DESALINATION has become an important tool for effective water management.
Desalination has become an important tool for effective water management. It can be seen as one solution to overcome rapid shortage in municipal water supply. But as I’ve stressed before, there is no single silver bullet to address the water issue overall. Desalination actually requires large amounts of energy and can be expensive.
In this blog post, I want to provide some insight and ideas on whether desalination can become a significant lever to combat water scarcity. It will not offer a complete answer but I do hope that the discussion generated here will provide a more comprehensive picture on this highly topical subject.
The impact of desalination is significant. In 2012, the installed global capacity for desalination totalled 27.3 cubic kilometres of generated fresh water annually. About 60% of desalination capacity treats seawater, and the rest treats other, less saline sources.
Today, capacity has increased from 17.4 cubic kilometres compared to five years earlier. This means it’s been growing at an average annual rate of 9.5%. Extrapolating with this growth rate, global capacity may reach about 140 cubic kilometres by 2030. Theoretically, the assumed increase between 2010-2030 may cover close to 5% of the estimated gap between fresh water withdrawals and sustainable supply.
But is 5% significant? Is it too high – or is there room to do more?
Price of desalination
Cost is a vital aspect to be considered. Today it has reduced to about USD 0.5-1 per cubic metre of fresh water. One cost-effective example is an installation in Singapore. In its first year of operation in 2013, the cost of desalinating water was as low as USD 0.45 per cubic metre.
But desalination cost depends on different factors. The most important is the type and stage of development in which technology is being used, and the cost of the amount of energy consumed in the process.
The table below shows the energy consumption and percentage of total installed capacity of the main types of seawater desalination:
Energy efficient reverse osmosis
Seawater reverse osmosis (SWRO) used in desalination is the most energy efficient process, and has the largest market share today – 63% of the total worldwide installed capacity. Over the past 20 years, technological progress has helped to reduce the energy requirements used in SWRO (pdf document):
The graph to the side from Yale University in 2012 suggests that such energy consumption is as low as 2 kWh (pdf document) per cubic metre. But this is still considerably higher than the 0.2 kWh per cubic metre, or even less, required for local fresh water supplies.
There is still potential for further gains in energy efficiency of desalination. In 2013, researchers from the Massachusetts Institute of Technology designed a new type of membrane composed of a one-carbon atom thick sheet of graphene. Until now, it’s only a computer model but it may pave the way for further improvements in desalination energy efficiency.
The theoretical minimum of energy required to remove 35 g/l of salt amounts to 0.8 kWh per cubic meter freshwater. This is the equivalent of the energy that would be generated when 'dissolving' the same amount of salt in water (law of conservation of energy). But getting to 0.8 kWh would need a perfect thermodynamic and mechanical system, which is not possible to build. Rather than trying to reach this limit, engineers are trying to get closer to it.
Bigger is better
Total cost also depends on the size of projects. As a rule of thumb, water produced in smaller plants – a few thousand cubic metres per day – would cost 3-5 times more than larger plants.
A new plant due to start operations early this year will be located in Ras Al Khair in Saudi Arabia. It will be the largest hybrid desalination plant in the world combining thermal and membrane technologies to serve about 3.5 million people in the city of Riyadh.
Part of the water debate
There seems to be opportunities in desalination, especially as seawater is available in nearly unlimited amounts. Desalination looks set to become more energy efficient, with a further reduction in costs, and high growth rates for new desalination capacity will continue.
But desalination is mainly a solution for communities based near a coast, as long-distance transfers of freshwater can be quite expensive.
More desalination not only comes with high investment and running costs but also has direct and indirect environmental impact from the different types of energy used.
From such insights, seawater desalination surely has its place in the global water debate. I welcome your thoughts, comments and ideas.
*data from Desware; **data from IDA