304 DISTRIBUTION OF BACTERIA 



one. It is perfectly satisfactory from a sanitary standpoint. 

 The septic tank was first built by Cameron of England. This 

 tank is a tight cement chamber through which the sewage 

 slowly flows. The point of entry is below the surface, so as 

 not to disturb the scum that soon forms, and not near enough 

 the bottom to interfere with the sediment. The rate of flow 

 for the sewage is such that it requires about twenty-four 

 hours to pass through the tank. The anaerobic bacteria 

 find conditions favorable for their activity and gradually 

 liquefy the solid matter. This reduction may amount to 

 80 per cent. The work of the anaerobic or putrefactive bac- 

 teria in the septic tank is supplemented by the work of the 

 aerobic bacteria in the contact filter beds. These are filled 

 with coarse material, such as cinders and coke. The beds are 

 covered with the effluent from the septic tank for a few hours 

 and are then drained and allowed to stand a few hours. This 

 process is repeated several times a day. This method of 

 sewage disposal is most satisfactory and is widely used at the 

 present time. The amount of land required is small, the 

 attention required slight, and the degree of purification high. 

 The use of chemicals, such as chloride of lime, for the pur- 

 pose of sterilizing the effluent of the septic tank or polluted 

 waters is being advocated. This method may be of the 

 greatest service in treating water supplies during epidemics 

 of water-borne diseases, especially typhoid fever. 



