RO Water: The Purest Water That Can Harm Your Health!

Manoj Pandey*

The web and social media, and opinions of people at large, are full of contradictory claims about the overall quality of water cleaned by RO machines.

I hope that you already have an RO system at home. RO machines are so ubiquitous now that visitors feel safe in drinking water in hostels/ hotels/ restaurants only if it is bottled water or purified by RO. We also see advertisements on television, print and social media about RO water purifiers that claim to give the purest water on earth and thus keep our loved ones free from water-borne diseases. 

Look at this from the perspective of those who swear by natural lifestyle. Many of such people tend to believe that RO water has been rendered unnatural and this can result in many metabolic diseases. The belief gets support from opinion leaders and influencers on social media platforms who play on people’s fears and doubts, and satisfy them with extreme opinion.

If such contradictory opinions make you wonder whether RO is essential for our health or is it just hype, let me share that most of us get confused until we dig a bit deeper.

Science is value-neutral, and, therefore, let us call upon it to find a dispassionate answer to this question. Before that, let us briefly refresh our understanding of the concept behind RO water purification.

RO – the science of sucking clean water out and leaving the solids behind

There is a process called osmosis that happens all the time in nature. In this, a solvent (say, water) of different concentrations is separated by a membrane, and the membrane allows only the liquid to pass through it but not the solids dissolved in it. An interesting thing happens: the solvent from the thinner solution passes through the membrane to the other side until solutions on both sides become equally thin/ thick. It is this osmosis process that helps cells in our food pipe absorb water when we drink it. 

In reverse osmosis, this process is artificially reversed by putting pressure on the side where the solution is more concentrated. Suppose you have a tank with a tap at its bottom. On the tap’s mouth you put a sieve that has so small pores that only water can pass through them. Now if you fill the tank with water, only plain water (without any dissolved solids) will flow through the tap.

It is interesting that this process, which is nearly as simple as straining water through a sieve, gets this scientific name, reverse osmosis, thus creating a mysterious halo around it. There are two conditions in which this can happen – the membrane has to be so fine that only water can pass through it; and there must be some pressure on water (which has chemicals dissolved in it) so that its molecules are pushed through the pores to the other side.

In RO systems, exactly this straining process takes place, of course with a lot of technological sophistication. The membrane is extremely fine and yet sturdy, and composed of many layers, and huge sheets of this membrane are rolled into cylinders so that large amount of water comes in contact with the membrane. Before the water goes through these RO cylinders, its impurities are removed through filters that have bigger pores. The water pressure needs to be regulated, various functions need to be standardised, and so on. These make the RO system costlier than other water purifiers.

This also explains why RO systems need to throw a lot of water away: when too much of chemicals remains behind and water becomes too salty, the process becomes extremely slow. Therefore, such water needs to be flushed away. This is also the reason why RO membranes need to be replaced after some time: its tiny pores get clogged with solids that are stuck there while trying to go to the other side.

While we are refreshing our technical knowledge, let us talk of TDS, a term dished out by RO sellers and service engineers at the drop of a hat. 

TDS (=Total Dissolved Solids) is the quantity (measured in mg/ litre) of solids dissolved in water.  

All natural water, including rain water, has some solids dissolved in it because water is a great solvent. When water passes along rocks and soil, whether on the surface or underground, it keeps dissolving solids present in the rocks. So, depending upon the chemical constitution of rocks that it encounters, chemical reactions among the dissolved chemicals, temperature and other factors, water can have a high or low TDS.

Please note that TDS only tells us the quantity of solids dissolved in water; it neither talks about the nature of these chemicals nor does it take into account suspended impurities such as mud and germs.

Even as a poor measure of water quality, TDS does serve a purpose from taste and health points of view: too much TDS makes the water salty/ brackish in taste, and if consumed over a long period, it can lead to excess of some minerals in the body. Consuming water with too much TDS also increases the chances of getting kidney stones, and some minor ailments. On the other hand, too less TDS can lead to deficiency of some minerals in the body. The amount of TDS also affects the absorption of water and nutrients by the body.

