SEWAGE PURIFICATION 291 



consistency and can be stripped off in pieces and examined ; 

 the smell is not offensive, being similar to that of an exposed 

 river bank. 



If some of the material is placed in a glass tube and air led 

 over it, considerable volumes of C0 2 are given off, through the 

 respiration and other changes of the organisms present. If 

 the deposit is covered with water and air is excluded, it very 

 soon putrefies and becomes offensive ; it is evident, therefore, 

 that in the working of such tanks care must be taken that the 

 conditions at no time become anaerobic. When the sewage 

 first enters the tank a considerable amount of air is dissolved 

 in it, as it falls through the slates, and a further quantity 

 is trapped underneath the slate surfaces ; this is sufficient to 

 maintain aerobic conditions for an hour or two, which is 

 the length of time which should be allowed to elapse before 

 the tank is emptied. On flushing out the deposit and 

 allowing it to drain and weather in the air, it is gradually 

 converted into a brown inoffensive mass, resembling garden 

 mould. 



(d) Chemical Clarification. In the chapter on the 

 chemistry of albumins it was shown that colloidal substances 

 of this nature could be coagulated and precipitated by addition 

 to their solutions of hydrated precipitates, such as those of iron 

 and aluminium hydroxides. This precipitation is made use of 

 for the clarification of sewage. The chief precipitants used 

 are aluminium sulphate, ferric sulphate, lime and ferrous 

 sulphate (green copperas) used in conjunction. The choice 

 of precipitant depends on the relative market price of the 

 particular chemicals, and on the facilities available for their 

 efficient use. 



The right adjustment, e.g., of lime and copperas so as to 

 keep the lime always slightly in excess, requires constant 

 attention, whereas salts of alumina are precipitated directly 

 by the carbonate of ammonia present in the sewage. Ferric 



u2 



