose tds remover it will effect in future for health purpose.? Or tds remover and controller r same. Kindly suggest better one purifier.

    • Vijay PadiyarVijay Padiyar04-07-2016

      Every RO purifier is basically a “TDS remover”. It is not a feature but rather a description. TDS controller allows you to control how much TDS to remove based on inlet water TDS level.

1 7 8 9

Leave a Reply