Clean Water Desalination dilemma

An article of POWERMAG written by Kennedy Maize and published on 1 March 2016 is about water desalination, and how to go about producing it.  Some sort of Clean Water Desalination dilemma is on going. 

Excerpts of the article titled  Desalination Expands, but Energy Challenges Remain are proposed here below.

Unfortunately like for most good things, desalinated water comes at a fairly high price in terms of amount of energy used usually of the fossil type and its consequent carbon footprint.  Ways and methods of alleviating these are being researched throughout the world and this would obviously interest the MENA countries perhaps more than any other countries in the world.

. . . At the ballyhooed Paris climate conference (COP21) last December, a little-noticed event occurred that could lead to important developments for electric generators.  At the Paris meeting, some 80 signatories— including national governments, energy and water industries, research groups, universities, and nongovernmental organizations— launched the Global Clean Water Desalination Alliance.  The group’s focus, which it calls “H2O minus CO2,” is on how to reduce carbon dioxide emissions from the energy intensive process of turning seawater into a potable product. A press release from Paris announcing the organization’s founding noted that access to clean water is “already a major challenge for as much as one-quarter of the world’s population,” and that some forecasts are “predicting that by 2030, 47% of the global population will face water scarcity.” It’s not that the world is short of water—which covers some 70% of the planet’s surface and is entirely renewable—but that most of it is seawater. . . .

Desalination Technologies

Separating salts and other impurities from H2O is a well-understood process with a long history.  Desalination of seawater, brackish water, and recycled water is widely practiced around the world in a variety of ways.  Generally, two types of separation technologies—thermal and membrane—dominate, each with about half of the global market (for a more detailed discussion, see “Adding Desalination to Solar Hybrid and Fossil Plants” in the May 2010 issue online at powermag.com).

Thermal Desalination

Thermal technologies use heat to vaporize seawater, condensing the steam as pure water.  The three approaches used with thermal desalination are multi-stage flash distillation (MFD), multi-effect distillation (MED), and vapor compression distillation (VCD).  In MFD, feedwater is heated under high pressure and then flows as a liquid into a successive series of chambers with progressively lower pressures.  Because each stage is lower in pressure than the one before, the liquid water continues to flash to steam, which is collected by heat exchange tubing running through each stage.  MFD technology dates to the 1950s.  MFD plants dominate the thermal sector, and many have been built in the Middle East, where water is scarce but energy resources are cheap and plentiful.  MED was first used in the late 1950s and early 1960s. In MED plants, a series of evaporator vessels are held at progressively lower temperatures and pressures. Because the boiling point of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next, and only the first vessel requires an external source of heat. Three MED plants with combined capacity of 3.5 million gallons (13,250 m3) per day operate in the U.S. Virgin Islands, serving as the principal water supply.  VCD uses heat from compression of vapor, rather than an external heat source. Typically, a mechanical compressor is used, often powered by a diesel engine. These desalination units are generally small and can be used at hotels, resorts, and in industrial applications.

Membrane Desalination Membrane technologies separate salts from water using exceptionally fine screens or membranes. The two categories are electro dialysis (ED) and reverse osmosis (RO). ED, introduced in the 1960s, is voltage driven and generally used for treating brackish water. Most salts dissolved in water are negatively or positively charged ions. The technology uses electrodes of opposite charge to attract the ions, with membranes to permit selective passage of either positively charged cations or negatively charged anions.  An ED stack consists of several hundred such cells that the feed water is pumped through.  RO is the latest technology, commercialized in the 1970s and based on Israeli research and development.  It is the most widely used desalination technology in the U.S.  This process reverses normal osmosis—in which a solvent moves from zones of low solute concentration to zones of high concentration—by applying pressure to the zone of high concentration.  This causes the pure solvent—in this case, purified water—to flow continuously to the low-concentration side of the membrane.  RO works for both seawater and brackish water, and removes all impurities, not just salt. ED and RO can be used together, with the ED stack treating both the RO feed water and its brine stream. . . .

That’s where the alliance announced in Paris—led by Masdar, the United Arab Emirates’ (UAE’s) renewable energy company, and the International Desalination Association—comes in.  The alliance said its “goal is to seek solutions that will substantially reduce the projected increase in CO2 emissions from the desalination process, as global demand for drinking water continues to grow.” The group said it is seeking “a decrease in emissions from 50 [million tons of CO2] up to as much as 270 [million tons] per year by 2040.”. . .

Government-owned Masdar last November began development on a pilot seawater desalination plant using solar energy, which the company says it will run at small scale for 15 months. “These technologies have never been used on a utility scale anywhere in the world,” said Masdar. Just days after the announcement at the Paris COP21 meeting, the UAE and China signed a deal to work together to combine Masdar’s desalination technology with low-cost solar photovoltaic technology developed in China.

There are currently about 15,000 desalination plants operating around the world, with the largest in Saudi Arabia, the UAE, and Israel.  The Saudi Shoaiba complex produces over 232 million gallons (880,000 m3) daily, while the Al Jubail complex produces over 211 million gallons (800,000 m3) per day. The big Saudi plants use a variety of desalination technologies. . . .

What’s Ahead for Desalination ?

Massachusetts Institute of Technology (MIT) researchers are testing a new approach to desalination that relies neither on energy-intense thermal distillation nor RO membrane technology, which can clog and decrease the efficiency of the process. According to a university press release, “Instead, the system uses an electrically-driven shockwave within a stream of flowing water, which pushes salty water to one side of the flow and fresh water to the other, allowing easy separation of the two streams.”

MIT professor Martin Bazant says the approach is “a fundamentally new and different separation system.” It is a continuous process that Bazant claims may be relatively easy to scale up. According to MIT, one of the uses for the technology could be to clean up the large amounts of wastewater generated by hydraulic fracking for gas and oil.

More conventionally, researchers at Egypt’s Alexandria University are looking at a combination of low-tech filtration and evaporation, which could lower desalination power requirements. In a paper in the September edition of the journal Water Science & Technology, the researchers describe a filtration technique known as “pervaporation,” which passes saline water through a fairly simple membrane to remove large molecules and then vaporizes the filtered water.  The technology is now used in wastewater treatment to separate organic solvents from the water stream.”

Further reading and details are at http://www.powermag.com/desalination-expands-energy-challenges-remain/