EXAMINATION OF WATER AND SEWAGE 81 



like mass effectively holds back the bacteria. Thus their removal 

 is largely a biological process due to the bacteria themselves. 



The action of a mechanical filter is strictly a straining. A chemical 

 coagulant is added, generally sulphate of aluminium, and the water 

 is passed rapidly through a layer of sand. The coagulant clears 

 the water much as the white of an egg clears coffee. Bacteria 

 are enmeshed and deposited on the surface of the sand, forming 

 thus an artificial inorganic carpet in place of the natural organic 

 one of the slow sand filter bed. Mechanical filtration is a compar- 

 atively quick process and especially suitable for turbid waters 

 containing much clay ; its action is somewhat less uniform than 

 slow sand filtration. It removes from 95 per cent to 99 per cent of 

 bacteria. 



Household filters of the ordinary type cannot be relied upon to 

 make infected water safe. They are serviceable in rendering a 

 turbid water clear, but they should not be depended upon for more 

 than that. 



Addition of Chemicals. Ozone is an effective purifier of water, 

 but its use is limited in that it does not clarify, and the expense 

 of producing it is comparatively large. 



Chlorinated Lime, Chloride of Lime or Bleaching Powder. 

 The germicidal action is due to liberated chlorine 'which acts on 

 the water, setting free nascent oxygen. So effective is chlorinated 

 lime that one part per million parts of water will destroy 99 per 

 cent of the bacteria in water containing little organic material. 

 It does not clarify water nor remove discoloration, but it is a cheap, 

 efficient, and harmless method and is widely used. 



Copper Sulphate. It was first claimed that the addition of 

 copper sulphate in small amounts to water would destroy both 

 the algae which produce objectionable tastes and odors and the 

 pathogenic microbes. Later it was found that while even in great 

 dilution it destroys algse and many microorganisms it has little 

 effect upon typhoid and dysentery bacilli. It is generally used 

 in the proportion of one tenth to one quarter part per million parts 

 of water. The copper combines with the bodies of the organisms 

 and both settle to the bottom as a sediment. 



