SEPTIC TANK METHOD 



169 



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

 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. 



Metal; 

 Ecreenl 



i 



^Conduit to Beds 





-Filter Bed full of 

 Burnt Clay 



Exit Pipe 



- Filter Bed empty 

 of Filtrant.siiowing: 

 Ex it Pipe at the 

 bottom 



FiQ. 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 



