222 MICROBIOLOGY OF WATER AND SEWAGE. 



This action is prominent although of less importance in coarse-grained 

 filters. Actual experiments by the writer have indicated that while the 

 liquid may pass through a trickling filter in half an hour, small suspended 

 particles such as ultramarine and B. prodigiosus cells require an average 

 of over twenty-four hours. In this way the actual time of passage is 

 greatly delayed even when coarse broken stone is the filter medium, and 

 the times that are now known to be necessary for the passage are ample 

 in themselves to account for the reductions that have been noted. 



It may therefore be stated as a conservative view of the efficiency of 

 purification processes in the removal of pathogenic bacteria, that there are 

 no strongly inimical processes at work in the tanks or filters, and that the 

 rate of decrease is not materially greater than would be observed in the 

 same period of time under the conditions of a running stream. 



THE CULTIVATION OF SEWAGE BACTERIA. 



There are two general methods employed for the cultivation of those 

 bacteria which are of assistance in sewage purification. They may be 

 cultivated in so-called filters of sand or coarser material, or in specially 

 constructed tanks such as the septic or the hydrolytic tank. In the for- 

 mer case the bacterial growth occurs upon the special medium provided, 

 the sand or stone; in the latter, it takes place in the liquid 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 brief 

 period of treatment with sewage suffices to produce an active state of 

 "nitrification." By this term is indicated all 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 over- 

 dose of sewage or continued flooding of the surface due to poor drainage, 

 will soon destroy the activity of the filter. The addition of germicidal 

 substances has a similar effect and cold weather somewhat reduces the 

 efficiency. From all this it is apparent that a filter is a biological culture 

 medium upon which the various types of bacteria are growing and carry- 

 ing out their functions and that such a medium requires careful control 

 and is sensitive to unfavorable changes in environment (Fig. 64). 



