ARTIFICIAL METHODS OF SEWAGE DISPOSAL. 



253 



forked over to let in the air. The sewage is passed into the bed at the 

 bottom and allowed to rise to within six inches of the top. After 

 remaining in contact with the filter for two hours, the sewage is drawn 

 off, and then passed in the same way to a fine grained contact bed made 

 up of the same material. After this treatment the sewage is considered 

 to be pure enough to be allowed to escape. There is no preliminary 

 treatment by passing the sewage through a septic tank or by chemical 



Fig. 133. — To illustrate a common method of sewage-treatment. Septic tank (a) 

 followed bjs contact bed (b). (After Lafar.) 



'treatment, because it is 'assumed that the anaerobic work has been 

 accomplished sufficiently for the purpose in the sewers. Definite 

 experiments have been made to ascertain whether this preliminary 

 treatment can be dispensed with. It was found that it was 

 unquestionably beneficial to the disposal of sewage. In the third 

 method, it will be noticed that the sewage is placed in subsidence 

 tanks, after the preliminary chemical treatment and before the 

 subsequent treatment in contact beds. A subsidence tank differs 

 from a septic tank in that little or no "septic" action takes place. 

 They are used to avoid the clogging of the contact beds which would 

 otherwise happen if the precipitate formed during the chemical treat- 

 ment were allowed to enter the contact beds. 



In the remaining methods (Nos. 6-10), whilst the preliminary treat- 

 ment is the same as in one or other of the first five methods, 

 the method of dealing with the filtration is different. 1 his is accom- 

 plished by continuous filtration. The liquid is brought into filter 



