MICROBIOTA OF SEWAGE TREATMENT PLANTS IN STREAMS 11 



the reversed flow is enhanced by the introduction of forced air through 

 openings in a pipe grid system imbedded near the base of each filter. The 

 procedure requires about 12 minutes and is effective in removing huge 

 quantities of sewage solids and biological film. Judging by the myriads of 

 filter flies in all stages of development which are visible on the surface of 

 the wash water leaving the filter, this procedure may be cited as a means 

 of fly control. 



The samples taken near the beginning of the backwash contained more 

 than twice as many binding organisms as at the close of the operation, but 

 the reverse was true of the free-living organisms and the scouring com- 

 ponent. Apparently, the first masses of binding organisms were loosened 

 quickly from the upper surface of the filter, but the majority of the other 

 organisms resided deeper in the bed and required more time to be dislodged. 

 The control sample from the effluent of the filters under normal operating 

 conditions indicated some natural sloughing of solids with about one-third 

 the number of binders, one-tenth as many free-living forms and but a trace 

 of scourers in comparison to the average concentration of the effluent from 

 the backwash. 



The series of 13 samples which represent the vertical section of a 

 trickling filter disclosed several interesting facts. But, first, it is to be noted 

 that the concentrations given in Table III represent numbers per mm of 

 film but do not give any clue to the thickness of the film or its mass per 

 unit volume of stone media. Tests have shown, however, that efficiency 

 of the film is a function of surface area and not mass. Table III indicates 

 that among the binding organisms, Zoogloea ramigera is the predominant 

 organism throughout the filter, but it is most abundant, and, from its 

 appearance, most active in the section from 6 to 30 inches in depth. The 

 large numbers of sewage fungus, Sphaerotilus natans, present on the surface 

 of the filter on the underside of the upper rocks, may be attributed to the 

 fact that the carbohydrates, upon which this organism feeds, are assimi- 

 lated largely in the upper portion of sewage filters. The abundance of the 

 white sulphur bacteria throughout the remainder of the section is con- 

 sistent with previous tests on high-rate filters. Among the free-living organ- 

 isms, predominance is shared by ciliates, rhizopods and flagellates alike. 

 Although the minute nematodes outweigh the filter flies in numerical pre- 

 dominance, the latter are probably the more important component of the 

 scourers. Moreover, the oligochaetes, such as Dero sp., although few in 

 number, exert a potent influence on the reduction and stabilization of 

 sewage solids and assist in clearing the voids between the rocks to improve 

 ventilation. The importance of these and other scouring organisms in sew- 

 age filters and some recent developments in methods for their control should 

 not be overlooked. 



Recent Developments in Selective Control of Microbiota in 



Sewage Plants 



Among the experts on sewage treatment in England, there are many 

 who advocate the cultivation of a balanced scouring fauna on sewage filters 



