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

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

Pros:

  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).

Cons:

  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 →

233 Comments

  1. Prasanna RPrasanna R08-17-2015

    Hi,

    Very good … I would like to appreciate you for the wonderful job you have done here clean clear and perfect thanks for making thing so clear.

    Must read article thanks again

  2. MarutiMaruti08-17-2015

    Hi Vijay,
    Amazing article with numbers, images, links! It was great help for me.

    Regards,
    Maruti.

  3. NARASIMHANNARASIMHAN08-12-2015

    1.Is it possible can we add separately minerals
    2. As u said if it is 30 to 40 ppm u can retain some minerals but we are not sure 100℅ so in which way we we can gauge the minerals level.
    3. Please recommend some standardized type n model for every one to use without checking TDS is there a product for that ?

    • Vijay PadiyarVijay Padiyar08-13-2015

      Instead of adding minerals it is easier and better to simply adjust the output TDS value and increase the output TDS to the recommended levels.

  4. VEDVED08-09-2015

    Hi Vijay, would you recommend to have a Zero B water purifier ?

    • Vijay PadiyarVijay Padiyar08-10-2015

      Sorry I am not aware of the feedback on Zero B purifiers (I assume you mean their RO model).

  5. PadmaPadma08-03-2015

    Hi Vijay,

    Happy to read your article. thanks for sharing. I have one question: I have a Tata Swacch. How effective is it, since its non-electric but has a filter and a silver nano technology to remove chemicals and liquid pollutants? Would you recommend a electric one generally speaking?

    • Vijay PadiyarVijay Padiyar08-03-2015

      Electric models are required to remove bacteria and soften hard water. If you don’t have water contamination or hardness issues then a non-electric water purifier or filter is fine.

  6. SreedevSreedev07-31-2015

    Hi Vijay
    Current TDS level of borewell water in my house is 630 and I have purchased RO + UV water. After that TDS level reduced to 160. Will it safe for drink?

    Thanks

  7. Pushpendre GuptaPushpendre Gupta07-30-2015

    Hi Vijay..I live in Gurgaon sector 17c. We are currently using the simple non-electric water purifier. How safe are these ones please tell. Your blog only describes about the wall mounted electrical ones.

    • Vijay PadiyarVijay Padiyar07-31-2015

      Non-electric models mostly remove sediments. They don’t reduce TDS or kill bacteria (generally speaking).

  8. JaveedJaveed07-11-2015

    I have bore well at my house which water purifies I must use. Can I use a simple within 5K water purifier or do I have to go for an expensive ones pls suggest.

    • Vijay PadiyarVijay Padiyar07-12-2015

      No one can answer this question. You have to measure the TDS content of water and then decide accordingly.

  9. pradeep kumarpradeep kumar07-09-2015

    Dear Vijay,

    In our house, Cauvery water is of 120 ppm and bore well water is around 350 ppm. When we mix and pump it to the tank, it is around 150 ppm. What should we use – ro or uv ??

    Regards

    Pradeep

    • Vijay PadiyarVijay Padiyar07-10-2015

      You have two options: (1) use a RO+UV purifier for the mixed water, or (2) use a water softener system for the borewell water (before pumping into the tank) and then use a simple UV purifier.

  10. ayaanayaan06-30-2015

    hi my name is ayyan my tap water tds is 254 which purifer I should use.

  11. HemanthHemanth06-23-2015

    Dear Vijay,

    It’s Hemanth again. Today I checked my Water Purifier and found it is only RO.
    Please suggest if only RO (Acquaguard Reviva) is Fine for bore well water stored in water tank.
    My worry is TDS levels of 1200-1400 being used for drinking purpose after only RO treatment.
    Please help

    • Vijay PadiyarVijay Padiyar06-23-2015

      As I said earlier, please check the output TDS and verify if it’s within the acceptable limits as mentioned in my blog post. If yes, then you don’t need to worry. UV is required only if you suspect biological contamination of water.

    • HemanthHemanth06-24-2015

      Thank You Vijay.

      The TDS after filtering water through RO is 70 (Output TDS).

      Your blog says beyond 500 TDS the water is not fit for drinking.
      So just to understand correctly.
      If my output TDS irrespective of input value is below 100 is the water safe for drinking?

  12. HemanthHemanth06-22-2015

    Dear Vijay,
    Is it safe Using RO+UF+UV filter for TDS levels of 1300? Will the high TDS levels affect the filtering effectiveness?
    I would like to know about the loss of minerals during RO filtering. Remedy for it.
    One of my friend has suggested to add one small bottle cap of Raw Water in the Purified water to avoid loss of minerals. What do you suggest?
    Also please suggest me how to check if the BOREWELL water in my locality is safe after RO process?

    • Vijay PadiyarVijay Padiyar06-23-2015

      You can use RO purifier with 1300 TDS water. The life of the membrane may be slightly reduced but it will work. Loss of minerals can be adjusted by setting the output TDS to the desired value. You can check the output TDS using a TDS meter.

  13. Davinder Pal SinghDavinder Pal Singh06-20-2015

    Hi, Vijay
    is there any kind of difference in the filters of Branded and Local water purifiers ?? Even when i asked from a mechanic, he said that both the local and branded companies use same water filters……is it true or not ??
    If there is any kind of difference between the filters of Branded and Local water purifiers ???
    And second question is some mechanics say that assembled water purifier is better option than branded water purifiers……..what would u say on this ???
    Shall be thankful to U….
    Thanx

    • Vijay PadiyarVijay Padiyar06-21-2015

      The semipermeable membrane is the most important part of RO purifier. Local manufacturers may not use the same quality of membrane as branded manufacturers. Do check up on this part.

  14. babubabu06-20-2015

    is it all right to use RO filters if you have regular Cavery water supply ? i mean does the water lose any nutrients/minerals value or taste if RO system is used.

    • Vijay PadiyarVijay Padiyar06-21-2015

      Decision on whether to go for RO or UV should be guided purely by TDS content of water and not by the source of water (Cauvery or bore well).

  15. santoshsantosh06-16-2015

    Hello Vijay,

    Thanks for the article.
    The Cavery water supplied to my home in Bengaluru has TDS of 150. The same water is pumped to overhead tank. TDS of water from overhead tank is 170. What kind of purifier I must buy for a clean and healthy potable water? UV+UF will suffice or RO+UV+UF is required? Please advise.

  16. pradeep christianpradeep christian06-10-2015

    Hi Vijay,
    Indeed very helpful post. But still I am confused a bit. I am living at Gandhinagar (Gujarat) and we are provided water of Narmada Canal by the authority. And water contains 215 tds level. I intend to buy simple RO system but I need your advice so kindly help.

    • Vijay PadiyarVijay Padiyar07-14-2015

      As is clearly mentioned in my blog post, for TDS between 150-300 ppm, you can go for either UV or RO+UV. The latter will give a sweeter taste but the former too is adequate.

  17. vanithavanitha06-09-2015

    Hi vijay, I wants to know the work of TDs controller. We are having 60-70 as TDs in our RO system. Is it give enough minerals ( calcium and magnesium) for our body?

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