2 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



organisms, and the scouring or grazing organisms. The members of the 

 binding group comprise the fibrous-gelatinous matrix of the biological film. 

 The slime bacterium, Zoogloea ramigera, certain molds and fungi, and fila- 

 mentous algae are among the representatives of this group. The free-living 

 organisms include the higher bacteria, protozoans and rotifers. The rotifers 

 and larger protozoans feed upon bacteria and solids in the sewage. Many- 

 smaller protozoans subsist largely upon dissolved nutriment in the sewage. 

 Those which feed upon bacteria help to prevent overpopulation among the 

 bacteria and some function as colloiders of dissolved material. The scour- 

 ing or grazing group includes insect larvae, round worms, annelid worms, 

 snails and adult insects (Collembola). As the name suggests, these organ- 

 isms feed directly upon the biological film and sewage solids. But scouring 

 organisms may sometimes create a nuisance or interfere with normal plant 

 operation; consequently, control measures have been developed in recent 

 years (Page 11). Nevertheless, they play an unquestioned role, particularly 

 in low-rate trickling filters,* by consuming solids in the sewage, by reducing 

 the volume of the film and by inducing sloughing, which opens the voids 

 between the rocks, thereby enhancing aeration. 



The studies on the microbiota of sewage treatment plants have been 

 made intermittently over a period of five years. The following projects are 

 included: 



1. Investigations on the biology of high-rate trickling filters were made 

 over a period of three months in 1942. The results of this work were pub- 

 lished as a supplement to the report of the engineering investigations of 

 filters of this kind (Walton 1943). This project was sponsored by five 

 State Health Departments: Wisconsin, Minnesota, Iowa, Illinois and 

 Indiana. 



2. Studies on the biology of contact aerators (Hays Process) were made 

 during a brief period in the winter of 1944. This work was under the aus- 

 pices of the Repairs and Utilities Branch of the Seventh Service Command 

 in Omaha and was performed as a supplement to engineering investigations. 

 Five plants were included in the survey. 



3. Investigations of the sewage treatment plant at South St. Paul, 

 which handles a large volume of wastes from meat packing plants, were 

 made this last summer (1947). The biological studies included the filter 

 back-washing operation and a vertical section of a filter. 



Most of the reactions which take place in a sewage treatment plant may 

 also be observed to some extent in a polluted stream; therefore, let us first 

 consider stream self-purification from the microbiotic standpoint. 



Microbiota of Stream Self-Purification 



In his book entitled "Stream Sanitation," Professor E. B. Phelps (1944) 

 uses a financial simile in a most apt manner to describe the self-purification 



* For the purpose of this paper, the following distinction is made between low-rate 

 and high-rate trickling filters: Low-rate trickling filters are operated with intermittent 

 application of sewage, whereas high-rate filters are dosed continuously. The total 

 amount of biochemical oxygen demand applied per unit period of time on a high-rate 

 filter is much higher than on a low-rate filter. 



