The Conversation/Werner van Zyl: Desalination: global examples show how Cape Town could up its game

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Desalination: global examples show how Cape Town could up its game
February 8, 2018 4.49pm SAST
Author

    Werner van Zyl

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

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Werner van Zyl receives funding from the NRF, Eskom (TESP) programme, and the Technology Innovation Agency (TIA).
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The Conversation is funded by Barclays Africa and seven universities, including the Cape Peninsula University of Technology, Rhodes University and the Universities of Cape Town, Johannesburg, Kwa-Zulu Natal, Pretoria, and South Africa. It is hosted by the Universities of the Witwatersrand and Western Cape, the African Population and Health Research Centre and the Nigerian Academy of Science. The Bill & Melinda Gates Foundation is a Strategic Partner. more
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Cape Town has started down the road of desalination. Shutterstock

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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.

In 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.

    Drought
    Desalination
    Cape Town
    water shortage
    drought response
    Cape Town drought

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2 Comments
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    Peter Ravenscroft

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    A superb effort and analysis,Werner. Cape Town owes you heartfelt thanks, and here is some of that. Thank you.

    My guess from a distance is that in the long run, this is going to be a large part of the solution. But day zero just over three months away, so this will not solve the crisis now looming.

    Multiple poly lines running from the intakes on the big dams, to smaller valves on the remaining 10 percent ponds, is the . quickest and cheapest option.

    Next, Table Mountain, Devil’s Peak the Twelve Apostles, and all other Cape Peninsula and Cape Fold Belt mountains, are huge natural pure water condensers. The quartzite rock is not porous, with water between the quartz grains, but in the fractures and joints and faults  And that is why Cape Town was called Camissa - the Place of Sweet Water - and had 31 natural springs. Much of that runs . See the tablecloth in the summer south-easters - the white stuff is all condensed water. Some is caught, but much of it has never been noticed as it runs into the sea all along the coast, particularly through the white sand beaches and under the sand.

    We now have a sea surface temperature map, from NASA/Fishtrack, updated daily, that picks up and maps that flow, as it is lighter and a degree or two warmer than the very cold Cape seawater .I sent a copy of the first to the Cape  Town Council Deputy Mayor, but no response or even acknowledgement yet.The map is of 5 megabytes, and at my end at least, is accurate for temperatures to every last single pixel. The coastline NASA puts up is less perfect, but not seriously so. We have been using these maps to get warm water going down rivers and via groundwater flows, to detect pending earthquakes days to months in advance, for a few years now. It works, so this is just borrowing a known technology. The results match similar estimates from the geology and topography, very well.

    That pure fresh water just before it reaches the saline sea-derived groundwater, can be carefully skimmed off by pumping, at thousands of intakes right on the strandline, all linked to poly lines running along the coast feeding into the mains grid. Avoiding of course those places with high populations and inadequate piped away sewerage.

    Unfortunately as The Conversation cannot post attachments, I cannot put up the map here. It is however public domain and free, so I can email it to anyone sending an ordinary email address. p.s.ravenscroft@gmail.com

    Peter Ravenscroft, geologist, Brisbane, but ex-CT for 25 years and UCT
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    Neville Long

    With everyone criticizing Cape Town for the mess they are in regarding their lack of preparedness for “Day Zero”, two things to note, the South African government are responsible for the supply of bulk water to the country as a whole and the provinces are responsible for distribution and management of that water provided. I’m pretty sure that the city manages we’re probably beating the door down with the department of water affairs and got little or no support. Secondly, Cape Town has a Company called GrahamTek- it’s been around for 30 years and are remarkabley good at water desalination. They have plants installed all over the world and are leaders in the field. It could have built a plant twice the size required to Cape Town. (They are fully capable of doing this) My guess, they are not BEEE, not interested in collusion politics and are therefore not asked or approached by Govt. to be part of the solution. Apparently, they have a small plant ready and waiting, but, there are no funds to pay for it. We don’t have to use over seas companies. We have one concerned Business in the heart of the city with the solution!!! Bet politics have been playing a serous part in this saga!
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