Measures to Desalinate Brackish or Salt Waters

Introduction:

Water is one of the most used resources on earth. Common uses of water including for domestic needs, industrial applications, and recreational and agricultural purposes (irrigation). The bulk or all of the water used for these purposes is fresh water whose availability is depleting.

The earth has large bodies of water (71%) when compared to the land space (29%) available for human habitation. It is expected that such a large body of water should support properly the population living on the land. However, this is not the case because the greater part of 71% of water is found in the seas and oceans and these are known as brackish/ or salty water because they contain sodium chloride.

Salty waters are not good for drinking and are not suitable also for many other purposes important to man. Hence, the few freshwater bodies found in rivers and lakes and those harnessed from rainwater are stressed.

It is possible to recover brackish water from its salty state to a pure form and make it suitable for human use. This can be done by eliminating the salt content of the water. Removal of salt from brackish/salty/saline water also called desalination is essential for the following reasons:

1. Fresh water at a given point may not be of sufficient quantity.
2. Where fresh water is available, it may not be adequate it is necessary to get water from another source which would be costly.

Measures to Desalinate Brackish Waters

The following measures are can be employed to desalinate brackish waters

A. Desalination by Evaporation and Distillation

In this process, the sea water is boiled in giant stills called evaporators, and the water vapour is caught and condensed into fresh water. The water produced is nearly pure and can be used for industrial applications and other needs provided the high cost is tolerated

Advantages of ‘A’
1. Produces pure water
2. Readily available and commonly used by modern sailors

Disadvantages of ‘A’
1. High cost
2. The problem of scaling (layers of hard crust deposited in the metal box evaporator during boiling).

B. Desalination by Electrodialysis Method

In this method, an electric current is passed through a salt solution. When this occurs, the sodium and chloride ions get freed from water molecules, and they start moving toward their oppositely charged electric poles. In other words, the positively charged sodium ions will move toward the negative pole which is the cathode while the negatively charged chlorine ions will move toward the positive pole which is the anode. If these cations and anions are allowed to segregate in different compartments, what is left is freshwater. The segregation is achieved by means of thin plastic-like sheets called ‘membranes’ which are made of particular chemical substances called ion exchange resins.

Advantages of ‘B’
1. Best method when too much purification is not needed.
2. it is a compact machine.
3. Low cost of buying and erecting the machine.
4. Easy to operate.
5. Ideal purificator for rural and remote areas.
6. It is adaptable.

Disadvantages of ‘B’
1. Not suitable when the water of very high purity is required.
2. Not capable to treat water for large populations.

C. Desalination by Reverse Osmosis Method

In this method, the water molecules and the salt ions are separated by forcing the salt solution against a semi-permeable membrane barrier, which permits the flow of water through itself but stops the salt from passing.

Advantages of ‘C’
1. It can be used for desalination with low-pressure membranes of moderately salty waters (Total Dissolved Solids (TDS) of 1000 to 10, 000 mg/l) and with high-pressure membranes for severely saline waters containing TDS above 10,000 mg/l.
2. Sufficiently good quality water containing TDS within 500 mg/l can be obtained by the method.
3. It is cheaper than other methods.

Disadvantages of ‘C’
1. It requires high osmotic pressure and does not work below 60,000 kN/m² (600 kg/cm²). The ideal pressure of operation is 100,000 kN/m² (1000 kg/cm²).
2. Not suitable for small-sized particles of about 0.001 μm.

D. Desalination by Freezing Process

This method is based upon the principle that when salt water freezes, the ice formed in the beginning is almost free of salt. The ice when melted can give good water.

Advantages of ‘D’
1. Satisfying quality of water.

Disadvantages of ‘D’
1. Its cost is high.

E. Desalination by Solar Distillation

Evaporation of water under the intense heat of the sun is a daily occurrence. Though evaporation occurs from both saline and freshwater bodies, billions of tonnes of water vapourizes from seas and oceans because they have very large surfaces and the larger the surface exposed to the heat of the sun, the more the evaporation. These vaporized waters which lose their salt content under vapourization, can be condensed into fresh and pure water.

Advantages of ‘E’
1. Free sources of vapourization.

Disadvantages of ‘E’
2. Appropriate method of condensation is still vague and may be costly.

F. Desalination by Chemicals

This method, though still under investigation, proposed that it could be possible to combine propane gas with salt water under controlled conditions of temperature and pressure. A chemical reaction would occur between the salt water and the propane gas at temperatures higher than the freezing point of water, forming ice-like crystals. These crystals reject salt and accept only pure water in their composition. These crystals are then separated from the brine, washed, and decomposed to form water. The propane gas released during the decomposition of the crystals can be re-used to form new crystals.

Criteria to Select the Right Method of Desalination

To select the suitable method of desalination, one should consider the following;

1. Expected TDS range (this is of high priority).
2. Process efficiency.
3. Plant capital and running cost.

Based on the TDS range which is of high priority, the table below shows the common TDS range and suitable desalination process.

Further Reading

Garg, S.K. (2007): Hydrology and Water Resources Engineering (14th Revised Edition). Khanna Publishers, New Delhi, India.