COAGULATION. This process depends upon the action of chemicals usually, bitter alum (Aluminium Sulphate Al2S03) is used for the purpose of forming the floc on top of the filters in order to provide satisfactory filtration. The floc forms when the filter alum, which is acidic, reacts with the alkali in the water forming a cloudy gelatinous precipitate which settles quickly on top of the filter sand. To form a satisfactory floc the pH condition of the water should be within the limits 7.2 to 7.6. The amount of alum required to form this floc is 0.5 to 0.8kg alum per square metre of filter surface. The addition of this amount should be made over a period of 1-2 hours immediately after backwashing the filter. At this pool it has been found to be advantageous to add the alum by slug dosing it to the balance tank. To do this the following procedure should be followed:-
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