MICEOBIOTA OF SEWAGE TREATMENT PLANTS IN STREAMS 9 



(Oligochaeta) . The change to parallel flow modified the volume and char- 

 acter of the sewage sprinkled on this filter. There was a 50% decrease in 

 volume of sewage but instead of receiving partially digested solids and 

 sloughed film from the primary filter, the filter received fresh material from 

 the primary sedimentation chamber. As a result, the binding organisms in- 

 creased in the upper section of the bed as well as in the efiluent from the 

 filter ; but the free-living organisms decreased steadily in the upper layer of 

 rock and increased in the lower strata toward the middle of April, as evi- 

 denced by the samples of filter effluent on April 17. 



On April 17, when the rate of dosage was increased on the central 

 portion of the filter, there was a widespread unloading of the microbiota 

 indicated by the effluent, while at the same time the numbers of organ- 

 isms increased in the upper strata of rock. The last investigation (April 24, 

 1942) indicated that the filter had assumed the biological characteristics 

 of a typical high-rate unit, whose microbiota are described as follows: 



1. The binding group composed of the omnipresent slime bacterium, 

 Zoogloea ramigera, large numbers of Beggiatoa alba (white sulphur bac- 

 teria) and fungi. 



2. The free-living group composed principally of bacterial-feeding and 

 saprophytic protozoans, such as Opercularia, Vorticella, Colpidium, Uron- 

 ema, Arcella vulgaris, and Bodo sjrp. 



3. The scouring group, with Nematode worms as the sole representa- 

 tive: This group plays a minor role in many high-rate filters, although 

 there are some notable exceptions. 



The trend of increase in the population of the entire microbiota toward 

 the close of these investigations demonstrated the importance of an ade- 

 quate supply of food in a sewage filter. This increase in numbers of organ- 

 isms was accompanied by an increased efficiency of the filter as a treatment 

 unit. 



Contact Aeration (Hays Process) 



This process of biological treatment is similar to activated sludge 

 except that with activated sludge, the sewage-sludge mixture is churned 

 freely in open aeration chambers, while in contact aeration large asbestos 

 plates are suspended 2 inches apart throughout the aeration tanks to pro- 

 vide a substrate for the growth of the microbiota. Most plants of this type 

 have two such aeration chambers placed in series with 3 sedimenation units 

 arranged as follows: Primary sedimentation tank, primary aerator, inter- 

 mediate sedimentation tank (clarifier), secondary aerator, and final 

 clarifier. 



The proponents of the contact aeration process claimed a real advan- 

 tage for their process owing to the establishment of a "biological gradient" 

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

 below a sewer outlet (Griffith 1943). This presupposed the maintenance 

 of straight-line flow throughout the plant with no recirculation, as occurs 

 in some other methods of biological treatment. Our studies showed that 

 where plants of this type were operating beneath their designed capacity, 



