Q. What are the requirements of drinking water? Explain various methods used for the sterilization of water for municipal supply.

ANS. Requirements of drinking water: The essential requirements of drinking water are as follows:

(1)         It should be clear, colourless and odourless.

(2)         It should be good in taste.

(3)         It should be perfectly cool.

(4)         Its turbidity should be less than 10 ppm.

(5)         It should be free from harmful gases like, H₂S.

(6)         Its pH should be in the range of 7.0-8.5

(7)         It should be free from harmful metals and its salts like, As, Cr, Pb and  manganese.

(8)         Total hardness should be less than 500 ppm.

(9)         Total dissolved solids should be less than 500 ppm.

It should be free from disease producing bacteria and E. Coil must be under permissible limits.   

Chloride, fluoride and sulphate contents should be less than 250 ppm, 1.5 ppm and 250 ppm, respectively. It should be reasonable soft.

The treatment of water for municipal supply (purification of water)

The table list the types of impurities and the process to be employed for removing them.

 Screening: It is a process of removing floating materials like leaves and twinges etc; from water. Raw water is passed through screens having holes, when the floating matter is retained by them and water is allowed to pass.

Sedimentation: The process of allowing water to stand undisturbed in big tanks for some time in order to facilitate the setting down of the coarse suspended particles due to the force of gravity is called sedimentation.

Sedimentation with coagulation: When water contains finely divided silica, clay and organic matter do not settle down easily and cannot be removed by mere sedimentation. Most of these are in colloidal form and carry define type of change. Thus, they do not coagulate due to mutual repulsions. For their removal by addition of requisite amount of chemicals (as coagulants) to water before sedimentation. Coagulants like alum or ferrous sulphate produce Al⁺³ or Fe⁺² ions which neutralize the oppositely charged colloidal and clay particles. After losing their charge, these particles come nearer to one another and combine to form bigger particles which settle down due to the force of gravity. The process is known as flocculation.

Some common chemicals used as coagulants are as follows:

1.      Alum

2.      Aluminium sulphate

3.      Sodium aluminates

4.      Ferrous sulphate [FeSo₄. 7H₂O]

S.NO                       CHEMICAL COAGULANTS       IDEAL Ph range of use

1.                             K₂SO₄.Al₂ (SO₄)₃.24H₂O                6.6-8.5

2.                                Na₂Al₂ (sodium aluminates)           5.5-8.0                                                                                                   

3.                                FeSO₄.7H₂O (ferrous aluminates)   above 8.5                                                                                                                                                                                                                                                                                                   Filtration: This is due to the process of removal of coarse impurities (e.g. coagulated colloidal material, suspended matter etc.) and some of micro-organisms by passing water through a porous material consisting of a bed of fine sand and other granular materials. The porous material used for filtration is called a filter. The filters used for water treatment are of two types (1) gravity type filters, and (2) pressure type filters. The gravity type filter classified further as slow sand filter and rapid sand filters. The water for municipal supply is usually filtered by gravity sand filter.

Sterilization (disinfection) of water: The filtered water still contains small amount of pathogenic (disease producing) bacteria's which must be removal or destroyed if the water has to be used for drinking water or municipal purposes. The process of destroying /killing of pathogenic bacteria are which must be removed or destroyed micro-organism from water to make it safe for use is known as disinfections. Disinfection does not ensure total destruction of all living organisms.  On the other hand sterilization means complete destruction of all living organisms. On the other hand sterilization means complete destruction of all living organisms which is possible by boiling the water over a period of time.

CHARACTERISTICS OF AN IDEAL DISINFECTANT:

(1)          It should kill the pathogens quickly at room temperature.

(2)         It should be inexpensive.

(3)         It should not be toxic to human.

(4)         It should provide protection against any contamination in water during conveyance or storage.