Desalination (or desalinization) – the process of removing dissolved salts from water – is a technology that has been used for centuries. References to desalination can be found as far back as the writings of Aristotle (320 BC) and Pliny the Elder (76 AD). It is widely used at sea to this day and has helped keep many early mariners alive during long ocean trips. In fact, a typical nuclear-powered U.S. aircraft carrier today uses waste reactor heat to desalinate 400,000 gallons of water per day.
Significant advances in desalination technology started in the 1900’s and took a major step during WW II because of the need to supply potable water to military troops operating in remote, arid areas. By the 1980’s desalination technology was commercially viable and commonplace by the 1990’s. Today there are more than 16,000 desalination plants worldwide, producing more than 20 billion gallons of drinkable water every day. This is expected to reach more than 30 billion gallons per day by 2020, with one third of that capacity in the Middle East. To put that number in perspective, current global water consumption is estimated to be just under 1,200 billion gallons.
Why is desalination so important? The earth is a water-rich planet, to the tune of about 300 million cubic miles of water, and each cubic mile contains more than one trillion gallons. The problem is that most of that water, approximately 97 percent, is in the oceans which have an average salt content (salinity) of 35,000 parts per million by weight, and drinking that water regularly can kill us. To quote ‘How Desalination Works’ by Laurie Dove: “Ingesting salt signals your cells to flush water molecules to dilute the mineral. Too much salt, and this process can cause a really bad chain reaction: Your cells will be depleted of moisture, your kidneys will shut down and your brain will become damaged. The only way to offset this internal chaos is to urinate with greater frequency to expel all that salt, a remedy that could work only if you have access to lots of fresh drinking water.”
What about the water that is not in the oceans? Three percent of 300 million cubic miles is still a lot of water. Unfortunately, most of that three percent is not easily available for our use. Some is tied up in icecaps and glaciers, some is tied up as water vapor in the atmosphere, and the rest is in groundwater, lakes and rivers. The other hard fact is that some of our freshwater supply is simply inaccessible due to its location and depth. The net result is that we make productive use of less than one percent of our global water resources.
Saline, salty water comes in different ‘strengths’ – seawater as mentioned above, and brackish water which has less salt than seawater but more salt than fresh water. It may arise from mixing of fresh water with seawater, a situation that is occurring more frequently as sea levels rise due to global warming, or it may occur in brackish fossil water aquifers that are quite old. Commonly accepted definitions of saline water are:
– fresh water: less than 1,000 parts per million (ppm)
– brackish water: 1,000-10,000 ppm
– highly saline water: 10,000-35,000 ppm (including seawater)
How does one separate salt from saline water to produce fresh water, and what are the barriers to more widespread use of desalination? The latter question is easily answered: the energy required to do the separation, the energy required in some cases to move fresh water to higher elevations, and the associated costs.
There are quite a few technologies today for removing salt from saline water, the oldest being sun-heated water that evaporates and is then condensed, leaving the salt behind. This is also a description of the earth’s hydrologic cycle. The most widely used desalination technologies today are reverse osmosis (RO/60%), multi-stage flash distillation (MSF/26%), and multi-effect distillation (MED/8.2%). Others include electrodialysis, electrode ionization, and hybrid technologies. Energy requirements (electrical + thermal) for desalinating a range of saline waters, expressed in kWh per cubic meter of fresh water and exclusive of energy required for pre-treatment, brine disposal and water transport, are: RO/3-5.5 kWh; MSF/13.5-25.5 kWH; MED/6.5-11 kWH. Reverse osmosis requires no thermal energy, just mechanical energy to force salty water through a membrane that separates the salt from the water. The laws of physics tell us that the minimum amount of energy required to desalinate seawater is about 1 kWh per cubic meter and under 2 kWh per cubic meter has been achieved in RO, leaving limited opportunities for further reductions.
Generally, costs of desalinated water are higher than those of other potable water sources such as fresh water from rivers and groundwater, treated and recycled water, and water conservation. Needless to say, alternatives are not always available and achievable desalination costs today range from $0.5-1 per cubic meter. To put this into perspective, bottled water at $1/liter corresponds to $3,785 per cubic meter.
Desalination projects can be found in about 150 countries, with many more being planned or under construction. Today’s largest users are in the Middle East – for example, Saudi Arabia derives 50% of its municipal water from desalination and Qatar’s much smaller fresh water supply is entirely from desalination. Currently under construction in Kuwait is a power plant-desalination combined facility that will produce 1.5 GWe and 486,000 cubic meters of fresh water a day. It is scheduled for completion in 2016.
As world population increases along with demand for clean water desalination will become an increasingly important part of our water supply in the 21st century. We will not run out of water but we will pay more for receiving it in potable form.