Will desalination affect our sea water?

Salinity impact negligible, regular monitoring and assessment of sea water carried out: PUB

Reader Andrew Ng wrote to askST after reading an article in The Straits Times about the construction of Singapore's fourth desalination plant.

"I understand desalination is a complex process where water is extracted from sea water. I wonder what happens to the salt. Is the salt dumped into the sea, making the sea water saltier than it should be?"

He added that the movie The Day After Tomorrow (2004) was built on a premise that salinity imbalance in the ocean can lead to the formation of catastrophic weather conditions.

He wrote: "I admit I am being slightly overdramatic here. But I am curious to find out if our quest for water independence will contribute to climate change, and thus global warming."

Environment reporter Audrey Tan answers.

Desalination is the process of turning sea water into potable water.

Key to this process is reverse osmosis, which involves forcing water through a membrane at high pressure so salts are removed from the water.

In Singapore, the salt retained through this process is discharged back into the sea. But it is unlikely to have any impact on a global scale, said a spokesman for national water agency PUB.

An artist's impression of part of the Keppel Marina East Desalination Plant, Singapore's fourth plant. PUB says that long-term monitoring of Singapore's first two desalination plants has shown no environmental impact on or change in sea water quality
An artist's impression of part of the Keppel Marina East Desalination Plant, Singapore's fourth plant. PUB says that long-term monitoring of Singapore's first two desalination plants has shown no environmental impact on or change in sea water quality. PHOTO: KEPPEL CORPORATION

"Because the ocean is so large, and with replenishment of fresh water from rainfall, the salinity impact of Singapore's desalination plants is negligible on a global scale," said the spokesman.

The salt in desalination brine - the residual solution from the desalination process - originates from the sea and the discharge returns it to the source, she added.

While this may not have a significant impact at the global level, it could have a localised impact, she said. This is why the desalination process is carefully studied, with mitigation planned and implemented.

For example, when planning for desalination plants, PUB carries out environmental impact assessments and modelling to ensure that any impact is within tolerable limits.

"Once the plant begins operations, PUB carries out regular sea water quality monitoring at the discharge points and surrounding waters. PUB complies with the National Environment Agency's guidelines for discharge from the desalination plants into the sea," said the spokesman.

She added that Singapore's first two desalination plants in Tuas have been in operation for 12 and four years, respectively, and that long-term monitoring has shown no environmental impact on or change in sea water quality.

Singapore's third desalination plant in Tuas is still being built. Construction of the Keppel Marina East Desalination Plant - Singapore's fourth - is expected to be completed in January 2020. A fifth desalination plant is also due to be built on Jurong Island by 2020.

At the plants, the desalination brine is discharged through an outfall pipe into the sea in a controlled way and is instantaneously mixed with sea water in the immediate vicinity. This process dilutes the brine and brings it to ambient sea water levels, thus preventing it from remaining concentrated in a localised area, said the PUB spokesman.

When asked if the salt can be collected for use elsewhere, PUB noted that the process is costly and uneconomical. Hence, Singapore does not do so.

A version of this article appeared in the print edition of The Straits Times on July 08, 2017, with the headline 'Will desalination affect our sea water?'. Print Edition | Subscribe