Using Desalination Plants to Cut Carbon Dioxide Levels

A new process for the decomposition of waste brine from desalination has been proposed by Ashton University researcher Phillip Davies. By using solar energy to decompose the reject brine, desalination plants could effectively aid the removal of Carbon Dioxide from oceanic waters and neutralise ocean acidity.

Desalination is a process by which salt and other minerals are removed from saline water, to produce freshwater. A by-product of this is brine (water highly saturated with salt), along with Carbon Dioxide. The demand for freshwater from desalination plants is increasing, with the current yearly production of 30 billion m³ expected to double within the next decade. This has increased pressure for a method of reducing emissions.

The research, published in the journal Environmental Science: Water Research & Technology, uses solar energy to hydrolyse Magnesium Chloride, a salt found in brine, thus forming Magnesium Oxide. This is then discharged into the ocean and neutralises ocean acidity due to its own alkaline nature. The Magnesium Oxide removes Carbon Dioxide present in the water through the formation of Magnesium Bicarbonate.

Hydrochloric acid is produced as a by product, but could be sequestered into silicate rocks. Another less favourable aspect of this process is that it would increase the energy requirements of the plants by 50%. However Davies calculates that this increase in energy demand would be offset by the Carbon Dioxide absorption capacity; each plant would remove 18,200 tonnes of Carbon Dioxide each year, rather than emitting 5300 tonnes as it would otherwise. This would equate to the absorption 0.4% of anthropogenic Carbon Dioxide emissions annually.

The major expenditure of the extra energy required is used to dewater the brine, in order to access the Magnesium Chloride. Davies acknowledges this fact, ‘Not much energy is needed to decompose magnesium chloride in brine to magnesium oxide, which makes the use of solar energy potentially very attractive’, he comments, ‘If we could find better ways to dewater the brine this would become very energy efficient as a means of avoiding carbon dioxide.’ He also warns of the possible effects of the Magnesium Oxide on local marine environments, stressing that this should firstly be assessed before any is discharged.

A major advantage of this new process is its impact on existing technology, as Phil Renforth, a geo-environmental engineer from Cardiff University, observes; ‘This approach may allow the industry to transform itself from a carbon dioxide villain into a force for good in the climate change debate.’

Reference: P A Davies, Environ. Sci.: Water Res. Technol., 2015, DOI: 10.1039/c4ew00058g

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Daniel Di Francesco

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