Clean Tech

Emerging desalination techprovides scope for solar play

M Ramesh | Updated on March 25, 2020 Published on March 25, 2020

Up till now, ‘solar’ has not had much of a play in desalination but the emerging water technologies might give it a large role.

When you soak a raisin in water, it bloats because water seeps through its skin — this, as high school physics tells us, is osmosis. Liquids separated by a membrane move from a rare medium to denser medium. ‘Reverse osmosis’, the most used desalination technique today, is the opposite of it — you push the denser liquid (sea water) through the membrane, so that water gets to the other side. The ‘pushing’ costs energy, and money. At the 100 million-litres-a-day desalination plant at Nemmeli, near Chennai, it takes nearly 4 kWhr of electricity to produce 1,000 litres of water, a burden of about ₹24.

If only such a plant could use solar energy to power its operations! After all, once the investments in the solar plant are paid back, the power is free.

The problem is, it would take about 60 acres of land to put up a solar power plant big enough to support the desalination machinery.

So, sigh, solar has generally not had a tie-in with desalination. But that could change in a few years.

When asked by BusinessLine to pick the two most promising alternatives to Reverse Osmosis, Syed Amir Basha, water technologist and Chief Technology Officer of the Chennai-based water MNC, Vatech Wabag, mentioned ‘forward osmosis’ and ‘membrane distillation’. Both need low-grade energy and hence you could bring in the sun.

Forward osmosis

‘Forward osmosis’ is based on natural osmosis, as in the bloating of a soaked raisin. In this, you basically need a ‘draw solution’ on one side of the membrane to pull in the water from the liquid on the other side — which could be sea water, brackish water, or industrial waste water, (or even, in other applications, fruit juice.) Over time, the draw solution becomes dilute. Then, in a secondary process, you extract the water from the draw solution and loop the solution back into the membrane chamber.

The secondary process typically needs heating. The amount of energy required depends on the type of draw solution. There are many of them, ranging from inorganic compounds (sulphatic fertilisers), to organic (i.e.carbon-based) compounds (including chemicals such as di-methyl ether and trimethylamine carbon dioxide) and functional solutions (such as magnetic nano particles).

However, regardless of the draw solution, the energy consumption to produce thousand litres of water is far less than RO. Companies engaged in this business, such as Forward Water Technologies and Trevi Systems, put the figure around 1.3 kWhr per thousand litres of water. The target is 1 kWhr. Some technologists have said this heat could come from solar collectors. UK-based Modern Water says forward osmosis plants need 30 per cent less energy than the conventional RO plants.

Forward osmosis has been ‘a fast-emerging water technology’ for about a decade, it still has to overcome challenges such as developing suitable membranes and draw solutions. But now some large plants are coming up, though none as big as the RO plants. Trevi Systems is building a 500,000 litres-a-day plant in West Asia, which “is powered entirely by renewables.”

The immediate application of forward osmos is is industries that end up with waste heat; solar heating is a little behind.

Membrane distillation

The other interesting technology is ‘membrane distillation.’ Here, instead of water, vapours of a liquid pass through an ‘un-wet-able’ (hydrophobic) membrane.

To make the vapours, you need heat. Basha puts it between 70 and 90 degrees C. Other estimates are more benign. Writing in the March 2019 edition of Applied Energy, a group of researchers say they could get 4 to 10 litres of water per sq m of membrane area at feed temperatures between 50 and 70 degrees C. Some residual heat is still available for other uses. “We envision that this design could be beneficially deployed on the rooftops of residential and commercial buildings — buildings that require a continual supply of both potable water and domestic hot water,” the authors say.

So, that’s the story. Where there is need for only low-grade heat, there is a play for solar. Sun is winking at water.

Published on March 25, 2020

A letter from the Editor


Dear Readers,

The coronavirus crisis has changed the world completely in the last few months. All of us have been locked into our homes, economic activity has come to a near standstill. Everyone has been impacted.

Including your favourite business and financial newspaper. Our printing and distribution chains have been severely disrupted across the country, leaving readers without access to newspapers. Newspaper delivery agents have also been unable to service their customers because of multiple restrictions.

In these difficult times, we, at BusinessLine have been working continuously every day so that you are informed about all the developments – whether on the pandemic, on policy responses, or the impact on the world of business and finance. Our team has been working round the clock to keep track of developments so that you – the reader – gets accurate information and actionable insights so that you can protect your jobs, businesses, finances and investments.

We are trying our best to ensure the newspaper reaches your hands every day. We have also ensured that even if your paper is not delivered, you can access BusinessLine in the e-paper format – just as it appears in print. Our website and apps too, are updated every minute, so that you can access the information you want anywhere, anytime.

But all this comes at a heavy cost. As you are aware, the lockdowns have wiped out almost all our entire revenue stream. Sustaining our quality journalism has become extremely challenging. That we have managed so far is thanks to your support. I thank all our subscribers – print and digital – for your support.

I appeal to all or readers to help us navigate these challenging times and help sustain one of the truly independent and credible voices in the world of Indian journalism. Doing so is easy. You can help us enormously simply by subscribing to our digital or e-paper editions. We offer several affordable subscription plans for our website, which includes Portfolio, our investment advisory section that offers rich investment advice from our highly qualified, in-house Research Bureau, the only such team in the Indian newspaper industry.

A little help from you can make a huge difference to the cause of quality journalism!

Support Quality Journalism
This article is closed for comments.
Please Email the Editor
You have read 1 out of 3 free articles for this week. For full access, please subscribe and get unlimited access to all sections.