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 →

303 Comments

  1. k v k nagarajuk v k nagaraju02-07-2016

    dear vijay ,
    Namaste your article is quite informative for buyers of purifiers instead of listening to words and stories of sales personnel blindly one can judge at one,s own . our ground water forms scale on water heaters etc . what type of purifier you suggest among ro, uv etc.
    nagaraj k v k
    ap

    • Vijay PadiyarVijay Padiyar02-08-2016

      You can install a water softener for your house. This will reduce the hardness of water and is adequate for non-drinking purposes.

  2. kritikakritika02-06-2016

    Hi Vijay,

    The TDS level of water supplied to my place is more than 500ppm and I’ve a Eureka Forbes UV system installed at my place. I want to upgrade the system from UV to RO. Can you please guide on this? Is it okay to go for this upgrade or shall I buy a new RO sytem.

    • Vijay PadiyarVijay Padiyar02-08-2016

      You can’t upgrade an existing purifier. You can buy a RO-only purifier and use along with the UV purifier, but the price of RO-only purifier is almost same as RO+UV.

  3. s faizi akhtars faizi akhtar01-31-2016

    Dear Vijay,
    Thanks for an informative article. I am from Muzaffarpur, Bihar which is on the banks of Gandak river.The underground water in our town is high in iron and calcium content. The borewell water (300ft deep) at my residence which is having a TDS 500ppm. We get rust colour deposition and scales in our utensils and water containers. Currently we are using USHA BRITA storage water purifier for drinking water. Kindly suggest as to what should be done so as to control the high iron & calcium.
    regards

  4. s v devs v dev01-29-2016

    Hi Vijay, I stay in Kopargaon dist ahmednagar Maharashtra Town is on the banks of river Godavari The Borewell water in my bungalow has 2300 tds Overhead water tank is at 25 ft height Please advise which of the standard/branded water purifiers would be best in quality, maintenance & after sales service at door. Awaiting reply for final decision Thank you. With blessings of Lord Sai

  5. manishmanish01-16-2016

    HI, Vijay,

    I found local brands like Aquafresh etc are much much cheaper than branded one’s like kent , Aquaguard, I’m doubtful if there is any duplicacy and compromise with water quality after filtration RO+UV if uses above local one’s available in the market . & whats could be the leverage to add on mineral cartridge recommended by the supplier . Pls advice .

  6. SrikanthSrikanth01-16-2016

    Hi, Vijay, I am planning to buy eureka Forbes aquasure RO , i stay in bangalore, T R Nagar, since i stay in rented house, here
    the water we get is bore well & bwssb water mixed, I don’t know about water ratings. Could you please suggest, will it be a good choice?

  7. PREMPREM01-09-2016

    i have government water supply having tds of 60.
    which one to go
    1. UF + UV
    2. UV
    what is main difference?
    Does water force matter?

    • Vijay PadiyarVijay Padiyar01-10-2016

      UF filters out microorganisms and impurities from water whereas UV kills microorganisms. Often municipal water can be muddy or dirty, so UF+UV might be a better choice than UV. Water force doesn’t matter much for UV or UF+UV.

  8. JeetJeet12-31-2015

    Thanks.

  9. Narayan KumarNarayan Kumar12-25-2015

    Dear Vijay ,

    Thank you so much for the wonderful info on potable water . we are getting metro water in our taps .thinking of not buying bisleri bubbletops. Do I need to do a TDS check or send water to some labs for analysis . kindly advice whether I need to go for UV filtration or RO .

    Thanks & Regards

    narayan

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