WATER 15 



When filtration begins, the sand, acting as a strainer, holds 

 back the coarser particles which fill the interstices, and so prevent 

 the passage of finer particles. Eventually the sand becomes covered 

 with a layer or carpet of organic matter, algae and bacteria. As 

 the formation of this carpet progresses, the rate of filtration 

 becomes slower and more efficient. It is this film of organic matter 

 that makes a filter efficient. When the filter has been in use for 

 some time the film becomes so dense that filtration is too slow, 

 then the filter is run dry, the film removed, the top layer of sand 

 washed and relaid, and filter is ready once more. The efficiency 

 of a filter depends largely upon the height of the superimposed 

 water, if it is too great then filtration is too rapid for satisfactory 

 results. The rate at which the carpet or film forms depends upon 

 the nature of the water; a filter may last a few days or as many 

 months. 



Mechanical Filtration. For industrial purposes it may be 

 necessary to have water that is free from sediment but which need 

 not be pure. All that is required is to remove gross organic matter 

 and sediment; the filtration is therefore done more rapidly and 

 without the formation of an organic film. Rapid mechanical filtra- 

 tion of high efficiency may, however, be effected by making an 

 artificial film. This is done by adding to the water a coagulant. 

 Sulphate of alumina is commonly used for this purpose. It reacts 

 with the carbonate of lime and of magnesia in the water, and forms 

 aluminic hydrate, a gelatinous substance that carries down bacterial 

 and other impurities as it settles. This method is. specially suit- 

 able for water containing much clay. Very soft water, to which 

 the sulphate of alumina alone is added, does not yield the necessary 

 film, as there is not sufficient calcium carbonate for the reaction to 

 take place. Lime is therefore added to very soft waters. The 

 following equation shows the chemical reaction : 



A1 2 (S0 4 ) 3 + 3CaCO,+ 3H,0 = A1 2 (OH) C + 3 CO 2 + 3 CaSO 4 



The above equation shows that the temporary hardness due to 

 the calcium or magnesium carbonate is converted into permanent 

 hardness by the formation of the corresponding sulphates. 



The film formed by the slow sand filtration method is an organic 

 one; that formed by the use of chemical coagulants is inorganic. 

 The latter is capable of standing more rough usage than the former ; 

 the water stands at a greater height in the bed and exerts a greater 

 pressure on the film. Thus filtration proceeds at a greater pace. 



Impure water when it is filtered is improved in appearance, 

 colour, taste and odour. Its organic matter is reduced and its 



