Water Purification


Natural sources of water

1. About three-quarters of the Earth's surface is covered by water, mainly oceans.


2. The natural sources of water and its contents are shown in Table below.

            The contents of natural sources of water

Water source

Contents of water


 The cleanest source of water. It contains dissolved gases and a little   dust.

 Groundwater,   wells, springs

 Quite clean as it originates from rain water that has seeped through   the ground. It contains dissolved minerals and microorganisms.

 Rivers, streams

 Dirtier. It contains dissolved and decaying substances, mud and   microorganisms.

 Lakes, ponds

 Much dirtier as it is stagnant. It contains more decaying substances,   mud and microorganisms.

 Seas, oceans

 The dirtiest source of water. It contains a lot of dissolved and   decaying substances and microorganisms.

 Glaciers, icebergs

 Depends on the origin of water


3. Water from most natural sources contains impurities.


4. Thus, we need to purify water for drinking so that it is free from harmful substances, disease-causing microorganisms, bad odour and unpleasant taste.


5. Water can be purified by filtration, boiling, chlorination and distillation.


Methods of water purification

(A) Filtration

1. It involves passing water through a filter to remove suspended particles such as clay, sand and plants from water.


2. The liquid that passes through the filter is called filtrate. The solid material trapped in the filter is called residue.


3. Normal filtration cannot remove microorganisms and dissolved materials from water. Thus, filtered water is unsafe for drinking.


(B) Boiling

1. The high temperature during boiling kills most of the microorganisms in water, thus making it safe for drinking.


2. This method is used to make water safe for drinking at home. However, this method is not practical to treat large quantities of water.


3. Boiling alone does not remove suspended particles and dissolved substances from water.


(C) Chlorination

1. In this method, a small amount of chlorine is added to water. The chlorine kills microorganisms.


2. Chlorination is used to treat water in swimming pools and in water treatment plants.


3. However, too much chlorine in water is bad for health.


(D) Distillation

1. Water obtained through distillation is called distilled water.


2. Distilled water is actually pure water. It is free from dissolved substances and microorganisms.


3. Distilled water is used in to prepare chemical solutions in laboratories and liquid medicines in pharmacies and hospitals. It is also widely used as battery water, in dialysis machined for kidney patients and in food industries.


4. Distilled water is not suitable for drinking on a permanent basis. This is because it does not have any dissolved minerals that our body needs.