09 August 2019 16:11:19 IST

Smart tech is changing the math for desalination

Several new technologies can make recovering water from oceans feasible and cost-effective

It is an irony that, when the earth’s surface is three times more water than land, there should be something like a ‘water shortage problem’ at all. If you feel thirsty, you could just go to the sea and help yourself, right?

Wrong, obviously, for that is not the water you’d care to drink. 97 per cent of the planet’s water is salty, of the rest, 2.5 per cent is frozen and only 0.5 per cent is available to drink.

But intelligent people will look at how to make a bit of the 97 per cent work for them. Yes, we are talking about desalination. If we could take just the fresh water from sea water, leaving the salts and other stuff behind, we will never have to worry about water shortage. People have been doing this for years. For instance, the country’s biggest sea water desalination plant is in Minjur, near Chennai — the reverse osmosis (RO) plant provides 90 million litres of water to the metropolis.

All you need is to put up more such plants, and no Indian will ever have to worry about water scarcity. And it is really no big deal. Ballpark basis, each such plant costs about ₹500 crore — so 500 of them will cost ₹25,000 crore, which is not deal-breaking number.

But (and there’s always a but, isn’t there?) there is a problem.

Reverse osmosis

Before we try to understand the problem, we need to know how a RO plant produces water. For that one must know the science of osmosis.

When you soak a raisin in a cup of water, it absorbs some of the water and bloats. Water outside the raisin gets through the skin of the raisin, in a process called osmosis. When separated by a filter — a membrane — liquid from a thinner medium flows to the denser medium. The raisin doesn’t discharge water, it absorbs it.

The opposite happens in reverse osmosis. The liquid is made to flow from where it is dense to where it is thin. The liquid — in this case, sea water — doesn’t make this movement on its own. It has to be pressurised, or squeezed through the membrane. And to pressurise, you need electricity.

RO desalination plants consume huge volumes of energy. Roughly, you would need 13 kWhr to produce 4,000 litres. A 100 million litres-a-day plant, like the one near Chennai, will, therefore, need 3.2 lakh kWhr, worth nearly ₹20 lakh a day.

Even so, water from desal plants is not very expensive. It costs about three-and-a-half paise to produce a litre. But the point is not the cost, it’s the electricity consumed.

There is a saying that ‘the greenest unit of electricity is the unit that is not produced.’ Desalination plants guzzle energy big time, and one can easily imagine the effect on climate if that energy is produced from fossil fuels.


There are quite a number of interesting technologies on the horizon.

An American company called Resolute Marine has come up with a solution. Its idea is simple: instead of using electricity to pressurise sea water so that the membrane separates soft water and the hard particles, why not get waves do the job?

Its technology uses the force of the waves to put pressure on salt water. The company insists that its technology is inexpensive. Using wave power is truly green. Also, waves are 24x7, so one gets an uninterruptible supply of power.

Closer home, the National Institute of Ocean Technology is toying with what is called ‘ocean thermal energy conversion’, or OTEC. The principle, again, is amazingly simple. Take warm water from the sea surface, put it in a chamber and depressurize the chamber. The lower the pressure, the lower the boiling point, therefore steam can be generated more easily, and then condensed into drinking water. Where do you condense the steam into water? In the deep seas. Pipe the steam down to the region where the sea water is pretty cool, and it comes back as liquid fit for drinking.

And then, there is the good old nuclear power. The immense heat energy that you get when you split the nuclei of certain atoms can be cheap, round-the-clock and clean. But if it is not cheap it is because of the numerous things you have to do to make it safe for human settlements in the neighbourhood.

However, it may not be too costly to put up a smallish plant, say 100 MW, on a barge and put it out at sea and, rather than cable down the power to the mainland, and use it to desalinate water. A tanker can shuttle from the plant to the shore, bringing us all clean drinking water.

The idea of a barge-mounted nuclear power plant is not so outlandish as it might sound to some. There are quite a few in operation already, in Russia. In any case, all nuclear submarines are, in essence, small nuclear power plants — only n-power enables the submarine to remain hidden under the waters for months on end. India’s own INS Arihant , for instance, has on board an 83 MW nuclear power plant.

Wave and solar power

One could go on fiddling with various technologies or combinations of them. For instance, a desalination plant could be powered by a combo of wave and solar power.

There is a lot of entrepreneurial scope in water. And what we have discussed is just production of water. Moving beyond it too, there is a lot one can do.

For instance, a start-up called Smarterhomes is a water metering company that measures water consumption in real time. Another, called Falcon Labs, has developed a product that mines data at water pumping stations in rural and urban areas, to improve efficiency. Yet another, called NextDrop, tracks water supply for utilities.

As the effects of climate change kick in, and water come under severe stress, it’s only logical that if you come up with feasible solutions, you’re in business.