In order to improve the design and to control satisfactorily the operation of the modern sewage disposal and industrial waste treatment plants, a great deal of work remains to be done by competent sanitary engineers, biologists, and chemists. It is the general belief that the stabilization of organic wastes is a biological phenomena. This indicates that a concentrated study of the organisms that are responsible for such stabilization should be undertaken and the part that each organism contributes should be evaluated The lack of specific knowledge has resulted in "rule of thumb" methods of design and control. A considerable amount of work on the biology of sewage disposal has been done by the New Jersey Experiment Station (4)(5)(8); by C.T. Butterfield and Elsie Wattie of the U.S. Public Health Services (10) (13) and by others, but a concentrated effort is still needed. James B. Lackey (1), has done an excellent job in summing up the work accomplished to date in the field of sanitary biology in his article “Sewage Treatment Biology”. In this article, Dr. Lackey lists four things that must be known before a clear picture can be presented on the subject of sewage and waste treatment. These points are as follows: (1) More precise information on the species of each group of organisms working in treatment plants. (2) The relative abundance of each so that no important (numerically or volumetrically) organism is neglected. (3) The range-not optimum- of environmental conditions under which the organism works. (4) The work accomplished by the organisms - whether a small segment of the stabilizing process, or a large one. Gerald A. Rohlick (2), in discussing Lackey’s article, places special emphasis on additional experimental work that should be done on the activities of protozoa and higher forms of life. The editors of Savage Works Journal (3) have summed up the problem in the following statement, “When we can answer completely the what, how, who, when and why of the organisms that populate our digestors, aerators and trickling filters, we shall simultaneously solve the problems of treatment plant design and operation that are of present concern”. In an effort to contribute something to the fund of knowledge of sanitary biology, the author undertook an investigation based on the for needs suggested by Lackey. The investigation is divided into three major parts: (l) A preliminary investigation; (2) the construction of a pure culture testing apparatus; and (3) the testing of a pure culture of organisms. The complete investigation is confined to studies of the organisms found in the aerobic phase of treatment, the precise source being the trickling filter at the Virginia Polytechnic Institute sewage disposal plant. This confinement of purpose was to allow the investigator to conduct a concentrated study of a single phase of treatment. The preliminary investigation consisted of taking frequent samples from the trickling filter and examining them under the microscope. The predominate organisms were noted and the development of pure cultures of each was attempted. The construction of a pure culture filter consisted of an attempt to duplicate in the laboratory as nearly as conditions would allow, the actual conditions that exist in the trickling filter at the plant, while at the same time observing pure culture requirements and techniques. The testing of the organisms consisted of measuring the amount of purification exerted by the organism on a synthetic sewage as it passed through the filter. The results of this investigation should answer two questions: First, can a pure culture apparatus be constructed and operated with such success that it can be used as a standard device for determining in the laboratory the degree of purification exhibited by organisms in pure culture, and Second, is it possible for a pure culture of organism to carry on the purification process. The answer to the second question will, of course, depend on a positive answer to the first question. / Master of Science