DESALINATION:
Turning Ocean Water Into Rain
Yudhijit Bhattacharjee

A novel technology may end the curse of bad drinking water on remote Indian islands--and offer an alternative method of desalination for mainland communities, too

 

KAVARATTI, INDIA--With its coconut palms and white-sand beaches, this coral island in the Arabian Sea seems like a tropical paradise--until you taste the water. For decades, the 11,000 people of Kavaratti have had to drink the brackish water from their wells, supplemented by a modest supply of monsoon rainwater. Now, however, the islanders are quenching their thirst with fresh water distilled from the turquoise expanse that surrounds them--thanks to a novel desalination method that's being held up as a model solution for water shortages along India's teeming mainland coast.

Most desalination plants either boil seawater and then condense the vapors (thermal distillation) or pump seawater at high pressure across a salt-retaining membrane (reverse osmosis). Both methods are energy-intensive and expensive to maintain. But the plant at Kavaratti, part of the Lakshadweep archipelago, is exploiting a third strategy that has been known for half a century but rarely implemented: using the ocean's own thermal energy to desalinate water.
The concept is simple. Water at the ocean's surface is warm, with a temperature that's typically between 26° and 30°C in the tropics. At a depth of 350 meters, it drops to a chilly 13°C or so. At the plant, surface water is pumped into an onshore vacuum chamber where the low pressure causes some of the water to vaporize. In another chamber, cold water drawn from the depths condenses the vapor into fresh water. "We are simply mimicking how nature makes rain," says S. Kathiroli, director of the National Institute of Ocean Technology (NIOT) in Chennai, which built the plant.

Known as low-temperature thermal desalination (LTTD), the technology is an offshoot of a more ambitious idea: to convert the ocean's thermal energy into electricity, first proposed by French physicist Jacques d'Arsonval in 1881. Competition from cheaper energy sources has prevented ocean thermal energy conversion from taking off, although experimental plants in Hawaii and Japan have shown that the concept works. LTTD has fared better--a plant in Italy operated commercially during the 1990s--but the technology has largely remained on the margins.

 

The Indian venture is a bold attempt to bring thermal-driven desalination into the mainstream by massively multiplying production. NIOT admits that the year-old Kavaratti plant, which produces 100,000 liters of fresh water a day, is not as energy-efficient as rival technologies: It consumes 30% more energy per unit water than a reverse-osmosis plant, for instance. But scaling up the technology 100-fold, officials believe, will unlock its potential.
To test that idea, NIOT has built a plant with a capacity of 1 million liters per day on a floating barge 40 kilometers off the coast of Chennai, on the opposite coast of India. Last month, NIOT engineers completed a 60-day trial of the plant, giving away drums of fresh water to passing ships. The institute is now inviting investors to help ratchet up the operation to 10 million liters a day by installing more condensers and evaporation chambers, which officials say would halve the cost to less than $1 per 1000 liters. That would be 25% cheaper than seawater desalination using reverse osmosis, says Kathiroli. There's a lower environmental cost too, he points out: Concentrated brine left over from reverse osmosis is often flushed back into the ocean to the detriment of local marine organisms.

Experts in India and abroad are watching the project closely. "It's a strategy worth pursuing," says Luis Vega, who designed an ocean thermal energy plant that produced electricity and desalinated water for the Natural Energy Laboratory of Hawaii Authority in the 1990s. But Vega doubts that scaling up will reduce costs much. Jayanta Bandyopadhyay, a water-policy expert at the Indian Institute of Management in Kolkata, says the government is right to experiment with desalination but must also invest more in low-tech solutions such as rainwater harvesting.

When NIOT researchers began working on ocean thermal energy a decade ago, electricity, not drinking water, was the prize they were after. But after multiple failures to install a deep water pipe at sea to draw cold water from a few hundred meters below the sea's surface, the government in 2003 pulled the plug. Kathiroli, who took over as NIOT director the following year, revived the project with the simpler target of desalination. This requires a smaller temperature differential than the 20°C needed to make electricity, and therefore water can be drawn from a shallower, more manageable depth. "We were driven by our ego," says Kathiroli. "We wanted to show that we could do it."

The government approved the proposal, and after completing a pilot project, NIOT engineers in 2005 began building the Kavaratti plant. The steep bathymetry of the island--the seabed plunges several hundred meters a short distance from shore--enabled accessing deep water without venturing far from land.

Since coming online in late 2005, the plant has pumped fresh water to a network of public taps for 2 hours every morning and evening. Islanders say they now use groundwater--which many have been drinking all their lives--only for washing and cleaning. "This water tastes better, and food cooked in it tastes better too," says M. Qasim, a schoolteacher.

Another benefit has been the prevention of waterborne diseases, once rampant on Kavaratti because of the many septic tanks near the shallow water table. P. S. Ashraf, superintendent of the island's only hospital, says he and his colleagues have witnessed around 50% fewer diarrhea and dysentery cases since the plant was commissioned.

Buoyed by the success, officials plan to build similar plants on Lakshadweep's 10 other islands. They expect that the Kavaratti experience will help make the new plants more cost-effective. "We are confident of streamlining the process considerably," says NIOT engineer Purnima Jalihal.

Although thermally driven desalination may be a good option for islands, it must pass a bigger economic test on the mainland, where the coast's gradual slope requires going several kilometers offshore to access deep water. NIOT's barge plant near Chennai will have to compete with a reverse-osmosis plant with a 100-million-liter capacity being built nearby onshore by a Spanish waterworks company, Befesa. The Chennai plant will have the added expense of transporting fresh water from the barge to the mainland, says Ravi Bondada, a business manager for Befesa in Chennai: "They are making a good attempt, but the economics will have to be proved."

Kathiroli agrees that the government should continue to pursue conservation strategies such as better river management and improve rainwater collection for drinking water. Nevertheless, he emphasizes that the need for fresh water is enormous; the shortfall for Chennai alone is 300 million liters a day. Hopes for ocean thermal technology are running high because it is young: "Reverse osmosis has been fine-tuned for over 40 years or more," Kathiroli says. "We are just starting out."