SEPTIC TANK STUDIES 1^ 



2. Bacteriological Tests of Effluents. The total counts of bacteria indicated 

 no advantage for any of the retention periods. The most probable numbers of 

 coliform bacteria favored the normal 24-hour retention period so far as the quality 

 of the effluent was concerned because fewer coliform bacteria were recovered 

 from the effluent. Diflferentiation of the isolated coliform cultures into the 

 sewage and non-sewage types did not favor any of the tanks. 



3. Chemical Tests of Effluents. The biochemical oxygen demand test indi- 

 cated that, with respect to digestion of the sewage solids, the efficiency ratings of 

 the three tanks were closer together than the 3:2:1 volume ratio of sewage passing 

 through the tanks. However, the normal 24-hour retention period would be 

 preferred in practical operation because the tests indicated more complete di- 

 gestion of solids in the tank. The ammonia tests indicated greater digestive 

 activity in the normal-retention tank and so agreed essentially with the bio- 

 chemical ox>gen demand results. 



4. Bacteriological Study of Soils of the Disposal Field. Results did not 

 favor any retention period. In fact, so few sewage-type coliform bacteria were 

 recovered from the soil that the question is raised: "What becomes of sewage- 

 type coliform bacteria which are abundant in the effluent but are not recovered 

 from soil which receives it?" This will be the subject of future investigation. 



Conclusion. A consideration of the data as a whole favors the normal 24-hour 

 retention period usually recoinmended for septic tanks. The too-rapid accumula- 

 tion of scum for the short-time retention periods, and the presence of undigested 

 sewage solids in this scum, would in the writer's opinion outweigh anj' favorable 

 evidence obtained from the laboratory examinations. The probable need for 

 more extensive disposal fields for these tanks, as compared to the normal-retention 

 tank, wouJd more than balance economy of space or construction costs that 

 might be gained by installing smaller tanks. Even though some of the laboratory 

 evidence indicated that the shorter retention periods were as satisfactory as the 

 normal 24-hour period, the management problems encountered argue in favor 

 of the normal period. Also, the chemical tests and the most probable numbers 

 of coliform bacteria of the effluent indicate preference for the 24-hour retention 

 usually recommended for septic-tank installations. 



REFERENCES 



1. Committee Report: Recommendations of Joint Committee on Rural Sanita- 



tion—Rural Sewage Disposal. U. S. Pub. Health Rpts. 58:417-448, 1943. 



2. American Public Health Association: Standard Methods for the Examination 



of Water and Sewage, 8th ed., 2nd printing, 1936. 



3. Ruchhoft, C. C, Kallas, J. G., Chinn, B., and Coulter, J. G. Coli-Aerogenes 



Differentiation in Water Analysis. Jour. Bact. 22:125-181, 1931. 



4. Parr, L. W. Sanitary Significance of the Succession of Coli-Aerogenes 



Organisms in Fresh and Stored Feces. Amer. Jour. Pub. Health 26:39-45, 

 1936. 



5. Werkman, C. H., and Gillen, G. F. Bacteria Producing Trimethylene Glycol. 



Jour. Bact. 23:167-181, 1932. 



