Drinking Water and Water

Seawater and Desalination

The seas are made up of water, but that water is not drinkable, requiring treatment in desalination plants prior to distribution in drinking water systems. To understand how and why it is not drinkable, and the complexities of desalination, it is necessary to look at the chemical and physical properties of seawater. Following this, two different desalination methods are discussed.

Chemical and Physical Properties of Sea Water

Seawater is made up of water and various salts. Water, which makes up approximately 96.5% of all seawater (Anthoni, 2006), is believed to originate mainly from water which condensed in the earth's early atmosphere, falling to the ground when the crust of the earth solidified (Pidwirny, 2006). Additional water has been added through volcanic activity, while some scientists speculate comets entering the atmosphere may also have been a water source (Pidwirny, 2006).

The dissolved salts in the water have a continental source, released into the water as rocks were weathered and carried to the sea by rivers (Pidwirny, 2006). There are up to 82 different elements which may be present in seawater (Turekian, 1968), however, only 6 elements make up 99%, with chlorine making up 55% of all sea salts and sodium making up 30.6%. The main constituents of water are detailed below.

Element

Parts per million

Oxygen

883,000

Hydrogen

110,000

Chlorine

19,400

Sodium

10,800

Magnesium

1,290

Sulfur

Calcium

Potassium

Bromine

63.7

(Anthoni, 2006)

Physically, the salinity of water can vary, but the proportion of the different salts remains very consistent (Pidwirny, 2006). However, other physical properties of seawater depend on the salinity (Pidwirny, 2006). Seawater usually boils at 100.56° Celsius at normal atmospheric pressure (Bullard, 2015). At 3.5% salinity seawater freezes at -1.91° Celsius, with most of the salts forced out during the freezing process (Pidwirny, 2006). When frozen, water has a lighter density of between 0.84 and 0.91 Mg m-3 when located above the waterline, and 0.90 to 0.94 Mg m-3 for ice located below the waterline, meaning ice floats (Timco & Frederking, 1996)

The mean conductivity of seawater is 3.27 S. m-1 excluding shallow oceans, and at a depth pf 400 meters the conductivity increase, so it is 6% greater than the surface (Bullard, 2015).

Desalination

There are three main types of desalination process; thermal distillation, the use of electric current, and reverse osmosis (Cipollina, Micale, & Rizzuti, 2009). The world's largest desalination plant currently in operation; Sorek, located in Israel, ten miles south of Tel Aviv, uses reverse osmosis to provide Israel with 20% (627,000 cubic meters) of all its drinking water requirements (Talbot, 2015). Reverse osmosis uses high levels of pressure to force water through a permeable membrane, usually in a single process which removes the salts from the seawater, allowing only the water to pass though (Cipollina et al., 2009). This is often criticised as incurring the highest costs of all three dilatation methods, with costs increasing as the salinity level increases (Karagiannis & Soldatos, 2008). The plant, which cost U.S. $500 million to build, is able to overcome the cost disadvantages by obtaining economies of scale, such as the use of larger than usual pressure tubes at 16 inches rather than the usual 8 inches, as well as the leveraging new technology (Talbot, 2015).