Desalination: Water for an Increasingly Thirsty World
This is part of our special feature on Water in Europe and the World.
Within the next decade, water shortages are projected to affect 40 US states and effectively all Americans.
Desalination: Water for an Increasingly Thirsty World
This is part of our special feature on Water in Europe and the World.
Within the next decade, water shortages are projected to affect 40 US states and effectively all Americans.1 The issue of water accessibility is not one limited to the US, however, as the problem of clean water availability has become more widely recognized in recent years. For example, the US National Academy of Engineering has recognized the urgent need to provide access to clean water as one of the “Grand Challenges for Engineering.”2 Desalination, or the separation of salt from water, will play a key role in addressing this challenge, as the vast majority of water sources on Earth are too salty to be used for agricultural or domestic purposes.3
Coupled to this need to provide access to clean water is the need to do so in an energy efficient and environmentally friendly manner. The intrinsic coupling between water and energy resources has been recognized broadly over the last decade.4-10 Ultimately, there is a fundamental energy cost associated with purifying water, and many widely used energy generation technologies require clean water (e.g., for cooling and/or to operate boilers). For example, in the United States, more than 133 billion gallons of water per day (roughly 41 percent of the total freshwater and saline-water withdrawal) is used for thermo-electric power generation.11 Furthermore, the laws of thermodynamics set the required minimum energy for desalination; 1.06 kWh is required to produce one cubic meter of desalinated water from two cubic meters of seawater containing a “standard” amount of salt (35 grams of sodium chloride per liter).12 This energy requirement, which is slightly less than the amount of energy required to operate a modern household refrigerator for a day (typically around 1.5 kWh), is simply the minimum energy required to separate the salt molecules from the water molecules and does not account for inefficiency in the process or other operating expenses associated with the desalination process. Desalination will always involve an energetic cost, but improvements in efficiency and process design over the last few decades have pushed desalination technology much closer to the fundamental limit for the energy cost of the overall process.12
Desalination technology is neither new nor perfect. Several technologies currently are used to desalinate water, and a combination of different challenges and application needs creates a situation where no single technology is a perfect fit for all situations. Desalination is performed on a very large scale, and the volume of water desalinated worldwide each day exceeds the volume of oil produced worldwide each day by roughly a factor of six.13-14 Let me discuss the common desalination technologies and technological challenges.
What is desalination and how is it done?
Desalination generally refers to any process that separates salt and water. Often, the salty water in question broadly can be characterized as either seawater or brackish water, depending on the salinity of the water. The salt content of water varies depending on the source, but seawater typically has a total dissolved solids, TDS, (or salt) concentration of 35 to 45 grams per liter, while brackish water sources typically have TDS concentrations of 1 to 10 grams per liter.15 The specific desalination process used typically depends on the salt concentration of the water source as well as other factors, such as the specific contaminants found in a given water source.
Desalination technologies often fall into two broad classes: thermally-driven or barrier desalination processes. Thermally-driven processes remove water from salt by boiling or otherwise vaporizing water; salt is not volatile, so water can effectively be removed from salt by this mechanism.16 Barrier methods typically involve a selective membrane to perform the separation; in processes such as reverse osmosis (RO) and nanofiltration (NF), water selectively passes through the membrane leaving the salt behind (Figure 1A), but in electrodialysis (ED) processes, electricity is used to drive selective transport of charged salt ions through special charged membranes, effectively removing the salt from the water (Figure 1B).17
Figure 1. (A) In reverse osmosis (RO) or nanofiltration (NF) desalination, water selectively passes through the membrane leaving salt (e.g., sodium chloride, NaCl) behind. (B) In electrodialysis (ED) desalination, ions are moved through special anion exchange or cation exchange membranes (AEMs or CEMs).
In general, ED processes are most efficient in situations where the source water is not particularly saline, and RO processes are used more widely for desalinating higher salinity water.17 If the salinity is too high, however, thermally-driven desalination methods can become the most efficient desalination method.17 For particularly hard water sources, NF desalination can be used to selectively remove the calcium and magnesium ions that contribute to water hardness.17
Reverse osmosis technology currently dominates the desalination market, and more than half of the worldwide installed desalination capacity is based on RO technology (F