Desalination: global examples show how Cape Town could up its game

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 meters 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 $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 meters per day. It supplies drinking water to around 1.3 million people in the region. It caters to 20 percent 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 meters per day or 26 000 kiloliters 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 kiloliter. 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 deionization, 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.

Werner van Zyl is Associate Professor of Chemistry, Lecturer in sustainable energy and water systems, University of KwaZulu-Natal. This article is published courtesy of The Conversation (under Creative Commons-Attribution / No derivative)