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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424 Comments

  1. AjayAjay11-24-2016

    Very Nice Article Vijay. Thank you for sharing.

  2. RohanRohan11-19-2016

    Excellent. Please suggest what treatment is suitable for water to be used in cement bricks plant. Would softener be suitable even in case raw water exhibits permanent hardness?

  3. ThiruThiru11-18-2016

    I have been using aquaguard Enhance (RO+UV).
    Now shifted to another house where TDS is 2300.
    Can I use same purifier here or not?

    • Vijay PadiyarVijay Padiyar11-18-2016

      Check the product manual. It will tell the maximum TDS that it can work with.

  4. Deepak TiwariDeepak Tiwari11-17-2016

    Hi Vijay,

    Your article was excellent. Thanks a lot for all the relevant informations.
    I would like to know for services our RO / UF Purifier, which membrane / filters (Sediment, Pre and Post Carbon) are good, because lot many brands are available but all are fake. On which spare parts we have to trust ?

    Regards
    Deepak

  5. VishalVishal11-15-2016

    Hats off to the author / Vijay. Great article.
    From the comments, looks like people either don’t read the entire article or do not understand it. All the questions can be addressed if the article is read thoroughly. If people don’t understand English, how did they even land on this article in the first place.:)
    – Vishal

  6. Arpit SuranaArpit Surana11-13-2016

    Hi Vijay ,

    Thanks for advance for providing best answer and solution confusions regarding RO.

    My question is we are recently shifted in new house where society provides borewell 700 TDS water and the same is also use for drinking . i want to know which RO is suitable for me i.e. RO+UV, RO+UV+UF,RO+UV+UF+TDS controller ,

    which RO company is most trustable or we can purchase it from local market like Aquagrad and Aquagrad plus.

    who many times cycles for change membrane and other filter and others.

    Thanks

    • Vijay PadiyarVijay Padiyar11-13-2016

      Everything is clearly explained in the blog. Please get a lab test done for your water and then choose the appropriate type of filter from the table provided in this blog.

  7. SimmiSimmi11-08-2016

    Vijay,
    TDS in my water is around 180 but keeps fluctuating.. Which purifier should I choose??
    RO or UV + UF??

  8. namranamra11-07-2016

    hi
    Can you plz guide me for the water which smell bit odd due to some iron/any other content.
    Kindly guide me whether I should go for RO+UV+UF or only RO+UV or only RO water purifier?

    • Vijay PadiyarVijay Padiyar11-08-2016

      Everything is covered here, please go through the entire article first.

  9. Aparajita DasAparajita Das10-27-2016

    Hi sir in my home water tds level is 270. After storing of water the lower part of the water it seems like some small brown things. For which purifier shell I go pl help me.

  10. sanjay guptasanjay gupta10-25-2016

    Please confirm, ultra filtration can purify the water without removing water’s natural minerals or not because its similar to RO membrane.

    please suggest which is best purifier for MCD water with 125 TDS without loosing water minerals. Purifier with UV or purifier with UV+UF.

    • Vijay PadiyarVijay Padiyar10-26-2016

      If you read the blog post it is clearly written that UF membrane cannot remove TDS and therefore UF is suited for soft water only. Please refer to the table to decide whether to buy UV or UV+UF.

  11. RKRK10-24-2016

    Hi,

    RO+UV is better or RO+UF for borewell water?

    • Vijay PadiyarVijay Padiyar10-25-2016

      Please refer to the table in the blog post and decide the suitable purifier for your case.

  12. RajendraRajendra10-24-2016

    Hi,
    Can u sujjest me a water purifier whrein we have both corporation water and also borewell water supply aternatively. So this need is a good water purifier for a residential purpose.

    Will be greatful for your suggetion.

    tegards

    • Vijay PadiyarVijay Padiyar10-25-2016

      All the information you need to choose the proper water purifier is here! Read the blog post.

  13. SherelSherel10-21-2016

    Very Informative and elaborate description of water purifiers and its work. Thank you.

  14. riddhimariddhima10-17-2016

    Very Nice.. You are RO and UV water purification technology is effective. Thanks for sharing this blog..

  15. sheetal agrawalsheetal agrawal10-17-2016

    Its Indeed a very helpful articles sir. Thanku soo much. Going for RO +UV.

  16. Sachin MehlaSachin Mehla09-27-2016

    You have done an excellent job Vijay. I have done my part of google with regard to water purifier however was not satisfied unless i have come across your article.
    Self explainatory.

    Thanks Man

  17. Srinvasa ChaitanyaSrinvasa Chaitanya09-26-2016

    Very Informative Post Vijay Padiyar Sir,

    As you mentioned in the post, an RO water purifier is the only one that is capable of removing the Hard metals from the water. So, we have to opt for an RO water purifier, if the TDS levels in the Ground water is high. I am awate of RO, UF and UV purifiers. Recently, Whirlpool has launched Destroyer water purifier that works with EAT technology. They claim that their non-electric water purifier can work with TDS levels up to 500 ppm. What’s your say on this!

  18. Faizi AkhtarFaizi Akhtar09-25-2016

    Hi Vijay
    Very informative blog…. I seek your help in choosing a water filtration unit and got my borewell water test reports for the same.Send me your mail id so I may attach it for your perusal.
    Regards

  19. AdityaAditya09-25-2016

    Very nice elaboration Sir,
    Cleared all doubts.

  20. viralviral08-26-2016

    Hi i am using Pure It RO Plus UV..municipal water is showing TDS 56 and after filter its is 7..is the TDS level low and harmful

  21. MeenakshiMeenakshi08-23-2016

    Hlo sir can u suggest me a good water softner ?
    May i use ro+uv water to wash my hair?
    Here tds level is 800 ppm

    • Vijay PadiyarVijay Padiyar08-23-2016

      Sorry I can’t recommend any brands. You can check the market for available products. RO+UV is unnecessary for washing purposes. Simple softened water should be enough.

  22. ManishManish08-14-2016

    hi Vijay,

    I just bought new flat where no corporation water available currently until we get completion certificate. so for First 1 year I’ll have borewell water ( TDS 500+) after that Corporation water will be available (~150-200). Please suggest best purifiers so no need to change after one year when corporation water will be available. Also don’t want to compromise on water taste and minerals.

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