Desalination no silver bullet, some wonder if exercise is worth its salt
WATER, water everywhere – but not a drop to drink. Desalination can provide an important additional source of water for Cape Town as our existing freshwater sources become more stressed under conditions of drought and climate change. But removing salt from seawater is no silver bullet. Here’s why.
How does desalination work?
Water is desalinated when it is treated to reduce the level of salts in it. Seawater typically has about 35 grams of salts (about seven teaspoons) dissolved per litre of water.
This is a concentration of 35 000 parts per million (ppm). The healthy limit for drinking water is 1 000ppm and the upper limit is 5 000ppm, so to make seawater fit to drink we have to remove at least 30g (six teaspoons) of salt out of each litre of water.
Separating the salt from the water takes energy. If seawater were to be left under a transparent dome with the energy of the sun shining through, about 3 litres would evaporate per day per square metre of water surface. If that water condensed again on the dome roof, you would have a solar still and you could separate small volumes of freshwater from seawater.
To make large volumes of freshwater from salty water needs large quantities of energy. Depending on which method is used to desalinate (reverse osmosis/vapour compression/ membrane distillation) between 2 and 12kWh of electricity is needed per 1 000 litres.
Where is desalination most common?
The International Desalination Association estimates that there are nearly 20 000 desalination plants worldwide, producing water for over 300 million people.
The highest levels of use are in arid countries with few other options and relatively cheap and subsidised energy costs. What about South Africa? In South Africa, desalination is used quite widely by mines to clean up polluted mine water and acid mine drainage. Small to medium-scale desalination has also been used in coastal towns during times of drought. Six municipalities are currently using small-scale reverse osmosis plants to desalinate water for bulk water supply.
They are Mossel Bay, the Knysna municipality, Plettenberg (Bitou municipality), the Cederberg municipality and Richards Bay, and there are two desalination plants in the Ndlambe municipality, namely the Bushman’s River Mouth and Cannon Rocks plants.
What desalination is in the pipeline for Cape Town?
Four of the seven augmentation projects that will bring new water online for Cape Town are desalination plants. They are based at the Waterfront, Cape Town harbour, Monwabisi and Strandfontein (the latter two on the False Bay coast).
These are relatively smallscale operations. The City has been criticised for initially trying to bring on smaller and quicker plants to provide water during a Day Zero scenario, as these are more expensive.
At one stage we were going to bring in desalination barges. Barges have worked successfully in more sheltered sea areas in the Red Sea and the Gulf. There is very little international capacity in this market at the moment, and this is currently not a viable option for Cape Town.
Barges are also generally more expensive than landbased desalination.
Why aren’t we doing more desalination in Cape Town?
For arid and droughtstricken coastal cities, desalination can be an important source of water which is completely independent of local rainfall. However, desalination remains the resource of last resort for most cities because it is the most expensive.
Desalination takes longer to bring online than drilling the shallower boreholes in Atlantis and the Cape Flats. Desalination would also be more cost-effective at larger scales – between 150 and 200 million litres per day would be the best economy of scale for Cape Town. Other constraints? The availability of electricity is also a potential issue. Concerns have been raised in the US that electricity supplies in local grids are not able to accommodate new desalination plants built in response to the Californian drought.
South Africa is not long out of an electricity crisis. A further energy crisis at the same time as a water shortage would place this source at risk.
Some plants overseas are starting to operate with solar energy, and this could be an option for Cape Town in the long term. At the moment, most of our energy is generated using coal-fired power stations. This means our energy generation puts a lot of greenhouse gases (GHGs) into the atmosphere, which is a cause of climate change – which in turn would fuel future droughts.
What is climate-neutral desalination?
Climate-neutral desalination relies solely on renewable energy which doesn’t produce GHGs and contribute to climate change.
There are some examples of solar-powered desalination in California, however these costs are even higher than conventional desalination at this point.
Other environmental concerns?
Desalination plants need to take in twice as much seawater as they produce freshwater. Often the intake points take in small organisms (fish larvae, plankton, etc) which can reduce local fish populations. A quantity of very concentrated brine is then produced, which generally will be disposed of back to the sea. As well as being very salty (which is toxic, even in a marine environment) this brine contains biocides and anti-fouling chemicals used in the desalination process which can also harm marine life.
How expensive is desalination compared to other water sources?
Globally desalination is between two to four times as expensive as most other sources, depending on the relative cost of capital equipment (how much has to be imported etc), the cost of energy and the cost of labour to implement other water savings. By way of comparison, the costs for Cape Town would be:
Raw surface water – between R1 and R4 per kilolitre.
Alien clearing to release more water from our catchments – from R6 to R15 per kilolitre.
New groundwater around R15 per kilolitre.
Reclaiming and reusing treated waste water – between R10 and R20 per kilolitre.
Large-scale, permanent – desalination – between R10 and R22 per kilolitre.
Smaller, short-term desalination – R34 to R44 per kilolitre. Long-term lessons? Cape Town may need one larger-scale desalination plant in the long term. This would add a drought-proofed water source into our bulk supply.
However, we have been warned about overinvesting in a source that other (more wealthy countries) have had to mothball because of excessive energy costs.
In Australia, the severe drought from the mid-1990s until 2012 prompted the construction of six large-scale seawater desalination plants at a cost of 10 billion Australian dollars.
The plants took years to build. Meanwhile, the National Water Initiative implemented water policy reforms and improved efficiency measures that led to cheaper water supply alternatives.
By the time the plants were operational, the drought was over and the more cost-effective alternatives made desalinated water prohibitively expensive. Most of these facilities have stood idle, and operated at a significantly reduced capacity.