Though there cannot be a fixed TDS level that can be called the gold standard, experts have come to a consensus on the desirable TDS levels in drinking water. TDS below 100 and above 900 are considered not desirable. The range between 150 and 350 gm/litre is taken as the most desirable. Water of TDS between 600 and 900 too is safe for drinking but it tastes salty.

Let’s now come to the facts and fallacies relating to RO water.

RO means pure water; that is indisputable. 

All naturally occurring water has some salts dissolved in it, and it also has some impurities suspended in it. Impurities are almost always harmful in some way or the other, and they can be easily strained with filters. If there are germs in the water, these can be killed by boiling, chemical treatment or irradiation with ultraviolet rays. 

When it comes to dissolved salts, some of these cannot be removed by straining or filtering. These can, however, be removed by chemical treatment or boiling but not always with high efficiency. Here comes the RO water purifier – one that can remove almost all dissolved salts from water. In addition, unlike chemical treatment (which can leave harmful chemicals in water) and boiling (which is cumbersome and inefficient), RO is simple and highly efficient in removing dissolved salts, and it does not leave harmful residues.

It is understandable that when RO membrane can filter away even dissolved chemicals, there is no question of harmful germs and impurities passing through it.  

RO removes almost all TDS

Most RO systems bring the TDS level to around 100, which – as said above – is not too desirable a level. As expected, RO companies often spread the notion that TDS of 50 or up to 100 is the best level.

Removing dissolved salts can lead to mineral deficiencies in humans, especially if the naturally occurring minerals (such as calcium, potassium and manganese) are not available in sufficient quantities in other food and drinks. Since these minerals are very important for metabolism, their deficiency can lead to weakening of muscles and fatigue, blood pressure not being controlled properly, goitre and many other ailments.

When the water has very low TDS, it can cause harm to our health in another unseen way. Such water tends to dissolve chemicals from the container in which it is stored. So, if it is stored in copper bottles, more copper can leach into water than in the case of normal water – which can sometimes reach undesirable levels. When it is stored in plastic containers, plastic can dissolve into water, causing severe health problems over time.  

Some expensive RO systems compensate for the lost TDS by mixing purified normal water (called TDS regulation) or adding salts (called re-mineralization). In RO systems with TDS regulator, the TDS of RO water is brought back to a slightly higher level by mixing RO water with purified normal water. So, RO companies win both ways: they force us to buy an RO system by scaring us about TDS, and then sell us an expensive machine with the provision of TDS regulator or re-mineralization!

Considering that in places where TDS is less than 500, the use of RO is an unnecessary burden on people’s pockets, the National Green Tribunal had, in 2019, ordered a ban on the sale of RO systems in such areas. The tribunal had also said that RO companies should keep TDS in their water at least 150 mg/l. Interested parties took the matter to the Supreme Court where it is languishing for four years, and there is no chance of it getting resolved soon.  

RO is highly efficient in removing harmful germs and chemicals

As discussed earlier, RO membrane does not allow germs of all kinds to pass through it. However, this may not happen a hundred percent in practice. For example, if the filtered water remains stored in the RO tank for a long time, it might develop germs and mould. Any admixing of ordinary water with filtered water due to breakage, leakage or use of sub-standard material can contaminate the water with impurities and germs.

RO does a good job in removing harmful minerals (such as lead and arsenic). Therefore, in regions where such minerals are found in excess, RO beats other methods of water purification by a big margin. It, similarly, serves well in areas where water is contaminated with pollutants.

More filters in the RO system need not add to goodness of water

A typical RO system has a number of filters so that water does not have suspended impurities before it reaches the RO membrane. These filters are such that they remove the impurities in stages, starting from the gross to the minutest. The RO membrane would not be able to work properly if these filters are not there, and so, if an RO company counts all these filters to show that it takes extra measures to purify water, it is just a marketing gimmick. 

Of course, RO systems may have an ultraviolet filter, and if so, it does ensure that germs that might leak due to some reason are killed.  

