18 MASS. EXPERIMENT STATION BULLETIN 446 



Differential Tests of Collform Bacteria 



A summary of the results of the differential tests on the cultures from the 

 soils appears in table 8 and the differential tests of that table are expressed in 

 table 10 as percentages of coliform organisms isolated from each tank effluent 

 for each sampling season. 



It is significant that, of the coliform bacteria recovered from the soils, very 

 few were either Escherichia coli or their close relatives, Citrobacter, which table 

 4 shows were present in considerable numbers in the effluents and which are 

 considered to be indicative of sewage pollution. The lack of E. coli and Citro- 

 bacter was observed in both sampling seasons and was consistent with the fact, 

 already mentioned, that a preponderance of the gas-positive tests obtained re- 

 quired 48 or 72 hours of incubation. The percentages of methyl-red positive 

 intermediates were generally lower for the second sampling season than for the 

 first. The highest percentage of these positives in the first season was obtained 

 from the south tank (24-hour retention) and the lowest percentage was from the 

 controls. Accepting the opinion of certain authorities in the field of sanitation, 

 this observation could argue in favor of the 8-hour and 12-hour retention as being 

 more efficient than the normal of 24 hours in re.moving sewage types of coliform 

 bacteria from the effluents. However, in the second sampling season the lowest 

 percentages of methyl-red positive intermediates were obtained from samples 

 adjacent to the south-tank line and from the control samples. Aerobacter aerogenes 

 and methyl-red negative intermediates predo.minated for all samples and for 

 both sampling seasons. There were no differences that the writer can call sig- 

 nificant. In the second season there were more of the A. aerogenes cultures than 

 of the .methyl-red negative intermediates. 



In general, it may be said that the data agree with those for the gas-positive 

 tests and indicate that the bacteria recovered from the soil samples may well 

 have been native to the soils rather than invaders from the septic-tank effluents. 

 This raises a question: what becomes of the sewage-type coliform bacteria when 

 they enter the soil? This will be the basis of a subsequent study. 



SUMMARY 



The experimental results may be summarized as follows: 



1. Management. As the tanks were being operated for two seasons it became 

 evident that decidedly too .much scum accumulated in the two tanks with re- 

 tention period of 8 and 12 hours respectively. The layer of scum in each of 

 these tanks formed rapidly and became so thick that the inlet and outlet pipes 

 to the tanks became plugged in about three months and use of the tanks had to 

 be suspended. This was in spite of special construction in the second season 

 designed to permit much greater accumulation of scum than would be anticipated 

 rn normal tank operation. The scum in these two tanks contained much un- 

 digested solid matter of the sewage. A normal amount of scum accumulated in 

 the tank with 24 hours retention. 



The amount of effluent passing from the tank with 8 hours retention appeared 

 to be greater than the capacity of the disposal field could take care of. The 

 ground about the two lateral lines, particularly the more distant one, was water- 

 logged and water stood on the surface part of the time. This effect may have 

 been influenced in part by the grade drop of the lines. 



Thus, fro.m a management point of view, the shorte: retention periods would 

 seem to have limitations that would render them unsuitable for practical sewage 

 disposal. 



