DISPOSAL OF SEWAGE 265 



inefficient and it must then be scraped, finally the sand must be 

 removed and washed. 



A sand filter is a highly efficient means of water purification. It 

 often converts a foul dirty water into a bright, clean, wholesome 

 water of low bacterial content. 



Mechanical filters depend for their efficiency upon the addition 

 of aluminum sulphate to the water. This is decomposed by the 

 carbonates and aluminum hydroxide is produced, which is a white 

 jelly-like flocculent precipitate, which mechanically entangles bac- 

 teria and removes them from the water. Mechanical filters, as a 

 rule, are highly efficient. Domestic filters, even the Pasteur, are 

 often unreliable. 



In time of epidemics of cholera and typhoid even filtered water 

 should be boiled before use, as it was found by experiments in the 

 Medico- Chirurgical Laboratories that typhoid bacilli live longer in 

 filtered water than in bouillon; they may even live three months. 

 The fewer the number of other bacteria the longer will typhoid live. 

 They can live many days in ordinary river water. 



Ice may contain great numbers of bacteria; it is well known that 

 freezing does not destroy pathogenic bacteria, such as the typhoid 

 bacillus. Prudden found typhoid bacilli in ice after 100 days, 

 although the number was greatly reduced over that placed in the 

 ice originally. Many are squeezed out by contraction of the water. 

 The greatest danger from ice is in dirty handling. 



Disposal of Sewage, is a bacteriological process in many cases; 

 either the sewage may be treated in sand filters or it may be run out 

 on land where over 200,000 gallons may be disposed of on an acre 

 of land a day. As far as possible nature should be imitated in every 

 way and the breaking up of masses of matter in sewage may be 

 accomplished in the septic tank process in which active oxidization 

 of the matter is accomplished by bacteria. It appears from the 

 observations of many sanitarians that both aerobic and anaerobic 

 bacteria are necessary to finally reduce sewage to the elementary 

 gases and pure water. 