Boiling water is good but it has its limitations

Boiling is the most popular traditional method of water purification in homes, and it is highly effective in removing some types of dissolved solids (such as calcium bicarbonate). It can kill harmful germs, but only if the water is boiled adequately well. 

However, boiling of water does not always remove suspended impurities and harmful chemicals. Boiled water also needs to be strained to remove salt deposits that settle at the bottom of the container or float on the surface. Boiled water is also prone to contamination during straining and when kept stored for a long time.

When only purification, not TDS, is a concern

The above discussion leads to some valuable decision points, and one of them is whether one should buy an RO system if the water taken from municipal taps or tubewell is visually clean, its TDS is within desirable range and there is no evidence of lead/ arsenic/ pollutant contamination.

The answer would be to rather buy a water purifier that has a UF (= ultrafiltration) filter and provision of ultraviolet purification. If the water is clean, it is chlorinated by the municipality, and there have been no incidences of water-borne diseases in one’s area, just filtering should be enough. However, to err on the right side, we can boil the water or install an ultraviolet filter..

People living in villages in the hills often use water from streams or springs, and those in other villages use underground water pumped up by hand pumps and tubewells. Such water can have a high level of TDS and some impurities. However, to preserve the natural goodness of such water, the water should not be put through unnecessary purification. A UF filter should be enough for such water, and RO should be used only if the TDS is beyond 900 or so. 

However, in areas where water is polluted, RO is a good option. 

RO system wastes a lot of water

We know that in reverse osmosis, dissolved substances remain on the other side of RO membrane, and these solids need to be flushed away. Thus, the RO machine continuously throws away a lot of water that has high concentration of dissolved solids. For every one litre of RO water, about 3 litres of water goes waste.

Since the reject (=waste water) is full of dissolved solids, it is not good for irrigation. It also cannot be used for cleaning utensils or washing clothes. Even if it is used for flushing purposes, it can cause heavy salt deposits in the water tank and pipes. 

Alkaline, kangen and structured water – more marketing than truth

A number of expressions are in vogue relating to the quality of drinking water. Put this herb in water, or put water in this container; it will become alkaline and develop great healing properties. Treat water this way to improve its structure. Ionize water so that its oxygen becomes active and thus very good for health… 

Such notions are either not true or greatly hyped for commercial reasons. Sometimes such water can become toxic too, due to impurities or excess leaching of metals into the water.

Let us take the example of alkaline/ acidic water. It is true that water’s natural acid-alkaline balance slightly turns towards acidic when salts of calcium and magnesium are removed from it. However, this change is too insignificant to have any impact on the human body. It can be a consideration only in very rare cases when alkaline water is required due to some disease.

Some RO companies, interestingly, take advantage of this technical fact that RO water is slightly acidic. They sell more expensive systems in which water is re-mineralized and thus becomes alkaline. 

To RO or not to RO?

All things considered, the RO is an excellent system for removing excessive TDS from water and also making it absolutely clean. However, it need not be used where water does not have high TDS levels. Other less expensive water purifiers often serve the purpose equally well, and boiling water is also an easily-available option. 

If you are considering buying an RO machine, keep in mind that it will waste a lot of water, and will need regular servicing/ change of filter(s). Also check whether you need the extra features available in expensive models. Do not let the TDS level go too low; and do not store RO water for too long, especially in plastic bottles.

You can watch a detailed video on the working of RO systems, in which many myths relating to RO are debunked in simple हिन्दी, at this link:

*Manoj Pandey is a former civil servant. He does not like to call himself a rationalist, but insists on scrutiny of apparent myths as well as what are supposed to be immutable scientific facts. He maintains a personal blog, Th_ink

Disclaimer: The views expressed in this article are the personal opinion of the author and do not reflect the views of which does not assume any responsibility for the same.


  1. Congrats Manoj ji,
    You have written the article in a lucid style and everything has been explained from a layman’s point of view

  2. Exhaustive, rational, fluent and explained in simple language. It busts many myths, it takes us to maire informed level, and yet so impartial.


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