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 impurity Source of contamination Commonly found in Effect on drinking water
Undissolved solids such as sand and mud Loose soil or sand mixing with flowing water River water or piped water where pipes are damaged Muddy or turbid appearance
Dissolved inorganic salts like Sodium & Potassium Salt deposits from surface Bore wells and seawater Salty or brackish taste
Dissolved inorganic compounds like Calcium & Magnesium Water flowing over rocky terrain Bore wells and river water Hardness and scale formation
Organic compounds Absorbed from plant life growing inside water body Lakes and ponds Foul smell or odour
Decontaminants like Chlorine Added by municipal water suppliers to kill microorganisms Municipal piped water supply Bitter taste
Microorganisms such as viruses & bacteria Biological contamination of water source Piped water where pipes are damaged Biological 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 content Is water muddy? Is water biologically contaminated? Which purifier to use?
Low (below 300 ppm) No No Tap/faucet or storage filter
Low (below 300 ppm) No Yes UV
Low (below 300 ppm) Yes No UF
Low (below 300 ppm) Yes Yes UF or UF+UV
High (over 300 ppm) No No RO
High (over 300 ppm) No Yes RO+UV
High (over 300 ppm) Yes No RO+UF or RO+UF+UV
High (over 300 ppm) Yes Yes RO+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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484 Comments

  1. anjalianjali08-15-2017

    hey, you are a very good writer.thanks for sharing all this information. I wanna know how many liters of purifier is okay for 5 member family in a 3 bhk flat.

    • Vijay PadiyarVijay Padiyar08-16-2017

      An average family member will drink 1.5 liters of water per day. So a 7-8 liter tank capacity is adequate. You will need to refill it once a day.

  2. Aswin k jenaAswin k jena08-13-2017

    Hi,nice post,help me a lot,regarding where to get a tds meter is a matter of concern

    • Vijay PadiyarVijay Padiyar08-13-2017

      You can order it online. Most of the e-commerce websites have it.

  3. UpendraUpendra08-10-2017

    Great article. Thanks a lot for sharing with the world. Saved me and definitely many more from considering RO with blind instincts.

  4. AshAsh08-10-2017

    Hi Vijay – meticulously written and explained.
    thank you! quick question – should i prefer a (ABC Brand) RO + UV (with a 6-stage filtration process), or an RO+UV+UF (DEF brand) – price not being a constraint. the key point being the inability of the UV to flush out the dead bacteria, a step that the UF typically addresses.
    Would you think RO+UV would be good enough. also is Micro Filtration similar to UF. thanks!!

  5. José RocaJosé Roca08-07-2017

    Hi Vijay,

    I would like to know how long can a system be stored after use before it develops some kind of biological contamination.

    I’m thinking of acquiring an RO system for a vacation house that may be without use for months at a time. Could the membranes or carbon filters develop any contamination?

    The article is great!

    • Vijay PadiyarVijay Padiyar08-07-2017

      It’s not safe to store water in a RO purifier for such a long period. You can empty the tank before leaving so that there is no water to contaminate. Also turn off the inlet tap and leave the system on for some time so that all the water in the system is sucked out. Before reuse, you can flush the system by passing water for some time so that accumulated contaminants are removed.

  6. RAMESHRAMESH08-04-2017

    I have TDS 850 mgpl in my feed water ,what should I use whethere,UV FILTERATION,RO AND WHAT IS the best method for removing DISSOLED SOLIDS ,microorganism and bacteria.

    • Vijay PadiyarVijay Padiyar08-07-2017

      It helps to read the blog before posting questions :). You will find that your question is already answered.

  7. Mitul GabaniMitul Gabani07-31-2017

    I m use to water with 400 tds..is it harmful our health or not??? please give me drawback or benefits of 400 tds water

    • Vijay PadiyarVijay Padiyar08-01-2017

      Your question is already answered in this blog post. Did you read it before posting your query? 400 ppm TDS is technically within safe limit but not very healthy.

  8. NishaNisha07-29-2017

    Thanks Vijay! This was a great read! I am looking for a very specific application – a water softener for my coffee machine, to remove the ‘temporary hardness’ from calcium that could cause scaling in the boiler. My current water supply from the municipal corporation is 50-72 ppm as measured by TDS meter. The manufacturer recommends lower than 50 ppm. I use a UF solution (Unilever Pureit)which does not impact TDS. Do you suggest upgrading to an RO system – but I feel it’s overkill? Or is there a specific water softening methodology you would recommend? My machine is not plumbed or connected to a tap, but refilled normally. Thanks in advance for any help you may give me!

    • Vijay PadiyarVijay Padiyar07-29-2017

      There are small water softeners available for use with individual appliances. Check the regular water purifier manufacturers’ sites. You will see the products.

  9. Amit D.Amit D.07-20-2017

    Hi Vijay,

    Thanks for sharing such a valuable information which we can’t get online at single place.
    By reading article, it was clear to me that I will need UV filter to be installed at my home.
    I have filtered three models from two vendors aquaguard and pureit. Can you please share your recommendation which model is best of three?
    Dr. Aquaguard Magna NXT HD+UV
    Aquaguard Superb UV+UF
    Pureit Marvella UV

    • Vijay PadiyarVijay Padiyar07-20-2017

      Thanks Amit! But sorry, I don’t provide specific product recommendations. You can do your own analysis and choose one of them.

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