MICROBIOLOGY OF SEWAGE 341 



itself and a continuous life history within such a tank is possible only 

 when the rate of flow is sufficiently slow to permit of the inoculation of 

 the incoming stream by the contents of the tank. 



FILTERS. The filtering media most commonly employed are sand 

 or crushed stone or other coarse material. In natural sand beds a 



FIG. 129. Sewage Experiment Station, Mass. Inst. Technology. Trickling 

 filter in front, sand filter just behind filter, dosing tank just behind sand filter, and 

 septic tank just behind dosing tank. 



brief period of treatment with sewage suffices to produce an active 

 state of "nitrification." By this term is indicated alt the complex 

 processes of oxidation one index of which is the formation of nitrates. 

 After such a filter has once become active in this way it will continue, 

 with proper care, to oxidize sewage almost indefinitely. Improper care, 

 such as an overdose of sewage or continued flooding of the surface due 

 to poor drainage, will soon destroy the activity of the filter. The addi- 

 tion of germicidal substances has a similar effect and cold weather some- 



