Technological advances have made removing salt from seawater and waste water less energy-intensive, but will they simply encourage us to use more?

The average Briton uses 150 litres of water a day; the average American gets through 570 litres of the stuff. The world is getting thirstier and the global demand for fresh water is rising by 640bn litres a year.

Population growth is one factor, not only the need for drinking water and sanitation but also the need to produce more food. Agriculture accounts for 70% of water use.

Even the push for biofuels to reduce consumption of fossil fuels has an unexpected consequence: between 1,000 and 4,000 litres of water are needed to produce just one litre of biofuel.



While reducing consumption is one way of helping to address the water crisis on an individual level, it is far from the complete solution. Something on a larger scale is also needed: desalination.

As climate change makes rainfall less predictable and droughts more common, a growing number of countries are turning to desalination. The term is used to refer to removing salt from both seawater and subterranean “brackish” water, as well as the treatment of waste water (aka sewerage) to make it drinkable.

Some environmentalists have long opposed desalination because of the energy the process demands, as well as other considerations such as the impact of sucking in large quantities of seawater from the ocean.

Facebook Twitter Pinterest Huntington Lake in the High Sierra, California, has run dry following the four-year drought in the state. Photograph: Lucy Nicholson/Reuters

But technological advances in recent years have altered the equation. The most common form of desalination is reverse osmosis; it involves forcing water through cartridges that contain thin-film composite polyamide membranes, which trap salt and other impurities but allow the fresh water through.

Randy Truby, comptroller of the International Desalination Association, says that advances in manufacturing processes have allowed 450 sq ft of membrane to be crammed into each cartridge, compared with 300 sq ft when they first came on the market. But treating seawater still requires pressure of about 80 bar, 40 times more than car tyres. That is why treating seawater is more energy-intensive than brackish or waste water, which require less force.

The location of a seawater desalination plant also makes a difference, Truby adds: while the salt content of water off the coast of California is about 34,000 parts per million, the figure in the Middle East is more like 40,000.

No alternative

Saudi Arabia is the country that relies most on desalination – mostly of seawater. The US is in second place. It uses mainly brackish and waste water although later this year it will open one of the world’s largest seawater desalination plants in Carlsbad, San Diego.

Truby says: “In many places there is no alternative – certainly the Middle East and places like Singapore, the Canary Islands and the Caribbean have to look to the sea. Those that have a choice, like Europe and the US, China, Japan, will try conservation and re-use and brackish treatment and use [seawater] desalination as a way to top-up and provide some drought-proofing.”

Facebook Twitter Pinterest The desalination plant in Carlsbad will use the reverse osmosis process to produce fresh water. Photograph: Gregory Bull/AP

Desalination remains about twice as expensive as treating rainwater or waste water, at about $3 (£1.95) per cubic metre, but the economics depend on a number of variables, explains Professor Raphael Semiat of Technion, the Israel Institute of Technology, in Haifa.

He says 3.5 kilowatt hours (kWh) of electricity are needed to desalinate 1 cubic metre of seawater – 1.3kWh to pump seawater to the plant and 2.2kWh for the reverse osmosis process.

Pumping a cubic metre of fresh water distances of more than 200km requires more energy than desalinating the same amount of seawater, according to Semiat. In addition, many plants produce the bulk of their water at night when there is less demand for electricity, and thus utilise power that would otherwise go to waste.

Philip Davies, reader in mechanical engineering and design at Aston University in Birmingham, argues that desalination is not an expensive way of producing drinking water.

He adds: “The trouble is most distribution systems don’t allow us to distinguish between drinking water or water used for sanitation. It’s also very difficult to put differential costing on water to reflect merits of its use, because at the end of the day you’ve got to make water affordable to everybody. There are much cheaper ways to economise on water than desalination … we should be re-using water for sanitation or irrigation.”

Davies points out that reverse osmosis is not ideal for developing countries because the maintenance of the membranes required to keep them running effectively is more problematic in a country like India.

Most desalination on the subcontinent is of brackish water that contains high levels of impurities, meaning the membranes can easily become clogged: “They are just filters and they get blocked up like anything else unless you have the right sort of pre-treatment.”

The expense of operating a desalination plant is another issue in developing countries. While NGOs can provide seed funding, they are less able to cover running costs.

One solution could be a micro-enterprise project Davies has been involved with near Jodhpur in India. As well as producing desalinated water, it generates incomes from farmers who pay to have their seeds pressed to produce castor oil, and provides refrigeration for ripening bananas.

There is also a growing effort to reduce the environmental impact of desalinating brackish water. The salt recovery rate is typically about 50%, meaning that the waste salty brine is often injected back into the ground in places such as India and Pakistan.

Saltwater greenhouses

Such a strategy is not sustainable because it increases the salinity of soil or rivers further downstream. Davies says increasing the amount of salt being removed to between 70% and 90% solves that problem, but requires more energy – although he has devised a system of solar power to keep usage to a minimum.

Seawater greenhouse technology is well suited to arid parts of the world. Photograph: Seawater Greenhouse

The academic is also involved in a project in Somaliland, which faces the twin challenges of rapid population growth and limited water resources. It is one area using seawater greenhouses, which produce water for irrigation by pumping seawater into the greenhouse and piping it over honeycomb cardboard pads that provide a large area for evaporative cooling.

According to Charlie Paton, who founded Seawater Greenhouse Ltd two decades ago, a seawater greenhouse cools the air by up to 15 degrees and increases humidity to as much as 90% even in some of the world’s most arid places. Davies says they can reduce the amount of water needed to produce a kilogram of produce from hundreds to tens of litres.



Researchers continue to seek to improve the technology behind reverse osmosis. Adel Sharif, professor of water engineering and process innovation at the University of Surrey, co-developed a new form of desalination called manipulated osmosis in 2003.

Facebook Twitter Pinterest Thames Water’s desalination plant at Beckton in east London can produce 150m litres of water a day. Photograph: Peter Macdiarmid/Getty Images

He says the technique helps to remove impurities such as limescale before the reverse osmosis process, which reduces the amount of energy needed by as much as 30%, as well as increasing the lifespan of the membranes and reducing maintenance costs.

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The first plant using manipulated osmosis began operating in Gibraltar in March 2009. A second opened in Oman later that year, and another opened in 2010. The company founded by Sharif to commercialise the technology, Modern Water, is listed on the AIM stock market in London, but he admits it has struggled to win contracts and may end up being bought by a larger desalination player.

Many desalination companies are eyeing China, which has just 7% of the world’s freshwater but a fifth of its population. About 400 cities face serious water shortages and Beijing aims to quadruple its seawater desalination capacity to 3.6bn litres a day by 2020.

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Even London now has a seawater desalination plant. The city and the southeast of England is a “water-stressed area”, says Simon Earl, head of water production for Thames Water. However, the plant is not routinely used and is there “in case of severe drought”, like the one that occurred in 2011 and 2012 after two dry winters.

Facebook Twitter Pinterest The pressure cartridges at Thames Water’s desalination plant in Beckton, east London. Photograph: Peter Macdiarmid/Getty Images

Surprisingly, there is no wastewater recycling in London, although Earl says building that capacity – or a new reservoir or pumping water from other parts of Britain – are options for coping with rising demand.

As the world’s population continues to rise, the pressure on water resources are only going to increase. It is yet to be seen whether technological advances will be able to meet the demand.