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Desalination: How Cape Town could up its game

Global examples show South Africa that desalination could increase water output.

Day Zero is looming for Cape Town and a dedicated and efficient long-term solution to South Africa’s water woes must be found. The weather can’t be controlled and drought patterns for the region are set to worsen. It’s time to stop relying solely on rainfall and dam levels for clean water as a critical resource.

South Africa boasts a coastline of over 2500 kilometres so it should be considering the oceans as an abundant water supply. Converting seawater to clean drinking water can be achieved by desalination, a proven technology that’s been used around the world. Desalination involves removing the salinity (dissolved salts and minerals) from water. There are a number of ways of doing this, but the only process that ticks all the boxes in terms of catering for large volumes, environmental impact and cost, is reverse osmosis.

South Africa has around 10 desalination plants dotted along the coast from Lambert’s Bay in the west to Richards Bay in the east. The output from each is quite small and caters only for households in the immediate vicinity.

Cape Town has started down the road of desalination. A temporary desalination plant is due to start producing 2000 cubic metres of water a day (going up to 7000 in phase 2) starting in March. The city will buy the water at a cost of around R30 per kilolitre. The contract is due to run for 2 years after which the equipment will be removed and the area rehabilitated.

But the city needs to develop far more ambitious plans. With a population of around 4 million people it needs a water output of around 500 000 cubic metres per day to supply each individual with roughly 100 litres. Desalination plants have dramatically increased in number and sophistication around the world due to membrane technology breakthroughs and energy saving equipment.

Three global examples in Saudi Arabia, Spain and Israel show that South Africa could increase water output in a timely and cost effective way.

State-of-the-art desalination plants

The Ras Al Khair (Saudi Arabia) hybrid desalination plant has a drinking water output of 1 036 000 cubic metres per day. It is a hybrid plant because it relies on both multi-stage flash distillation (which is highly electrical and energy intensive) and reverse osmosis technology. It was built at a cost of US$7.2 billion between 2011 and 2014.

In response to the worst drought Spain had in years, Barcelona built a seawater desalination plant with drinking water output of 200 000 cubic metres per day. It supplies drinking water to around 1.3 million people in the region. It caters to 20% of the population of Catalonia. Additionally, more than 5200 photovoltaic modules are installed on the roofs of eight buildings of the plant which generate approximately 1 MW of electricity annually. It is the largest reverse osmosis based desalination plant in Europe. Construction started in 2007 and the plant was inaugurated in 2009 at a cost of 230 million euros.

The plant was built by Aguas Ter Llobregat, the public utility responsible for the supply of water to the city which also contributed €28 million to the cost. The public utility’s involvement is remarkable given that it would be comparable to the Water and Sanitation Department for the City of Cape Town being in charge of the entire construction of a desalination plant.

The Sorek desalination plant in Israel sets significant new industry benchmarks in desalination technology, capacity and water cost. It provides drinking water output of 624 000 cubic metres per day or 26 000 kilolitres per hour, which is more than all desalination plants in South Africa combined produce daily. It is the largest seawater reverse osmosis desalination plant in the world, built by IDE Technologies. It opened in 2013 at a construction cost of around USD$500 million. Sorek will sell water to the Israeli water authority for 2.00 NIS which is around R7 per kilolitre. By comparison, the eThekwini municipality that serves the greater Durban area charges customers R16.20 per kl consumption.

Challenges

Challenges remain despite the increase in global investments in reverse osmosis desalination plants. The only proven technology that is economically viable and uses manageable levels of electricity is desalination by reverse osmosis (and energy demanding multi-stage flash distillation).

The main challenges for reverse osmosis are:

  • Most plants in the world are still powered by fossil fuels. This isn’t good for energy sustainability. The use of solar and offshore wind could help.

  • Reverse osmosis membrane fouling. This means over a period of time, substances deposit on the membrane surfaces and lead to lower performances. Some solutions to the fouling problem have been suggested.

  • The problem of dealing with brine (dissolved salt in water). This means once the water has been removed from the solution, a lot of salt remains that needs to be dealt with efficiently. All desalination systems eventually need to deal with the brine. One route is to return the salt to the sea with ample dilution. This has been done with minimal impact on the marine ecosystem.

