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

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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!


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489 Comments

  1. ShravanShravan07-19-2017

    Hi Vijay,

    Do UV filters need to be attached to tap which draws water from overhead tank, or can it just work as table top /gravity based filter too ?

    Thanks,
    Shravan

    • Vijay PadiyarVijay Padiyar07-19-2017

      UV purifier works on running water, so it needs to be connected to tap. It can’t be used on stored water.

  2. Giridhar LGiridhar L07-09-2017

    Thanks from bottom of my heart. You helped us decided. We have 250 TDS in our bore water. After reading your blog and other. We have decided to go for UV+UF, NO RO as we have smelly water and which becomes muddy during rainy season. Any options to stop mud flowing into filters. Some thing cheaper than UF filters, so that they become first cleaners. I have heard of sponge, how good are those. Thanks again Sir.

  3. ansariansari07-08-2017

    hello sir i stay in hyderabad i was using Green RO from 7yrs because of the tap water tasted salty, but recently i change to pure it classic, because of health issues with RO. Now my tap water tds is 287 it is tasting salty is it safe to drink or uv uf combination filter wil be better.

    • ansariansari07-08-2017

      and if RO giving out put tds 174 wil the essential minerals like calcium, magnesium etc wil remain in it or RO wil waste it out…

  4. kunalkunal07-04-2017

    very helpful artcle

  5. RukhsarRukhsar06-29-2017

    Hello Sir,
    I want to buy Water purifier and i an confused Plz Help me

  6. Alok SahaAlok Saha06-22-2017

    I am having a Aquagard water purifier without RO. This I bought during my stay in Mumbai, and it was working good as the tds of Mumbai tap water was low.
    Now I had shifted to Thane. Here the water I gate is high on TDS. Is there a attachment available in market to add a RO upstream of my present Aquagard water purifier.

    • Vijay PadiyarVijay Padiyar06-22-2017

      You can buy a RO-only purifier and attach it upstream of the UV purifier, however cost of RO-only purifier will be almost same as RO+UV. So better to sell your UV purifier and buy a RO+UV one.

  7. Majid DhokleMajid Dhokle06-22-2017

    is there any purifier in which we can switch off RO system and use UV+UF? whenever Municipal drinking water is not available we can use RO system to filter borewell water

    • Vijay PadiyarVijay Padiyar06-22-2017

      You can adjust the output TDS using TDS controller. If the output TDS is same as input TDS then RO will be bypassed and it will effectively become a UV+UF purifier.

  8. AniketAniket06-08-2017

    Thanks for writing such an informative article in easy to understand terms. Helped me understand and select the correct purifier for my need.

  9. sridharsridhar05-25-2017

    Hi Sir

    We use Manjeera water (Supplied by Municipal) . No Mud but not sure wat they mix . The TDS level is 200 . From above my understanding is that Tap facet is good enough for my case.Please advise

    Thanks in advance

  10. KarthickKarthick05-23-2017

    Hi,

    I’m from Bangalore. We are getting tank and borewell water. Till date we are using CAN water for drinking purpose. Tank/borewell water TDS is around 700. CAN water TDS is around 35.
    Aquaguard guys are pushing to buy Aquaguard NXT HD RO+UV. It cost around 19k. Online has mixed reviews. Please guide me in buying a good water purifier.

    Thanks,
    Karthick

  11. Sushmit ShahSushmit Shah05-22-2017

    Hello Vijay Sir,

    First of all, thank you very much for such detail and vital information on water purifier.
    I am planning to install water purifier in my home. Purifier will be installed on corporation water whose TDS level is 150-170. got checked with TDS meter. The representative suggested to install RO+UV+UF+TDS controller purifier as TDS is very high. I am really confused now. Please help.

    • Vijay PadiyarVijay Padiyar05-22-2017

      Upto 150 ppm RO is not required. Between 150-300 ppm it is optional but not necessary. Choice is yours.

  12. ANIMESH PANDYAANIMESH PANDYA05-12-2017

    dear sir,
    good information about ro+uv+uf I have shift in new house in Vadodara builder provided water provided though bore well the TDS is 750 and but water is not contaminate which
    water purifier is better

  13. DJDJ04-25-2017

    Hi Vijay,
    Just want to understand the following:
    Company X in India use RO+UF+UV+TDS technology. But Ro and UF is parallelly working and not in series.
    Company Y uses RO+UF+TDS+MES technology in series.
    Now, What is the point using RO and UF parallel by Company X ? and why Company Y is using the same in series and adding MES technology ?

    • Vijay PadiyarVijay Padiyar04-25-2017

      UF and RO are in parallel for controlling output TDS. UF does not reduce TDS, so output TDS can be adjusted by adjusting the ratio of RO and UF water in final mix. I am not sure what is the purpose of having RO and UF in series.

  14. DivyDivy04-20-2017

    The TDS of water in our home is 330. Its not muddy. Looks clear. What should i use ? RO or UV ?

    • Vijay PadiyarVijay Padiyar04-20-2017

      Your question is already answered. Please refer to the table in the blog post.

