12 MASS. EXPERIMENT STATION BULLETIN 446 



Organisms not conforming to the reactions in this tabulation are termed inter- 

 mediates. For the purpose of this report the intermediates encountered were 

 placed in two groups, methyl-red positive and methyl-red negative, to relate 

 them to either E. coli or A. aerogenes. 



It may be noted here that the term "Citrobacter" is employed by sanitary 

 bacteriologists for the purposes of sanitary classification but is not recognized 

 in biological classifications. 



Reference to table 4 shows that more than 75 percent of all the cultures isolated 

 were methyl-red positive, as was to be expected. Certain differences appeared 

 between the results for periods I and II. In period I the Citrobacter percentages 

 were considerably higher and those of E. coli were lower than for period II, but 

 the totals of methyl-red positive cultures were similar for both periods. Also, 

 no A. aerogenes cultures were isolated in period I, but they were encountered in 

 period II. The total percentages of methyl-red negative cultures naturally 

 were in accord with those of the methyl-red positives. 



The result of the differential studies did not appear to favor any one of the 

 tanks over the other two for either of the collection seasons or for the totals of 

 the two. 



CHEMICAL EXAMINATION OF EFFLUENTS FROM TANKS 



The chemical examinations of the effluents, reported here, were made in the 

 local laboratory of the Massachusetts Department of Public Health by Mr. 

 Ernest A. Snow, Senior Chemist of the Division of Sanitary Engineering. The 

 scope of the examinations was limited by the time at Mr. Snow's disposal for 

 the work, so it was decided to make the tests that it was fe t would give the best 

 indication the effic'ency of the tanks in digesting the solid matter of the sewage. 

 These tests were the determination of the biochemical oxygen demand, com- 

 monly called B.O.D., and of the free-ammonia nitrogen in the effluents from 

 the three tanks. The tests were made on the samples collected for the bacterio- 

 logical examinations of effluents reported in this paper. 



Biochemical Oxygen Demand 



The test for the biochemical oxygen demand depends up>on the fact that oxygen 

 in a sewage suspension is used up in the process of digesting suspended organic 

 matter. Thus, the higher the B.O.D. value in a test, the greater is the amount 

 of undigested matter in the sewage or, in this instance, the effluents from the 

 tanks. The techniques employed for the tests were those of the Standard Methods 

 for the Examination of Water and Sewage (2). Tests were made only in the 

 second operating period, fall and early winter, 1945-6. Results are summarized 

 in table 5. 



On the basis of the averages and of the score (explained in note at bottom of 

 table 5) the biochemical oxygen demand for the south tank was considerably 

 less than for the other two tanks, which had values close together. These results 

 indicate that the most nearly complete digestion of the sewage solids took place 

 in the south tank which had the normal retention period; but it should be noted 

 that the B.O.D. values and the scores for the three tanks were closer together 

 than the 3:2:1 volume ratio of sewage passing through them. This could indicate 

 that, considering greater volumes of sewage treated in the north and middle 

 tanks, digestion of sewage solids was relatively more efficient in them than in 

 the south tank. 



