The purest form of natural water is the rain water. The reason for this is because rain is formed as a result of condensation of water vapour. It is obvious to see that the process is similar to distillation.

All the types of natural water or the water bodies such as sea, oceans, streams, lakes and rivers are constantly undergoing circulation which result in gigantic water cycle.

Treated Water

This type of water is treated artificially by man for special purposes. We can easily find the following examples:

Water from a river, lake or stream is stored in reservoirs. Thereafter, the water is purified through various stages that include: coagulation, sedimentation, filtration and disinfection (See figure).

As shown in the figure, the untreated water is first passed through settling tanks where alum is added to induce flocculation and coagulation. The particles in the water will now come together or clump to form larger ones orflocs.

In this state,it can now be passed through a filter bed to remove the rest and finer particles. After this filtration stage, the water can be considered clean, but definitely not pure.

To rid the water from germs and dangerous pathogens that may lurk inside it, it is treated with chemicals like chlorine in calculated amounts. In areas where they are prone to goitre and tooth decay due to lack of iodine and fluorine in their diet, these useful chemicals (that is, chlorine and fluorine) may be added to the water. This latter stage is calleddisinfection stage