Seawater Chemistry and Desalination Methods Explained

Introduction

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 seawater is unfit for drinking, and to appreciate the complexities of desalination, it is necessary to examine the chemical and physical properties of seawater. Following this, two different desalination methods are discussed.

Chemical and Physical Properties of Seawater

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

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

(Anthoni, 2006)

Physically, the salinity of seawater can vary, but the proportion of the different salts remains very consistent (Pidwirny, 2006). Other physical properties of seawater, however, depend on the salinity level (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, which is why ice floats (Timco & Frederking, 1996).

The mean conductivity of seawater is 3.27 S m⁻¹, excluding shallow oceans. At a depth of 400 metres, conductivity increases to approximately 6% greater than at the surface (Bullard, 2015).

Desalination Methods

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 metres) 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 that removes the salts from the seawater, allowing only water to pass through (Cipollina et al., 2009). This method is often criticised for incurring the highest costs of all three desalination approaches, with costs increasing as the salinity level rises (Karagiannis & Soldatos, 2008). The Sorek plant, which cost U.S. $500 million to build, overcomes these cost disadvantages by achieving economies of scale — for example, using larger-than-usual pressure tubes at 16 inches rather than the standard 8 inches — as well as leveraging new technology (Talbot, 2015).

The oldest form of desalination is thermal distillation (Cipollina et al., 2009). This is the process used at the Curacao Netherlands Antilles plant, built in 1928 (IETC, 1997). In this process, seawater is heated to create water vapour; the vapour is then condensed to produce fresh water (Cipollina et al., 2009). The cost of thermal desalination is reduced today by lowering the pressure in the distillation chambers, which decreases the temperature required to create water vapour (IETC, 1997). The process also conserves energy through the interchanging of condensation and vaporisation within units, with heated brine recirculated throughout (IETC, 1997).

Conclusion

Seawater provides an excellent potential source of fresh water. Through different types of desalination, seawater may be transformed into safe drinking water by removing its salts and minerals.

References

Anthoni, J. F. (2006). The chemical composition of seawater. Retrieved from http://www.seafriends.org.nz/oceano/seawater.htm#composition

Bullard, E. (2015). Physical properties of sea water. Retrieved from

Cipollina, A., Micale, G., & Rizzuti, L. (2009). Seawater desalination: Conventional and renewable energy processes. New York: Springer.

IETC. (1997). Source book of alternative technologies for freshwater augmentation in Latin America and the Caribbean. Retrieved from http://www.oas.org/dsd/publications/unit/oea59e/begin.htm#Contents

Karagiannis, I. C., & Soldatos, P. G. (2008). Water desalination cost literature: Review and assessment. Desalination, 223(2), 448–456.

Pidwirny, M. (2006). Physical and chemical characteristics of seawater. Retrieved from http://www.physicalgeography.net/fundamentals/8p.html

Talbot, D. (2015). The world's largest and cheapest reverse-osmosis desalination plant is up and running in Israel. Retrieved from

Timco, G. W., & Frederking, R. M. W. (1996). A review of sea ice density. Cold Regions Science and Technology, 24(1), 1–6.

Turekian, K. K. (1968). Oceans. London: Prentice Hall.