MICROBIOLOGY OF SEWAGE. 223 



The other filters are similar to this and illustrate the true function of 

 filtration. In the case of the sand filter it might be maintained that 

 filtration or straining was an essential element in the process, but in the 

 case of these coarse-grained media straining action is eliminated. Here 

 there is nothing but a pile of stones, varying from one to three inches or 

 more in diameter, upon the surface of which the bacteria grow. The 



FIG. 64. Sewage Experiment Station, Mass. Inst. Technology. Trickling filter in 

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

 septic tank just behind dosing tank. 



sewage trickles slowly over the surfaces, or is held in contact with them 

 temporarily, according as we are dealing with trickling or contact filters. 

 Solids adhere to the stones or settle upon them, and soluble material is 

 " absorbed" by the surface growth and removed from solution. Within 

 these gelatinous growths to which the air also has free access, the pro- 

 cesses of oxidation take place and the products, the semi-oxidized organic 



