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Reverse osmosis (RO)

Topics Covered: Science and Technology- developments and their applications and effects in everyday life Achievements of Indians in science & technology; indigenization of technology and developing new technology.

Reverse osmosis (RO)

What to study?

For Prelims: What is Osmosis and RO? TDS in water.

For Mains: Why was it banned? Issues and alternatives available.

 Context: The Union Environment Ministry has issued a notification to comply with the NGT order which prohibited the use of reverse osmosis (RO) purifiers in places where total dissolved solids (TDS) in the supplied water are below 500 mg per litre.

The NGT had ordered a ban on RO filters on the grounds that they wasted water and that, in the process of removing salts, they often deprived drinking water of essential salts, which could affect the nutritional intake of the people.

Background:

Current BIS regulations consider 500 mg/litre—1,200 mg/litre of total dissolved solids, which consist of salts and some organic matter, as acceptable.

Osmosis and RO:

Osmosis involves ‘a solvent (such as water) naturally moving from an area of low solute concentration, through a membrane, to an area of high solute concentration.

A reverse osmosis system applies an external pressure to reverse the natural flow of solvent and so seawater or brackish water is pressurised against one surface of the membrane, causing salt-depleted water to move across the membrane, releasing clean water from the low-pressure side’.

What are the problems with RO plants?

Deposition of brine (highly concentrated salt water) along the shores.

Affects fauna and flora: Hyper salinity along the shore affects plankton, which is the main food for several of these fish species. The high pressure motors needed to draw in the seawater end up sucking in small fish and life forms, thereby crushing and killing them — again a loss of marine resource.

Construction of the RO plants required troves of groundwater. Freshwater that was sucked out and is replaced by salt water, rendering it unfit for the residents around the desalination plants.

Cost and time: On an average, it costs about ₹900 crore to build a 100 MLD-plant and, as the Chennai experience has shown, about five years for a plant to be set up.

Energy needed: To remove the salt required, there has to be a source of electricity, either a power plant or a diesel or battery source. Estimates have put this at about 4 units of electricity per 1,000 litres of water. It is estimated that it cost ₹3 to produce 100 litres of potable water.

Is RO water healthy?

There are concerns that desalinated the RO water may be short of vital minerals such as calcium, magnesium, zinc, sodium, potassium and carbonates.

Most RO plants put the water through a ‘post-treatment’ process whereby salts are added to make TDS around 300 mg/l.

Are there technological alternatives?

Low-temperature thermal desalination (LTTD) technique works on the principle that water in the ocean 1,000 or 2,000 feet below is about 4º C to 8º C colder than surface water. So, salty surface water is collected in a tank and subject to high pressure (via an external power source). This pressured water vapourises and this is trapped in tubes or a chamber. Cold water plumbed from the ocean depths is passed over these tubes and the vapour condenses into fresh water and the resulting salt diverted away.

Ocean Thermal Energy Conversion: It will draw power from the vapour generated as a part of the desalination process. This vapour will run a turbine and thereby will be independent of an external power source. While great in theory, there is no guarantee it will work commercially. For one, this ocean-based plant requires a pipe that needs to travel 50 kilometres underground in the sea before it reaches the mainland.

Sources: the Hindu.