On a very cool evening in the house, my friends and I got talking
about the growing demand of portable water in the world today
especially in modern cities. Portable water business is fast growing
and it seems that the fresh water sources are gradually Diminishing
especially in developed and developing countries and that got us talking about
sea water .i remember we do have a slang which says “water water everywhere non to
drink”.
Image source Pixabay
CC0 creative commons license. Image author dimitrisvetsikas1969
Sea/ocean water body are the largest water bodies in the world and yet
it is unfit for direct human consumption because of the high salt
content in these water bodies. The salt content in these saline water
when consumed directly into the body can damage the liver, intestine
and kidney as well and this brings us to what desalination means.
Desalination is a very important concept in hydrology and it can be
defined as all the chemical processes involved in the removal of salt
and other natural compounds from water in order to get fresh portable
water.
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A reverse osmosis desalination plant. Source Wikimedia CC3.0 license. Author: James Grellier
Desalination is a process that is undergoing continuous development
due to improvement in science and technology today and this is because
it consists of some main modes of operation which are water
intake(brackish and saline water), pre-treatment which is aimed at
removing solid particles from the water, desalination which is aimed
at removing salts and other forms of matter, post treatment which
involves the addition of chemicals in order to avoid water
tank/pipeline corrosion and concentrates of fresh water storage. Fresh
water storage is meant for distribution to the teaming population in
various sectors.
The purpose of desalination is to produce fresh water which is
suitable for consumption and other related purposes at a very
affordable rate for human use. Example of its uses are for irrigation
agriculture, industrial use, domestic use. This process is very
relevant especially in dry regions that experience little or no
rainfall. In addition, the by-product of desalination is salt which
can also be put into various uses such as preservation of food and
industrial use. Research has shown that desalination produces about
86.6 million cubic meters of water daily which supplies fresh water to
about 300million people worldwide.
Desalination plants are used in many ways which includes sea driving
ships and submarines of which without this technology life in the sea
will be very difficult
There are several methods of desalination that can be used to get
clean portable water and these include
The reverse osmosis technology:
This method is known to be one of the most widely used desalination
methods around the world today. In this method, pressure is applied in
the desalination plant trough a semipermeable membrane on the membrane
feed side of the desalination plant.(usually from high solute content
to low solute content)
Schematics of a reverse osmosis system (desalination) using a
pressure exchanger. 1:Sea water inflow, 2: Fresh water flow (40%),
3:Concentrate Flow (60%), 4:Sea water flow (60%), 5: Concentrate
(drain), A: High pressure pump flow (40%), B: Circulation pump,
C:Osmosis unit with membrane, D: Pressure exchanger. Image source Wikimedia.
Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic,
2.0 Generic and 1.0 Generic license. Image author ChrisThe capacity of water it can hold is larger than other methods of
desalination in use. Water is induced into the plant in order to
remove the salt content and other impurities in the water which may
include molecules and ions, bacterial, viruses, colloids as well as
other insoluble particles which can destroy the membrane. In the
reverse osmosis desalination plant, less energy is used up during
desalination process which can be as low as 3kwh/m3 for seawater
desalination and 0.2kwh/m3 or even lesser for fresh water
desalination.The main type of energy used during desalination process is the
thermal or electrical energy and it is mainly used for pre-filtering,
ancillaries and intake/outfall pumping. Even at this, the type of
energy dispensed in this method depends on the level of salinity of
the water.The higher the level of alkalinity of the water the higher the energy
needed to purify the water and the lesser the alkalinity of the water,
the lesser the energy cost of purification and vice versa. The
desalination plant size is also put into consideration as well as the
process type. The salinity of sea water is higher and that is why the
cost of desalination is higher unlike other fresh water bodies that
require less cost.The distillation method:
In the distillation method, the key method employed in these operation
is the vacuum distillation. In this process, there is a continuous
boiling of the water in order to be able to remove all the impurities
because when the atmospheric pressure is reduced, it in turn lowers
the temperature needed for distillation to take place but when the
vapour pressure is increased it increases the temperature which makes
the liquid to boil faster.Since temperature and pressure can be reduced in removing the waste
from the water then the heat from industrial processes and electrical
power generation can be adopted for use in this method of
desalination.The electrodialysis method(ED):
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Image source Wikimedia. Creative Commons Attribution 4.0 International license. Contributed by Yagneseni Roy and David M. WarsingerIn this method of desalination, electrodialysis optimizes the use of
electric potential to remove salt from the distillation plant membrane
and when the water passes through this pair of charged membrane, the
salt in the liquid is trapped and removed through another alternate
channel.This process is an electrochemical separation process and for the fact
that ionic salts dissolve in water which can be cationic and anionic
in nature. The energy required in this method must be direct current.
The direct current helps in the separation of the ions in the liquid.The freezing method or freezing-melting desalination:
This method of desalination employs the use of freezing to remove salt
from water and make it fresh water. The salt water is passed through a
vacuum and sprayed thereby exposing the salt water to freezing
conditions.The spraying of salt water is done usually into a pad where ice piles
can be built up and as it melts gradually, fresh water is removed and
stored for use. This technique depends on a long period of natural
freezing and thawing conditions in order to carry out the
process.Solar desalination:
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Using sun for distilling seawater. Image source credit: Wikipedia CC4.0 license. Image author Engelbert NiehausSolar desalination operates using solar energy in which sunlight can
be applied directly or indirectly to the desalination plant to achieve
desalination. In the direct sunlight method, the solar collectors are
heated up so that evaporation process would be fast and carried out in
one simple circle. The number of circles carried out will determine
the amount of fresh water that will be produced which is proportional
to the area of the solar surface.An estimated value of about 3-4L/m2/day can be produced on the average
while the indirect sunlight method employs the use of photovoltaic
(PV) thermal conductors which can produce about 2000liters of fresh
water per hour.
This method is one of the simplest natural form of desalination
because in this process all we need to do is to expose the salt water
to heat from the sun for evaporation to occur and as evaporation takes
place, the water vapour is then condensed into a cool surface and
collected. For evaporation to take place the temperature must be high
enough to generate enough heat needed for the process.
Summary
Desalination is becoming more popular and practical today due to
improvement in desalination technology around the world especially in
developed countries.it uses renewable source of energy such as solar
energy, biofuels, hydropower, biomass, wind energy and geothermal
energy for every desalination process and this has made desalination a
more efficient alternative to get fresh water supply from brackish or
saline water.
The process is mostly energy efficient because in most cases it
requires low heat energy to operate the desalination plants. The
process is reliable and simple to operate as well because of its
environmental friendly technological process.
If we look at the way population is increasing around the globe
today, we would also see that it’s the same way the demand for fresh
water is increasing and demand need to be meet because so many aspect
of the society is dependent on the use of fresh water for various
sectors in a country.
In order words, as a result of the growing demand for fresh portable
water both for domestic and industrial purpose the need to produce
fresh portable water must be feasible. Desalination process is
Relevant in so many countries where climate and climate change has
made the process unavailable.
However, research is still on going in order to get more improvements
in the various desalination technologies available.