We, along with others, are involved in exploring desalination processes to remove salts through emerging technologies such as capacitive deionisation, redox-active electrodes, and absorption studies. The advantages of these processes are that they don’t use much energy and that they’re economically viable. The main disadvantage is that they are best suited for brackish water and can’t yet handle the high salinity levels of seawater. They are also not ready to be scaled up to supply a city’s needs.

The ConversationIn the long term, new and more sustainable methods for water production by desalination and energy recovery must be found. Squeezing fresh water from the ocean might be the only viable way to increase the supply and as the examples show, there is now enough evidence to show that large seawater desalination plants are practical.

Werner van Zyl, Associate Professor of Chemistry, Lecturer in sustainable energy and water systems, University of KwaZulu-Natal

This article was originally published on The Conversation. Read the original article.

COMMENTS   16

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This was all known to CoCT for at least 10 years – their response – we don’t have a water problem! BTW check out the properties for sale in Constantia! Mansions now coming on the market. Someone has discovered that properties on Atlantic Seaboard can use seawater to flush their toilets (it goes straight into the sea apparently- charming). This luxury not available to residents elsewhere in wc.

In the late 70’s it was necessary to lower the water table where the Cuban Hat takeaway existed.
Pumps less than 100 m from high tide discharged water into a storm water drain on lower Marine Parade. Runners stopped to drink this water. The suggested that it did not taste salty.
It seems that by taking brackish water from a hight groundwater system the water will be brackish but not full of salt enabling cheaper production potable water.
Maybe this is currently the process?

Does Cape Town have the electricity to run these plants?

when I posted this over a year ago it was that the desal plant would be next to a nuclear power station. but a lot of water (sorry for the pun)under the bridge since then!!!!-

unless CoCT builds a desal plant capable of bringing water to all you can kiss your fancy house prices away – if that hasn’t happened already

https://www.youtube.com/watch?v=QqV8CBrS77M

I sense a bit of jealousy there. Is your 1 bed flat in Sydney not so nice?

With wind and solar now running at record low prices, and energy storage so expensive, it might make sense to combine massive renewable with massive desalination.

Wind farm plus solar farm on the west coast that feeds power to the grid and desalinates water that can store fairly easily and long term.

So, all energy the wind and PV can produce in peak hours goes to grid and the desal idles – avoiding filthy and R4/kWh diesel peakers. In off peak times that energy is only worth about 42c/kWh and we need less anyway, so all the energy goes to desalination.

That way the desal energy input cost is actually 42c/kWh and not the circa 90c/kWh blended average of time of day grid charges.

There is one problem with this. Desalination plants have to operate 24 hours to utilise their capacity. If you use renewables like wind or solar you have on average about 8 hours operation a day and it would mean that the plant has to be triple in capacity to produce enough in the 8 hours of operation. This would nearly triple the cost. So you either spend much more on the plant or pay more for electricity.

The article forgets to mention that the two temporary desalination plants which will sell the water for R30 per kl (in an other article the price was quoted R40), cost over 500 million to build which adds a bit over R50 per kl to the price. So we will pay over R80 per kl for the water.

Werner, you have a conversion error in you units that you use. 1 cubic metre = 1kl. Therefore e.g. CT using 500 000 cubic metres per day should be 500 000 000

Please disregard comment, my pre-coffee maths failed me this morning

Temporary desalination is expensive, permanent desalination not so much. I’ve heard permanent desalination can produce water for R16 a kiloliter (which includes financing cost). We don’t need a lot of desalinated water either, probably just a third of base load, 200 Ml per day.

Desalination requires lot of electricity. The quoted electricity requirement is between 4 and 10kWh/kl. If we take the average of these values R16 would be near to the electricity cost only without the capital repayment, maintenance, labour, etc.

Professor – kry jouself hier in Slaapstad, en maak ‘n afspraak met Mmusi – want hy dink ontsoutingsaanlegte is te duur en buite ons bereik.

The DA won’t listen to him….these arrogant know-it-all pillocks knew of the impending water disaster since 2007 and did nothing about it, apart from densify and over-develop Cape Town till the water ran out…

Sure there is less water in the main dams, however one set of farmers open up a valve and “Day Zero” is pushed a month away?

Fear tactics by politicians to get some money flowing IMO.

There is enough potable water underground to avert a crisis, but their is little money to be made for anybody in the RO plant business.

Please tell about the RO plant failures?? Plett, how many liters did it produce if ever?

Still the elite DA seems commited to have the middel class pay their masters..

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