Frequently asked questions (FAQ) on Reverse Osmosis (RO) systems

An advice to all those thinking of installing RO systems.

The Reverse Osmosis FAQ, is meant to provide a section dealing with TDS in RO Systems, clear what the term means and how relevant an RO system is to the readers. 

The most popular FAQs are listed below. Please click on a topic to view more detailed information:

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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.

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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.

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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.

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What points should be considered 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.

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