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Raw water as it is extracted from rivers, lakes, tanks, reservoirs or wells may contain many impurities. It need treatment for these impurities besides the color, odor and taste also need to be removed. Various uses of water may need different levels of purity. The following are some common water treatments methods.
Sedimentation:
Raw water contains suspended and floating solids giving it turbidity. Sedimentation is a process of completely stopping or slowing down the flow of water very much so that the suspended solids could settle down in what are called as settling tanks. Lighter solids however, float to the top.
The settled impurities are then removed from the bottom of tank and floating ones from top, to make it ready for next step of filtration. It is achieved in “fill and draw type tanks” or “continuous flow type tanks”. The latter is either a Rectangular tank with horizontal flow or Hopper vessel with vertical flow or a Circular tank with radial flow. Plain sedimentation is known to remove up to 60 % suspended load and 75 % bacterial load.
Coagulation:
When the solid load is too fine in the water, the time in settling tank may not be enough to allow them to settle. Certain chemicals called coagulants are generally added to water before sedimentation, when turbidity is more than 40 ppm. These chemicals react with fine solids and increase their size by coalescing into lumps called as flocs. The commonly used coagulants are Aluminium sulphate (Alum), Chlorinated Copperas, Ferrous sulphate and Sodium aluminate.
The coagulants are added to the water by agitating it with centrifugal pumps or compressed air or mixing tanks with baffle walls etc.
Filtration:
It is process of mechanical straining of water through a thick layer of filter sand. Filtration of water removes most of the suspended and dissolved impurities and bacteria besides altering the chemical nature of water. Apart from mechanical straining in sand, the impurities may settle in the voids of sand and stick to gelatinous layer on sand grains.
This layer is formed by colloidal matter in water and millions of bacteria that thrive on organic impurities in water and render them into harmless compounds. The filters are either slow sand filter type or rapid sand filter type (either gravity or pressure type).
Disinfection of Water:
Disinfection of water is done on filtered water to remove the residual harmful bacteria. This can be achieved primarily by Chlorination. At pH value of 7.0 or less, the chlorine gas can effectively remove bacteria. It is applied in the form of bleaching powder or chloramines or as free chlorine gas, with the former being very popular. Other methods of disinfection include excess lime treatment, Iodine and Bromine treatment, Ozone treatment, boiling etc.
Water Softening:
Hard water is not suitable for drinking and certain commercial uses. Hence, if hardness is present in water, it is removed either by Zeolite process or Lime-Soda Process or Demineralization Process (Ion Exchange method).
Aeration:
It is a process of agitating water to expose it more to the air, helping remove taste, odour and color. In this process, oxygen is absorbed and carbon dioxide is liberated. Removal of sulphur dioxide removes odour. To certain extent it removes bacteria and oxidizes iron and manganese. Aeration can be achieved by Direct Air diffusion, Cascading the water, Spraying with nozzles and running through trickling beds.
Other methods:
Other treatment methods include Activated Carbon method, copper sulphate treatment, removal of iron and manganese, fluoridation and de-fluoridation, UV ray treatment, radio activity removal etc.
In many cases, a little chlorination or other disinfection may be resorted to just before releasing water for distribution. Since, if the distribution system is contaminated, whole of the treatment would be useless.
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