296 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



bed must be of smaller dimensions than that in the primary 

 bed, if the best results are to be obtained. In special cases a 

 third treatment on stiU finer grade material, e.g., sand, may be 

 called for. 



The following are the principal changes which take place 

 in a contact bed. The suspended and colloidal matter, still 

 present in the liquid to be treated, is mechanically retained 

 by the filtering medium. It is evident, therefore, that the 

 fineness of this medium must increase, as the amount of 

 suspended and colloidal matter decreases, if this mechanical 

 effect is to be obtained. Besides the mere mechanical strain- 

 ing, a species of absorptive action also takes place between the 

 surface of the medium and the constituents of the sewage, 

 which increases within limits, as the slimy layer thus formed 

 on each fragment of filtering material becomes more well 

 defined. This slimy layer also acts as a sponge retaining an 

 appreciable proportion of the liquid applied, together with its 

 dissolved constituents. A considerable amount of purification 

 wiU therefore take place by purely mechanical and absorptive 

 action, immediately the liquid is applied to the filter. This 

 action, however, would very soon cease, and the contact bed 

 become clogged and foul, were it not for the biological activities 

 which are set up within it. These activities are exceedingly 

 various, and depend not only on the life of bacteria, but on 

 many higher organisms, notably small worms and many species 

 of infusoria. Recent researches, carried out more particularly 

 at the Government Experimental Station in Berlin, have 

 emphasised the functions of these higher organisms, and it is 

 here that the choice of the preliminary treatment, whether by 

 simple sedimentation, by anaerobic or aerobic tanks, or by 

 chemical precipitation, needs careful study. If a sample of 

 sewage be collected in a sterile bottle, and allowed to stand 

 freely exposed to the air, but protected from infection by a 

 plug of cotton wool, a film of organic life generally makes 

 its appearance. If this is carefully examined under the 



