Making desalinated water cheaper

Challenges remain in finding best way to boost capacity 10 times by 2060
Grace Chua & Lester Kok Straits Times 5 Jul 10;

DESALINATION, the process of removing salt and other minerals from water to make it drinkable, is set to play a growing role here.

By 2060, Singapore aims to boost its desalinated-water capacity 10 times, to meet 30 per cent of water demand, which will double to some 760 million gallons (3.5 million cubic m) a day.

National water agency PUB last week put out a tender for the building of Singapore's second water desalination plant, which will add 318,500 cubic m of water a day to the nation's water supply when it is completed in 2013.

The Republic's first such plant, the $200 million SingSpring facility in Tuas, began operations in 2005, and currently produces 136,000 cubic m of fresh water daily.

Desalinated water is also known as Singapore's fourth 'national tap', along with catchment areas, water bought from Malaysia and reclaimed water Newater.

But it is costly, the most expensive of the four 'national taps'. As Singapore gradually moves towards self-sufficiency in its water supply, Minister for the Environment and Water Resources Yaacob Ibrahim has already warned that higher water tariffs can be expected down the road.

Producing 1 cubic m of drinking water from seawater costs about 65 US cents (90 Singapore cents) to US$1.20, said the International Desalination Association's former president Lisa Henthorne. She is now the association's spokesman and treasurer.

In comparison, Newater from Sembcorp's plant, which started operations earlier this year and is the fifth such plant here, had a first-year price of just under 30 cents per cubic m. Water from Malaysia comes at a price of 3 sen (1.3 Singapore cents) per 1,000 gallons.

About 30 per cent to 40 per cent of desalination costs comes from energy, Ms Henthorne said, depending on the local costs of power and what kind of power (conventional or renewable) is used.

Another 40 per cent comes from capital costs, and the remainder from operation, maintenance, labour and so on.

Desalination is energy-hungry because the filtration method of reverse osmosis uses pressure to push water through a membrane and takes about 3kwh to produce a single cubic m of water.

About 60 per cent of global desalination is done by reverse osmosis, while multi-stage flash distillation (turning water into vapour at very high temperatures) accounts for another quarter of the desalination market. Both of these are very energy intensive.

Even Singapore's new desalination plant will use reverse osmosis to remove salt and other impurities from water.

So researchers and companies are experimenting with new, and cheaper, ways to get the salt out of seawater.

One way is membrane distillation, which, instead of flushing water through a membrane, heats the water so it turns into vapour first.

That operates at lower temperatures and leaves less residue on membranes than reverse osmosis, so membranes have to be cleaned less frequently.

Currently, PUB is working with local firm Keppel Seghers on a technology called Memstill. By using waste heat to push water vapour through a membrane, the Memstill technology cuts two- thirds of the energy needed for desalination.

A Memstill demonstration plant is currently being built on Jurong Island, while Memsys Clearwater, another membrane desalination firm, is building a small pilot plant, powered by solar energy at the Marina Barrage.

The trial, supported by PUB and the Nanyang Technological University (NTU), will last 12 months, from now till next July, and begin its production of 1 cubic m of water per day in September.

Unlike current reverse osmosis systems, which are made of steel, Memsys uses mostly plastic, which needs less maintenance and so lowers cost.

However, Memsys director Florian Bollen - who is also the chairman of the Singapore Flyer - declined to comment on the overall cost of the pilot, as test-beds tend to be more costly per unit of water produced, than commercially operating plants.

The distilled water produced at the Marina Barrage will be either used in nearby water fountains, for watering plants, or sold to the public.

Another desalination method, biomimicry, uses technologies that mimic nature. For instance, some research methods use a molecule called aquaporin, which shepherds water across a membrane while blocking other contaminants out.

Aquaporins are naturally found in red blood cells and in the kidneys, where the flow of water molecules in and out has to be controlled.

But aquaporin desalination is about 10 to 15 years away from testing and commercialisation, said Professor Ng Wun Jern, executive director of the Nanyang Environment and Water Research Institute at NTU.

However, Singapore's push to increase desalination and Newater treatment offers opportunities for businesses.

Companies which carry such cutting-edge technology, Prof Ng said, would have a commercial advantage in the region and further afield.

As Asian cities grow, their natural aquifers and catchments may not be enough to support their burgeoning populations, and so 'water reclamation will become a part of life', he added.

Still, when salt and minerals are removed from water, they have to go somewhere and this could have an environmental impact.

Typically, this waste is discharged back into the sea, so an environmental impact assessment has to be done before a desalination plant is sited and built.

Plant sites are often chosen based on the quality of the surrounding water - the higher quality the water, the less waste is produced, explained NTU researcher Chong Tzyy Haur.

But as desalination needs grow worldwide, lower-quality water sources may have to be used, he said.

To address that problem, the International Desalination Association started an Environmental Task Force this year to study discharge and energy issues, said Ms Henthorne.

In December, it will hold its first conference in Bahrain and come up with a Blue Paper that addresses such issues.