SEWAGE PURIFICATION 185 



is then usually treated to some aeration process or trickled 

 over coke filter beds in which a high degree of aeration takes 

 place. In the upper layers of the beds the breakdown of 

 organic-nitrogen is completed and organic matter extensively 

 oxidised. In the lower layers conditions are suitable for the 

 growth of autotrophes and nitrification takes place. The 

 bacterial population of the filter bed is very mixed and a 

 certain amount of denitrification may also take place, the 

 composition of the final effluent depending upon the design 

 of the bed, the speed of flow of sewage, the time since the bed 

 was rested, etc. If the filter bed is kept in continual use 

 the coke becomes " choked " with bacterial growth and the 

 purification process slows down. Consequently the beds 

 have to be rested periodically by stopping the flow of sewage 

 for a few weeks, when the filters clear themselves by auto- 

 digestion, during which the cycle is repeated on a smaller 

 scale, having bacterial protein as starting point instead of 

 sewage. 



FOR FURTHER READING 



Cattle Fodder and Human Nutrition, Virtanen, A. I. (C.U.P.). 



The Biochemistry of the Symbiotic Nitrogen Fixation Process, 

 Wilson, P. W. (Univ. Wisconsin Press, Madison). 



