Sea water forms part of the hydrological cycle – water is evaporated from the sea, and other water bodies, where it is held in the atmosphere as water vapor. When the water evaporates, any salts and other impurities remain behind. As the air cools, the water vapor condenses, forming raindrops, which are suspended in clouds. These raindrops have effectively been purified, having the salt, minerals, and other impurities removed, much like distillation. However, when rain falls to the earth, contaminants suspended in the atmosphere adhere to the droplets, which once again become contaminated.
Once it reaches the land, rainwater washes over urban surfaces and agricultural land, gathering more pollutants; it eventually reaches rivers and streams, where the water may become even more polluted. Rivers eventually empty out into the sea, bringing with them all the minerals and other impurities gathered along the way.
Consequently, sea water contains a vast amount of impurities, other than the obvious, salt. Sea water consists of many different components, with the common constituents being: chloride, sodium, sulphur, magnesium, calcium, potassium, bicarbonate, bromide, strontium, boron, and fluoride, whose concentrations tend to vary from sea to sea. In addition, sea water may contain many organic and inorganic toxins, such as industrial chemicals, pesticides, and other contaminants that wash into rivers, and finally flow into the sea.
We cannot drink sea water as with salinity levels ranging from 32000 mg/L – 45000 mg/L, it is far too salty, and because of this, it is bad for our health. Water with a salinity of up to 500 mg/L is considered normal for drinking, but we can drink water with a salinity of up to 1000 mg/L, although the lower the salinity level the better. Long term consumption of water with salinity levels higher than 1000 mg/L can cause health issues, such as high blood pressure, which can cause heart attacks and strokes.
However, it is possible to drink sea water if the salt is removed. There are a number of desalination methods that can be used to remove salt from sea water. These include: distillation, reverse osmosis, freeze-thawing, and electrodialysis; however, some methods are more effective for rendering sea water suitable for drinking than others.
Distillation – When water is heated, it transforms from a liquid to a gas state, leaving the impurities behind. The vapor can be removed, then cooled, to provide purified water, free from salt and other impurities when water transforms from a gas state back to a liquid through the process of condensation.
Reverse Osmosis – Pressure is used to force sea water through a semi-permeable membrane, which only allows water molecules to penetrate; salt and other dissolved or suspended impurities remain behind.
Freeze-thawing – Sea water is cooled transforming water from a liquid state to solid ice. The ice is then separated from the ice slurry and the concentrated brine water, and melted, transforming ice back into liquid water. However, this method has many drawbacks, including the need for complex refrigeration processes, and the logistics of handling and disposing of the highly concentrated brine slurry that results as a waste product.
Electrodialysis – By using an electrically charged current, salt ions (cations and anions) are attracted to negatively or positively charged poles of a charged electrode. Electrodialysis can effectively remove ionized salts from sea water, however, it is ineffective at removing other contaminants, and consequently many impurities remain behind. It is therefore not suitable for providing fresh drinking water, but sea water treated in this manner may be used for irrigation or industrial purposes.