BACTERIAL PURIFICATION 261 



The percentage purification found in Exeter in 1897 was : — 



The analyses of the raw sewage and tank effluent do not, 

 however, give a complete idea of the purification, since the 

 effluent is the sewage minus a certain quantity of organic matter 

 that has been removed, and plus another quantity that has been 

 dissolved from the sludge. 



In Cameron's patent 5671, of i8g8, the tank has two or more 

 decks or floors, to facilitate the separation of the solids, and 

 allow the deposit to come better in contact with the fresh 

 sewage. Depressions or pockets may be provided into which 

 the sediment may drift, and from which it may be removed 

 without emptying the tank. For the utilization of the gas 

 generated, two or more outlets are constructed at different 

 levels : when the lower ones are closed, the liquid in the tank 

 rises and so creates a pressure by which the gas can be 

 expelled. 



At Yeovil and other places it has been proved that the septic 

 tank process is not affected by manufacturing refuse. The 

 smoothing and diluent effect in the volume of sewage, and the 

 room for precipitation and neutralization by the ammonia, 

 seem to obviate these difficulties. The works at Exeter and 

 Yeovil are in the neighbourhood of houses, and are quite in- 

 offensive. 



With reference to the action of the grit chamber ; on enter- 

 ing, the heavy particles of gravel and sand at once sink, while 

 the organic refuse almost entirely floats and passes over the 

 submerged wall into the tank. The result is that very little 

 solid sewage remains in the grit chambers, and the gravel may 

 at intervals be dredged out without disturbing the contents of 

 the tank. This is not at all parallel to the action of screens or 

 straining filters, which arrest a large amount of solid organic 

 matter, and form a subsidiary sludge. 



As to the stay in the tank. The flow of the liquid through 



