Emerging desalination techprovides scope for solar play

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.

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