Water Management – Institutional Reforms, Conservation Efforts, etc.

Reverse Osmosis (RO)

Note4Students

From UPSC perspective, the following things are important :

Prelims level : Osmosis, TDS

Mains level : Drinking water crisis in India


  • The Supreme Court has refused to interfere with a NGT order and asked the Association of RO manufacturers to approach the Centre with its grievances.
  • The NGT had passed an order prohibiting the use of RO purifiers when Total Dissolved Solids (TDS) in water are below 500 mg per litre.

Total Dissolved Solids (TDS)

  • TDS is made up of inorganic salts as well as small amounts of organic matter.
  • According to the World Health Organisation, TDS levels below 300 mg per litre are considered excellent, while 900 mg per litre is said to be poor and above 1,200 mg is unacceptable.
  • The top court directed the government to regulate the use of purifiers and also passed directions to sensitise the public about the ill-effects of demineralised water.
  • During the hearing, the counsel representing the Association referred to a recent BIS report on standards of water and said it points to the presence of heavy metals in Delhi’s groundwater.

Explained: Osmosis and Reverse Osmosis

  • Osmosis is a phenomenon where pure water flows from a dilute solution through a semi permeable membrane to a higher concentrated solution.
  • Semi permeable means that the membrane will allow small molecules and ions to pass through it but acts as a barrier to larger molecules or dissolved substances.
  • As water passes through the membrane to the salt solution, the level of liquid in the saltwater compartment will rise until enough pressure, caused by the difference in levels between the two compartments, is generated to stop the osmosis.
  • This pressure, equivalent to a force that the osmosis seems to exert in trying to equalize concentrations on both sides of the membrane, is called osmotic pressure.
  • If pressure greater than the osmotic pressure is applied to the high concentration the direction of water flow through the membrane can be reversed.
  • This is called reverse osmosis Note that this reversed flow produces pure water from the salt solution, since the membrane is not permeable to salt.
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