Desalination is SA’s future – But do you know how it works?

When it comes to water scarcity, there is no silver bullet, no single solution that’ll address the current and future crises. At an individual level, we should all be implementing water saving behaviours in our households, recycling water where can (i.e. greywater), harvesting rainwater, and growing water wise gardens. With this said, one avenue that water scarce countries like Australia, Singapore, Israel, Saudi Arabia, Kuwait, the UAE, Qatar, Bahrain and the US are successfully exploring is seawater desalination. South Africa is now joining the list.

The situation in Cape Town

Despite the recent much needed rains, Cape Town’s water supply will still be under severe stress for the foreseeable future. And in the long term, water scarcity will most likely increase. Unfortunately, the process of getting the three temporary desalination plants – in Monwabisi, Strandfontein and the V&A Waterfront – fully operational has not been without difficulties. The large permanent plant in Cape Town harbour, set to supply the city with 50 million litres daily, has also encountered delays. Nonetheless, city authorities aim to use desalination to supply 25 percent of CT’s water demand by 2020.

How does desalination work?

There are two main methods to desalinate seawater. The first is not in any way new, and relies on evaporation and condensation to distil potable water from salt water. In many distillation techniques, salt water is heated until it evaporates; it then condenses on a cool surface and is collected. The salt doesn’t evaporate with the water, leaving the condensed water free of dissolved solids. The process, however, does consume a lot of energy.

The second main method involves passing water through a membrane that allows water molecules through but not the dissolved solids (like salt). Two examples of this method include Reverse Osmosis (RO) and Electrodialysis. Reverse Osmosis is, relative to the other methods, economical, produces large amounts of water and has been used in many of the countries listed above.

Watch: Reverse Osmosis explained in plain English

1. Seawater intake

  • Intake pipes are placed in the ocean, collecting the amount of water that can be desalinated per day. To avoid capturing sea life, these intake pipes are often quite large, covered in a fine mesh and draw water in at a slow rate.

2. Pre-treatment

  • In order not to “foul” the fine Reverse Osmosis membrane, impurities in the seawater need to be removed. This includes non-organic solids (sand, clay, oil, plastic and other pollutants) and organic solids (seaweed, twigs, microorganisms, etc.). At this stage, the presence of bacteria is often also addressed (but disinfection/purification may also occur after the seawater has been passed through the RO membrane).

3. Desalination through Reverse Osmosis

  • Osmosis is a process in which water with a low concentration of dissolved solids (like salt) naturally moves through a semi-permeable barrier to water with a high concentration of dissolved solids. In Reverse Osmosis, this natural movement is reversed by placing pressure on the side of the membrane that contains the saltwater. The water molecules of the saltwater are forced through the fine membrane, leaving the salt behind – making the water drinkable (after some further treatment). The saltwater left behind is then pumped back into the ocean; hopefully in a manner that doesn’t damage the local ecosystem.

4. Disinfection, pH balance and remineralisation

  • To remove all pathogens (e.g. viruses and bacteria) still in the desalinated water, it must be disinfected. To further ensure that the water is suitable for human consumption, the pH levels need to be neutralised. Lastly – as strange as this may sound – the water has to receive minerals to ensure that it isn’t “too clean”. This is because water without minerals can actually damage the body.

Watch: Israel’s desalination success story

Whereas desalination has incredible potential to mitigate the negative effects of global warming on our water supply, it poses problems: for example, due to high transport costs it’s only really feasible in coastal regions, and; it has high energy requirements. But, it can certainly be a significant contributor to a diverse and comprehensive solution that prevents crises yet to come.

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