300 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



receiving a dose of liquid at given intervals of time, depending 

 on the speed of rotation of the sprinkler. 



It is not necessary here to describe in detail the various 

 methods for applying sewage effluent to trickling filters, an 

 account of them will be found in text-books dealing with the 

 engineering side of the problem. It will only be briefly 

 mentioned that distribution may be effected by simple inter- 

 mittent discharge on to a surface of fine material, by rotary 

 distributors such as are indicated in Plate IV (ii), by spray 

 jets (Fig. 28), and by other mechanical devices of more or less 

 complexity. 



Supply Pipe 



Distribution 

 >/ 



Collecting 

 Channel 



FIG. 28. PERCOLATING FILTER WITH SPRINKLERS. 



. Plate IV (ii) shows a set of trickling filters at the Accrington 

 Sewage Works, which will sufficiently indicate their general 

 appearance. 



We have here carefully to consider, assuming equable 

 distribution of the liquid upon the filtering material, what the 

 physical and mechanical conditions are which result in the 

 production of a purified effluent. 



The efficiency of a sewage filter depends on the total effec- 

 tive surface area of the filtering material, together with a 

 sufficient air supply. By effective surface area is here meant 

 the sum of the surface areas of the fragments of material. 

 The surface area of the filter may be spoken of as the upper 

 surface area. Thus, if large-sized material is used, a greater 

 quantity of it is necessary in order to obtain the same total 



