Water treatment process:

Treatment
Done by
Purpose
Screening
Screening shutters with variable sized holes
Removes floating material like wood, plastic, papers
Aeration
Mechanical aerator
Removes Oxygen, Carbon-di-oxide , toxic gases, Fe, Mn salts
Sedimentation
Allowing the water to stand for 2 – 6 hrs in a tank
Removes 75% of suspended impurities
Coagulation
Adding alum,
Al2(SO4)3 à Al (OH)3
Removes 100% suspended and colloidal impurities, clay, silica
Filtration
Filter bed
Removes bacteria, colour, odour, small dust particles
Sterilization/Disinfection
Boiling, ozone, chlorine, UV radiation
Destroys bacteria

5.Filtration:
Filter bed consists of four layers i.e., Fine sand, coarse sand, fine gravel and coarse gravel. When water is passed through this, all the colour, odour, part of bacteria are removed.  The bed needs periodic regeneration.
Diagram:

6.Sterilisation or disinfection:
Removal of bacteria is sterilization or disinfection.
1. By boiling for 10 – 15 minutes, water can be sterilized .  But it affects the taste and not for higher volume water.
2. By using Ozone, we produce nascent oxygen which is powerful disinfectant.  But, it is costly.  It cannot be used and stored for long time as it is unstable.
O3  à O2 + [O], this nascent oxygen kills bacteria.
3. By passing UV radiations from mercury vapour lamp, the swimming pool water can be sterilized.  But it is costly and turbid water cannot be treated.
4. By chlorination:
a) Chlorine gas bubbling will kill bacteria.
b) Adding chlorine and ammonia (2:1) will form chloramines.  It decomposes slowly to release chlorine which kills bacteria.
Cl2 + NH3 à ClNH2 (Chloramine) + HCl
c) Bleaching powder (CaOCl2) reacts with water and forms hypochlorous acid which kills bacteria.
CaOCl2  +  H2O  à Ca(OH)2  +  Cl2
Cl2 + H2O  à  HCl + HOCl (Hypochlorous acid which kills bacteria)
It should be noted that chlorine should be always added a little higher than the break point chlorination for perfect disinfection.
Breakpoint chlorination: 
The point at which the added chlorine completely removes bacteria, NH3, Organic and inorganic impurities is known as “Breakpoint chlorination”.
The water contains:
 a) Bacteria 
 b) Ammonia 
c) Organic impurity (sewage)
d) Inorganic salt impurities ( Effluents, H2S , Fe salts)
·         When we add chlorine, first it kills bacteria and oxidizes some inorganic impurities. The added chlorine is completely consumed. (A).
·         Further addition of chlorine is used to combine with ammonia to form chloramines compounds.  So, the residual chlorine content is increased upto B.
·          As saturation level is attained, the chloramines begin to decompose to release chlorine which is utilized to remove all the types organic and inorganic impurities.  So, residual chlorine level decreases. (C).
·         At point C, the added chlorine removes all the types of impurities.  So, the point is known as “ Break point chlorination”. 
·         After this point, further addition of chlorine is not at all utilized and simply increases the residual chlorine content.

Diagram for break point chlorination: