SEPTIC TANK METHOD 



169 



a difficulty. In the bacterial processes it is reduced to one-third, and 

 often is so little as to be a negligible quantity. 



It is not possible to lay down exact limits as to where denitrifica- 

 tion ends and oxidation begins. To a certain extent, and in varying 

 degree, they overlap each other. But, generally speaking, we may 

 say that in the tank there is a breaking-down (denitrification and 

 decomposition) and in the filter-beds a building-up (nitrification). 

 The case is precisely parallel to similar changes occurring in soil, 



-Filter Bed full of 

 Burnt Clay 



Exit Pipe 



Filter Bed empty 



of Filtrant, showing 



"Exit Pipe at the 



bottom 



FIG. 22. Contact Beds (as used at Sutton). 



and with which we have already dealt. It is hardly necessary to add 

 that there is a marked reduction in the number of bacteria present 

 in the crude sewage, and the tank and cultivation-bed effluents. One 

 investigation has shown that a sample of crude sewage contained 

 4,084,827 bacteria per c.c. ; the sewage precipitate, 1,344,925 ; the 

 tank effluent, 398,695 ; and the cultivation-bed effluent, 45,755 bacteria 

 per c.c. 



2. Multiple Contact Bacteria Beds. This method in its 

 simplest form has been applied, for instance, at Sutton, and in a 



