6 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



rationed diet. The microbiota is partially "starved" during these slack 

 months. Just as a patient in a condition of physical weakness must be 

 strengthened before an operation, so it was considered advisable to enrich 

 the secondary filter before the experiments were conducted. Consequently, 

 the intermediate clarifier was bypassed and the effluent from the primary 

 filter flowed directly to the secondary filter. This procedure was started on 

 March 20 and continued 11 days (March 31), after which auxiliary piping 

 was installed to provide parallel operation of the two filters with low-rate 

 dosage. Three weeks later (April 17), the outer orifices of the distributors 

 were plugged as a means of increasing the rate of dosage on the central 

 portion of the filters. This schedule of operation is shown on the graphs 

 (Figs. 1, 2). The samples which were collected on April 17 represented the 

 second series taken after parallel flow was begun and they immediately 

 preceded the change to high-rate dosage. 



Fig. 2. Effect of changes in operation of the sewage plant at Owatonna upon the 

 microbiota in the effluent from the second stage trickling filter. 



These changes of operation caused quantitative as well as qualitative 

 changes in the microbiota within the secondary filter, which is the one to be 

 discussed (Figs, and Tables I and II) . The initial enrichment process caused 

 a noticeable increase both in binding and free-living organisms, but a 

 decrease in the scourers. Among the binding components of the film, the 

 white sulphur bacteria {Beggiato alba) almost doubled in numbers and 

 fungi appeared on the surface rock where they were not found before. 

 Almost all of the free-living organisms increased as a result of the more 

 adequate diet of bacteria and organic solids. Predominant among these were 

 two ciliate protozoans, Opercularia sp. and Paramecium sp., and rotifers. 

 Scouring or grazing organisms were not abundant at any time during the 

 tests. Those present were minute round worms and a few sludge worms 



