MICROBIOTA OF SEWAGE TREATMENT PLANTS IN STREAMS 13 



in order to increase aeration, to prevent ponding and to assist in stabiliz- 

 ing sewage solids. Nuisance from adult filter flies (Psychoda spp.) is at a 

 minimum where the population of other macrofauna such as chironomids 

 is sufficient to control them. Nevertheless, a certain amount of fly trouble 

 is considered to be inseparable from the low-rate filter (Lloyd 1945) . 



In this country, however, the benefits derived from these organisms 

 seem, in the opinion of the majority of sewage plant operators, to be over- 

 shadowed by the nuisance which they create in the immediate vicinity of 

 the plants. Moreover, from the southern part of this country Brothers 

 (1946) reported such enormous numbers of filter flies on certain high-rate 

 filters that not only was practically all of the growth consumed but the 

 filter bed and pipe lines became clogged. Numerous methods of control 

 had been tried through the years prior to World War II from chlorination 

 to flooding the filters for extended periods. One operator perfected the 

 unique, if hazardous, method of spraying gasoline on the surface of the 

 filter and igniting it. Most of the earlier methods had the one common dis- 

 advantage of injuring the biological film. 



During the war some significant experiments on the control of filter 

 flies by the use of DDT were conducted under the sponsorship of the U. S. 

 Army Sanitary Corps. Brothers' (1946) experiments at Camp Fannin, 

 Texas, demonstrated the effectiveness of DDT when application is by the 

 batch method and the chemical is introduced into the sewage as it is sprayed 

 on the filter. The optimum concentration of one part per million was effec- 

 tive when a holding period of one hour was provided. The effectiveness 

 of such a treatment should be apparent for about 30 days. 



Although the investigation of Carollo (1946) in this field was con- 

 ducted independently from Brothers' investigations, there is excellent agree- 

 ment in the results. Carollo, however, applied the DDT emulsion to the 

 filter with the sewage over a 24 hour period at the rate of 1 part per mil- 

 lion based upon the 24 hour flow. 



Examination of the microbiota of the test filters following the appli- 

 cation of DDT confirmed the selectivity of the chemical. According to 

 Carollo, snails became more abundant after each treatment, and sewage 

 filter bacteria were not killed in concentrations up to 100 parts per million. 



Since the time of these investigations, there have been others which 

 confirm them in similar situations and extend the range of possibilities of 

 DDT in the selective control of the microbiota in sewage plants. Our 

 knowledge, however, is insufficient with regard to the residual effect of 

 DDT on receiving waters. 



Summary and Conclusions 



1. The term microbiota is a collective one, including microscopic and 

 near-microscopic organisms from the higher bacteria through insect larvae. 

 For convenience in discussing the biology of sewage plants, the organisms 

 are divided into the following 3 groups on the basis of the functions which 

 they perform: Binding, free-living and scouring organisms. 



