SEPTIC TANK STUDIES 



By James E. Fuller, Research Professor of Bacteriology 



INTRODUCTION 



The septic-tank study that is the subject of this report originated as a joint 

 project between the Division of Sanitary Engineering of the Massachusetts 

 State Department of Health and the Department of Bacteriology of the Mass- 

 achusetts Agricultural Experiment Station. The design, location, and construc- 

 tion of the original tank installation werecarried out by the Division of Sanitary 

 Engineering, and the operating supervision was in the hands of their resident 

 engineer in Amherst. They planned to make an operational study of the installa- 

 tion, to make chemical studies of the effluent, and to study the efficiency of 

 different coarseness of fills about the tile lines in the disposal field. The De- 

 partment of Bacteriology undertook to study the bacteriology of the effluent 

 with particular reference to its content of coliform bacteria, which are accepted 

 as indicators of sewage pollution of water supplies. 



Shortly after the installation was completed, and before it was put into satis- 

 factory operation, the engineer in charge was called into military service and 

 no one was available to replace him. Consequently, the whole of the project 

 was taken over by the Experiment Station and assigned to the Department of 

 Bacteriology. 



The purpose of the project was to study the effect of the length of time sewage 

 is retained on the efficiency of a septic tank. Employment of a retention period 

 shorter than the usual 24 hours would enable a tank to handle increased quanti- 

 ties of sewage; or it might mean that tanks need not be as large as is commonly 

 recommended for normal household installations. The design of the installation 

 to carry out the study is explained in detail later in this paper. 



The function of a septic tank is not well understood by laymen, who often 

 believe that the effluent should be free from bacteria and harmless so far as its 

 capacity to pollute water supplies is concerned. That this is not true is illustrated 

 by the report of a committee representing governmental agencies of the United 

 States concerned with rural sewage disposal (1). The report states: "Contrary 

 to general belief, septic tanks should not be depended upon to remove disease- 

 producing bacteria from sewage." The report adds: "The septic tank should be 

 located where surface drainage from the site is away from all sources of water 

 supply." 



Regarding the size of tank and the retention period of the sewage, the report 

 states that a septic tank is intended to retain and permit the bacterial digestion 

 of the solids of the sewage. "The size of the septic tank should be based on the 

 average daily flow into it with a retention period of approximately 24 hours, with 

 due consideration to sewage sludge storage." The perusal of experiment station 

 bulletins from several states, and reference to standard textbooks on sanitation, 

 discloses recommendations for septic tank construction and operation substan- 

 tially identical with those in the bulletin just quoted. 



THE SEPTIC TANK INSTALLATION 



The septic tank installation, the supply line, and the disposal field, are shown 

 in diagram I. The installation was located in a field at the State University 



