askST: What happens to the salt collected from the desalination process?

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 seawater. But I wonder what will happen to the salt? Will the salt be dumped into the sea water that makes it even 'saltier' than it should be?"

He added that the movie blockbuster The Day After Tomorrow (2004) was built on a premise that salinity imbalance in the ocean led to the formation of catastrophic weather conditions: "I admit I am being slightly over dramatic 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 answered.

SINGAPORE - 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 from national water agency PUB.

"Because the ocean is so large, and with replenishment of freshwater 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 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 begin operations, PUB carries out regular seawater 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 (since 2005 and 2013, respectively), and that long-term monitoring has shown no environmental impact to or change in seawater quality.

Singapore's third desalination plant in Tuas is still being built. Construction of the Keppel Marina East Desalination Plant - Singapore's fourth - kicked off last month (June) and is expected to be completed in January 2020. A fifth desalination plant of similar capacity is also due to be built on Jurong Island by 2020.

At Singapore's desalination plants, the desalination brine is discharged through an outfall pipe into the sea in a controlled way, and is instantaneously mixed with the seawater in the immediate vicinity.

This process dilutes the brine and brings it to ambient seawater 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 while desalination brine can be further treated and salt recovered, the process is costly and uneconomical. Hence, Singapore does not do so.