14 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



2. Self-purification in streams and in sewage treatment plants is dis- 

 cussed. Many of the processes are similar, but in general the reactions 

 observed in a sewage plant are accelerated. This is attributed to the action 

 of surface, contact and interfacial forces which operate as a group in 

 trickling filters, contact aeration tanks and other biological treatment 

 units. The biological film, with its teeming life, removes food from the 

 sewage with which it comes in contact and discharges waste products to be 

 carried away with the eflBuent. 



3. Many of the organisms which are important agents of self-purifica- 

 tion in streams are also found in sewage treatment plants. 



4. The importance of an adequate supply of food in a sewage filter 

 was illustrated by the effect of changes in operation on the microbiota of 

 the secondary filter at Owatonna, Minn. 



5. Studies on the biology of contact aeration showed the importance 

 of an adequate supply of dissolved oxygen in the aerators. Where plants 

 were operating beneath their designed capacity, the biological gradient, 

 which is apparently akin to the succession of organisms in a polluted stream, 

 could be observed. Experience has shown, however, that where septic condi- 

 tions have been created by overloading, the most effective means of correct- 

 ing the condition is to alter the succession of organisms either by inter- 

 posing additional treatment units or by recirculating some of the sewage. 



6. An unusual opportunity for examining the abundant and diverse 

 microbiota on trickling filters was afforded at South St. Paul, not only at 

 the time of backwashing, but also when a filter was opened for repair. 

 Examinations of the microbiota at the beginning ai>d toward the end of 

 the 12 minutes required for the backwashing process indicated some differen- 

 tial in the rate of removal of the organisms. A control sample of the com- 

 bined effluent from the filters confirmed the efficiency of the process as a 

 means of removing excessive microbiota and solids. Moreover, this was 

 an excellent demonstration of control of filter flies. The examination of 

 the vertical series of samples showed some noticeable variation in the con- 

 centration of certain organisms, as for example Sphaerotilus natans, Zoo- 

 gloea ramigera and Chlorella vulgaris; otherwise, the population of micro- 

 biota was reasonably uniform throughout the section. 



7. Recent experiments in the control of filter flies in trickling filters 

 indicate that an amount of DDT emulsion equivalent to one part per mil- 

 lion of the total daily flow of sewage is highly effective in controlling the 

 flies (larvae, pupae and adults) in the dosed area. When applied with the 

 sewage, DDT does not destroy the biological film or cause it to unload or 

 slough off. There are, however, insufficient data on the residual effect of 

 the DDT on aquatic life in receiving waters. 



