4 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



observations on the self-purification of polluted streams, yet, in the process 

 of contracting a stream to the dimensions of a sewage plant, forces other 

 than those normally active in the self-purification of a stream are brought 

 into play. Surface, contact, and interfacial forces become operative as a 

 group. For example, in a trickling filter, the sewage disperses a thin film 

 and passes from rock to rock with all substances in the sewage coming into 

 contact with the biological film. Particles in the sewage are adsorbed by 

 the film and digested particles are removed from the film to be washed out 

 with the effluent from the filter. The reduction in the strength of sewage 

 (biochemical oxygen demand removal) by biological contact processes at 

 work in a sewage plant approaches 90%. At normal summer temperatures, 

 this value is reached in the self-purification of polluted streams only after 

 days of flow. However, the usual time required for the percolation of 

 sewage through sand beds and trickling filters is 1 - 2 hours (Imhoff and 

 Fair 1940). In spite of this contrast in the rate at which self-purification 

 proceeds, many of the same organisms and many similar biotic relationships 

 may be observed in streams and sewage plants. 



MiCROBIOTA OF SeWAGE TREATMENT PLANTS 



Technique. Five series of samples were collected for the identifica- 

 tion and enumeration of the microbiota from the sewage treatment plant at 

 Owatonna, Minnesota, which was included in the cooperative survey of high- 

 rate filters (Walton 1943). These samples were taken from the upper sur- 

 face rock, one foot beneath the surface and the filter effluent. The samples 

 of effluent from the filter were taken during extensive sloughing or unload- 

 ing induced by stopping the distributor for a brief period. The rock medium 

 was analyzed on the basis of numbers of organisms per square cm of sur- 

 face area. The samples of effluent were enumerated on the basis of numbers 

 per liter of the original samples which were examined without concentration. 

 Quantitative determinations were made on samples preserved in formalin, 

 but in order to facilitate the identifications the living organisms were exam- 

 ined. The standard plankton counting cell (Sedgwick-Rafter) was used 

 with a maximum magnification of 210 diameters. 



Samples from contact aerators were usually obtained by scraping 

 growth from plates and sample boards in the aerators. Wherever possible, 

 at each plant, one plate was lifted from near the influent to the primary- 

 aerator, one from near the effluent, and two additional plates from corres- 

 ponding locations in the secondary aerators. This was done to determine the 

 nature of the biological gradient as postulated by the proponents of the 

 process. The samples were examined in the fresh state with the maximum 

 magnification of 440 diameters. The results were qualitative, but the rela- 

 tive numbers of each organism were recorded for comparative purposes. 



The method of quantitative analysis of the filter growth in the plant at 

 South St. Paul differed from the technique described above. Instead of 

 measuring the surface of several rocks from which the growth had been 

 scraped, the volumes of representative samples of growth were measured 

 in a graduate cylinder and diluted to a sufficient degree to permit examina- 