  15. Pankaj sharmaPankaj sharma04-14-2017

    Good article

  16. RickRick04-07-2017

    Dear Sir,

    Your article is a really helpful one regarding water purification system. I am aware of the TDS level for my locality input water feed i.e slightly more than 700 ppm. I do have a UV based machine which is running good with AMC. But still I hope the UV machine is not sufficient to reduce the TDS level of the aforesaid water. So I planned for a standalone RO machine without quitting my UV machine. I want to store 20 litres of water through my UV machine and that stored water will be independently feed to a 6 litres storage type RO machine. I am also aware that RO machines do reject some water during filtration process, so if feed water through 20 litres storage tank above the RO machine it should provide atleast 12 litres of water from that independent storage?
    Moreover can you please mention what should be the input pressure for a RO machine?
    If I am not wrong I think all types of RO purifiers come with a booster pump to balance the RO membrane feed, otherwise in low pressure it won’t work!
    Kindly revert back with your suggestion.

  17. Swarnali MandalSwarnali Mandal03-19-2017

    I am confused to choose water purifier. We use personal boring water. How do we know the level of TDS easily by ourselves? Please suggest… Thanks

    • Vijay PadiyarVijay Padiyar03-23-2017

      It is already mentioned in the blog. You have to use a TDS meter. It costs about Rs. 200 – 500.

  18. C.k.SreemohanC.k.Sreemohan03-16-2017

    Our drinking water supply is augmented with borewell water at present we are using u.v.water.since I understand that Delhi s under ground water supply is contaminated with heavy metals I am thinking of installing r.o.filter eventhough our t.d.s.level is 346.if we install one with r.o.+ u.f. is it alrigt.can the t.d.s level be adjusted without the help of a technician .

  19. Simon SawSimon Saw03-10-2017

    Thanks Vijay Padiyar Ji for your detail input on water purifiers. Indeed really helpful to decide one to purchase being very clearly explained. Thank you sooooo much.

  20. V vjoshiV vjoshi02-24-2017

    With borewell water about 1000 TDS what is estimated quality by using softening plant

    • Vijay PadiyarVijay Padiyar03-03-2017

      Water softeners do not reduce TDS, they replace calcium and magnesium ions with sodium so the water feels better for applications like washing clothes, bathing, etc. For drinking purposes, a RO purifier must be used to reduce TDS.

  21. yuneshwariyuneshwari02-24-2017

    hello ,
    i am yuna, in my home we fix bore . my parents thought to fix water purifier.what purifier is best and the water also good. till now the bore water use only other purpose but no drinking. please reply.

  22. shwethashwetha02-22-2017

    thanks a lot for giving such a good information, and my question is that in our house we are getting a water which is not in sweet taste it is like salty one so if i want to get sweet water from this salt water which water purifier we should buy ie UV, or UF or RO etc.

    • Vijay PadiyarVijay Padiyar02-23-2017
      • PawanPawan02-26-2017

        Sir great knowledge sharing but my question is that ro/uf/uv treated water also remove natural minerals of water also or not? And if yes than as per your experience which is the best purifier which add the natural mineral

        • Vijay PadiyarVijay Padiyar02-27-2017

          Your question is already answered in my blog post. Please read.

  23. Arun MehtaArun Mehta02-13-2017

    Can one shut RO in a RO+UV water purifier. I believe that RO is unnecessary and loses out on mineral from the water. Can we avoid RO from the machine. Vijay, your thoughts please….

    • Vijay PadiyarVijay Padiyar02-15-2017

      Yes this guide is written to discourage people from unnecessarily buying RO when UV is sufficient. You can shut out the RO module to some extent by increasing the output TDS using the TDS controller. But best option is to avoid buying RO in the first place.

  24. Rajeev SRajeev S02-10-2017

    Hi,
    want installed RO plant, But which water to connect the RO?
    i have borewell water
    and corporation water. Is it okay to connect borewell water its ppm is 445ppm ? or should connect corporation tap , to get more life of RO membrane?

    Thanks

    • Vijay PadiyarVijay Padiyar02-11-2017

      If you have corporation water then use that for drinking purposes. Use a water softener for borewell water and use it for other domestic purposes. The lower the water TDS, the longer will be the life of RO membrane.

  25. Vinit MishraVinit Mishra02-10-2017

    Respected sir,
    Your article is very help for me I’m very oblige to you for such real n good knowledge about purification tech.

    Please tell me about NF (Nano filtration) ?
    The TDS of my ground water is 265TDS.
    Any product in aquagard or any other which have NF
    Aquagard total NF is not in market 🙁
    Is Permionic.com product Pureflo NF is good ?

    • Vijay PadiyarVijay Padiyar02-12-2017

      Nanofiltration is nothing but filtering water using a finer membrane than ultrafiltration (but still not as fine as RO membranes). I am not aware of any nanofiltration-based products currently available from the major water purifier brands in India. Perhaps they may launch in the near future. I am not sure about Permionic.

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