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FAQ - Reverse Osmosis (RO) systems - Advice to all those thinking of installing RO systems

Over the months, one has seen, in this section dealing with TDS in RO Systems, numerous queries which indicate that the general public is not clear as to what the term means and how relevant an RO system is to them. One is astounded to find that companies manufacturing RO systems are advising people to install RO systems for water with TDS of 200 and 300 ppm to bring it down to 10 or 20. In other cases with higher TDS, they first reduce the TDS to below 10 or 20 and then advise them to have a valve through which they can add some quantity of the raw water in order to improve palatability. Cases of such advice are so many that a note is called for on what RO is, its efficiency and when at all it is needed in a domestic environment.

What is the desirable quality of drinking water?

As per the Bureau of Indian Standards, the desirable quality of drinking water is that which has TDS (Total Dissolved Salts) content of 500 ppm or less (ppm stands for parts of the salt present in a million parts of water). Where water of this quality is not easily available, the compromise level is water having upto 2000 ppm.

It is to be borne in mind that in some places, iron salts may be present and if the content of iron salts is more than the permissible 0.4ppm, even if the total salt content is less than the desired level, the iron salts will have to be removed before drinking that water. There are also some pockets West Bengal and U.P. where the water contains Arsenic. This is poisonous and so here also the same rule applies. In some pockets again, fluoride salts may be present which affect the bones if that water is drunk. Using this water for non-potable purposes is however not harmful.

Another point to be remembered is that water with very low salt content is not very palatable and therefore where the total salt content is less than 500, reducing it to 10 or 20 by RO is not only meaningless from the point of view of wastage of water but also from the cost and loss of palatability aspects. In cases where the salt content is not much higher than 2000 ppm, a simpler route would be to harvest rainwater which will dilute the salts and bring it within potable limits progressively.

What does the Reverse Osmosis (RO) process do? What is the mechanism?

RO is a process where water having more than the desirable salt content is put in one part of a vessel with two compartments separated by special media and pressure is applied on the water. This results in only the pure water going across the media to the other compartment with the salts remaining in the same compartment. Thus the process results in accumulation of salts in the first compartment. Beyond a certain concentration of salt the process will not proceed further and the water which contains all the salt is rejected.

Because of this, if one starts with say 100 parts of water, the process yields only about 70 parts of good water and the other 30 parts which contain all the salts present originally in 100 parts have to be thrown away.

What can we do with the RO reject water?

In RO systems of small capacity suitable for domestic purposes, the rejected component may be as high as 45%. The process therefore is a wasteful one with much of the water having to be thrown away.

Diverting large volumes of this highly salty water into the sewage line could result in acting against the smooth movement of its contents. The reject will not be tolerated by normal garden plants. It will form deposits on the floor and sanitary ware. It is also not advisable to divert it to the septic tank.

If the water subjected to RO has less than 1000ppm say, then the salt content in the reject water will not be much and it can be used for gardening or flushing. But the point is that this water need not be subjected to RO at all in the first place.

Points to consider before going for a RO purifier

  • RO is to be resorted to only in cases where the salt content of water to be used for drinking is much higher than advisable.
  • Even here reduction of the salt content to the level of 10 or 20 ppm is counterproductive. If the salt content of the water is very high even for non-potable purposes, rainwater harvesting often works wonders.
  • In the cases where the water contains coliform bacteria, the source for their presence should be traced and the contamination eliminated. While RO may be advised, elimination of the cause is the safer and preferred route and ultimately the cheaper route also.
  • Those who go in for RO for water with high salt content are well advised to assess the volume likely to be subjected per day and ask the supplier how long will the media work effectively with that volume, what is the cost involved for the replacement of the media and what are the monthly running costs, apart from the capital cost.
  • They also should question any proposal to reduce salt content to less than 500 ppm.
Name of Author: 
Indukanth Ragade

6. Need details of water testing labs in Chandigarh

I am from Zirakpur, Mohali. Need information on where we can get water tested for its mineral constitution in Chandigarh?

5. TDS level 80 ppm after RO, still with a bitter taste- Need help

Hi all,

First of all thanks for providing valuable information. Our locality ground water TDS is measured as > 2000 ppm. So I am using RO equipment to reduce TDS, and TDS level after RO is 80ppm. People suggested 75 to 150 ppm is safe to drink.

Even with 80 ppm also I am feeling the water has a bitter taste. Can anybody please explain if there is any relation between TDS and taste of water?


4. Impact of this on health depends on the nature of the salts

Mr.Kulloli mentions about adjustment of the human body to certain quantities of salt. People in some parts of Africa have been drinking water with 3000 TDS having no alternative. While the human body, no doubt, gets adjusted to salts, the impact of this on health depends on the nature of the salts consumed.

Regarding the disposal of the reject component from large scale RO systems, the tendency is to dump it in the sea which can have an impact - a localized impact on marine life.

Indukanth Ragade

3. Feedback

Dear Dr. Ragade,

Thanks for wonderfully information. However I was worried about the concern raised on management of left overs after RO process mainly with large plants. I was wondering any options/practices available for the same. I believe, our biological system is adjusted to certain quantity of salt present in the water. Sudden change in the water quality has its own side effect.

Thanks and regards

2. It is great to know that you have openly said the truth

Its good to read that you have openly said that there is no need to install RO for TDS less than 500 ppm. You are right by saying that people continue to sell RO system that gives water at a TDS of 20 (meeta pani) which is good for health.

Definitely, the general public need to be advised through mass media against such false information

1. FAQ - Reverse Osmosis (RO) systems - Advice to all those thinkin

 Dear Dr. Ragade,

Thank you for a comprehensive and informative article.

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