The rapidly growing desalination industry produces water for drinking and for agriculture in the world's arid coastal regions. But it leaves behind as a waste product a lot of highly concentrated brine, which is usually disposed of by dumping it back into the sea, a process that requires costly pumping systems and that must be managed carefully to prevent damage to marine ecosystems. Now, engineers at MIT say they have found a better way.

In a new study, they show that through a fairly simple process the waste material can be 

converted into useful chemicals—including ones that can make the desalinationprocess

 itself more efficient.

The approach can be used to produce sodium hydroxide, among other products. Otherwi

se known as caustic soda, sodium hydroxide can be used to pretreat seawater going into 

the desalination plant. This changes the acidity of the water, which helps to prevent fouling

 of the membranes used to filter out the salty water—a major cause of interruptions and 

failures in typical reverse osmosis desalination plants.

The concept is described today in the journal Nature Catalysis and in two other papers by

 MIT research scientist Amit Kumar, professor of mechanical engineering John. H. Lienhard

 V, and several others. Lienhard is the Jameel Professor of Water and Food and the directo

r of the Abdul Latif Jameel Water and Food Systems Lab.

"The desalination industry itself uses quite a lot of it," Kumar says of sodium hydroxide. 

"They're buying it, spending money on it. So if you can make it in situ at the plant, that could 

be a big advantage." The amount needed in the plants themselves is far less than the total 

that could be produced from the brine, so there is also potential for it to be a saleable product.

Sodium hydroxide is not the only product that can be made from the waste brine: Another 

important chemical used by desalination plants and many other industrial processes is 

hydrochloric  acid,  which  can  also  easily  be  made on site from the waste brine using 

established chemical processing methods. The chemical can be used for cleaning parts of

 the desalination plant, but is also widely used in chemical production and as a source of 

hydrogen.

Currently, the world produces more than 100 billion liters (about 27 billion gallons) a day of 

water from desalination, which leaves a similar volume of concentrated brine. Much of that is 

pumped back out to sea, and current regulations require costly outfall systems to ensure 

adequate dilution of the salts. Converting the brine can thus be both economically and 

ecologically beneficial, especially as desalination continues to grow rapidly around the world. 

"Environmentally safe discharge of brine is manageable with current technology, but it's much 

better to recover resources from the brine and reduce the amount of brine released," 

Lienhard says.