Purification of raw water for domestic use involve following process:
(1) SCREENING: The raw water is passed through screens having large number of holes where floating matter like leaves etc. Are retained by these holes.
(2) SEDIMENTATION: Sedimentation is a process of allowing water to stand undisturbed in big tanks about 5m deep, when most of the suspended particles settle down at the bottom, due to the force of gravity. The clear supernatant water is then drawn from tank with the help of pumps. The retention period in sedimentation tank ranges from 2-6hrs depending upon the particles size of suspended matter.
(3) Sedimentation with coagulation: when water contains fine clay particles and colloidal matter, it becomes necessary to apply sedimentation with coagulation for removing such impurities. In this process, fine suspended and colloidal impurities are removed by the addition of requisite amount of chemicals (known as coagulants) to water before sedimentation. A few commonly used coagulants are as follows-
(a) Alum (b) sodium aluminates (c) ferrous sulphate (d) poly aluminium chloride
(4) Filtration: Filtration is the process of removing colloidal matter and most of the bacteria's, microorganisms etc. By passing sediment water through a bed of fine sand and other proper sized granular materials. Filtration is carried sand filter.
(5) Disinfection: the water after physical treatment is subjected to sterilization for "Removal of micro-organisms". The process of destroying the pathogenic disease bacteria's micro organisms etc. From the water and making it safe for use is called disinfection. The disinfection of water can be carried out by the following methods-
(a) By boiling water for 10-15 min: Although this method provides the safeguard against existing germs only. Moreover it cannot be applied on large scale at municipal level.
(b) By adding bleaching powder: 1kg of bleaching powder per 1000 lit. Of water is mixed and water is allowed to stand for some time (2-5 hours). The chemical reaction produces a powerful germicide HOCl.
(c) By chlorination: The molecular chlorine can be applied to the water in liquid or gaseous form. Chlorine produces hypochlorous acid, which a powerful germicide.
Cl2 + H2O = HCl + HOCl
HOCl + bacteria = bacteria's are killed
Apparatus used for disinfection by chlorine is known as chlorinator. It is a large tower containing number of baffle plates. From its top raw water and chlorine solution are introduced. These get thoroughly mixed during their passage through the tower. Disinfected water is taken out from the outlet at bottom of the chlorinator.
(d) By using chloramines (CiNH2): Chloramines provides a greater safeguard against recontamination and its excess does not cause bad odour and taste in water. However it is weaker disinfectant compared to free chlorine.
CiNH2 + H2O = HOCl + NH3
(e) Disinfection by ozone: Ozone is an excellent disinfectant. It is highly unstable and breaks down, liberating nascent oxygen.
O3 (Ozone) = O2 + [O] (nascent oxygen)
The nascent oxygen is very powerful agent and kills all the bacteria. Disinfection of water by Ozone sterilizer is having separate inlets and outlets of water. Separate inlet for ozone is also provided. During the treatment, ozone is manufactured and bubbled through water contained in sterilizer. The sterilized water is removed from top through the outlet.