About 70% of the earth’s surface is roofed in water. 97% of it exists as seawater (saline water). Only 2-3 per cent are freshwater, but most of them are within the north and south poles in the form of glaciers. The remaining one per cent of the freshwater is usually groundwater, and few of it fills the lakes and rivers. Seawater has about 35,000 ppm of salt dissolved in it, whereas freshwater has less than 1000 ppm, and salt cannot just be filtered through any old filter, so it needs a unique treatment process.
Photo by Anderson Rian on Unsplash
Why can’t we drink seawater?
Drinking seawater is toxic to human beings because our bodies will not excrete the salt from the seawater. Though our kidneys typically remove excess salt by producing urine, our body also requires freshwater to dilute the salt in our body to work correctly. Thus freshwater is essential in metabolism for all organisms to survive.
There are two main ways of eliminating the salt from seawater: desalination (a primary process) that uses special membranes and the other one is distillation.
The reason to convert seawater to freshwater:
The freshwater will provide the way for the drinkable water, also known as potable water, directly consumed by Humans. The sources for converting into freshwater are seawater and brackish water or brack water which are more saline than the freshwater but are less saline than the seawater. Water is essential for life and is important for the metabolism of many organisms. Drinkable water is required every day for the proper functioning of the body. In order to achieve that, seawater which is an unlimited source can be used to provide freshwater by various processes.
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Thermal Distillation:
To distil water, heat is applied until it boils and then cools the vapour down. Thus, the water turns into vapour, whereas the salt does not separate the two, and the condensed water is condensed elsewhere.
Desalination:
Desalination is the common process of producing fresh water from seawater. The Shoaiba desalination plant located in Saudi Arabia is the largest of this type. If we are going to use a membrane to separate the salt and water, we need a selectively permeable membrane with pores so tiny that the water can pass through, but salt cannot. We need a reverse osmosis set or forward osmosis set up to enable the water to flow through the pores. Reverse osmosis uses a pump to pressurise the saltwater and force the water through the membrane, whereas the forward osmosis uses a charged nanoparticle that is easy to remove salt on the opposite side of the membrane takes advantage of osmotic pressure to move water from one salt solution to another.
The disadvantage of desalination is that it requires a lot of energy. Salt dissolves easily in water which forms strong chemical bonds that are very difficult to break. As a result, energy and technology to desalinate seawater are both large and expensive.
Did you know?
The Shoaiba desalination plant, located in Saudi Arabia on the coastal area of the Red Sea, produces freshwater of about 450,000m3 /day.
Try out:
We can easily experiment with this process. First, boil a bowl of saltwater using a Bunsen burner beneath an overturned metal bucket. In a few seconds, you are able to see drips of freshwater dropping off the rim of the bucket.