PURIFICATION OF WATER 701 



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The Clark process of softening water may also reduce 

 the number of organisms present, but is very uncertain 

 (Moor and Hewlett). By the Porter-Clark rapid process, 

 however, in which the precipitate of calcium carbonate is 

 removed by filtration through canvas bags, very con- 

 siderable purification is effected. 1 



Houston has introduced an " excess lime " method. 

 Enough lime is added to the water to render it decidedly 

 alkaline and germicidal for the colon bacillus in five to 

 twenty -four hours (for raw Thames water, about 1 part 

 of lime in 5,000 parts of water). At the end of this period 

 a sufficiency of water purified by storage is added so as 

 to precipitate the excess of lime. With Thames water, 

 3 parts of raw water with 1 part of stored water would 

 be the approximate quantities. By this treatment the 

 water is both purified and softened. 



The tables on pp. 702 and 703 illustrate the influence 

 of sand filtration and of storage on the bacterial content 

 of a water. 



The Bacteriological Examination of Water 2 



The bacteriological analysis of water affords valuable 

 indications of the purity or pollution of a water, and will 

 indicate a pollution so small in amount as to be incapable 

 of detection by chemical methods. 



The specimen of water should be collected in clean 

 sterile bottles of about 100-200 c.c. capacity. Failing 

 sterilisation, the bottles may be rinsed with a little strong 

 sulphuric acid, and then thoroughly rinsed several times 



1 Nankivell, Journ. of Hyg., xi, 1911, p. 246; Hewlett and Nankivell, 

 Rep. Med. Off. Loc. Gov. for 1911-12, p. 350. 



2 See Savage, Bacteriological Examination of Water Supplies (Lewis, 

 1906) ; Thresh, Examination of Water and Water Supplies (Churchill, Ed. 2, 

 1913) ; Houston, Gordon and others in Reps. Med. Off. Loc. Gov. Board, 

 1899-1904; Houston, Reports to the Metropolitan Water Board and Studies 

 in Water Supply (Macmillan & Co., 1913). 



