10 MASS. EXPERIMENT STATION BULLETIN 446 



lowest average. For the set of calculations with the extra high figures omitted, 

 the north and middle tanks changed places and the south tank again had the 

 lowest average. 



For period II, all tests included, the middle tank had the highest average 

 M.P.N., the north tank second, while the south (normal) tank had the lowest 

 average. The same order obtained for calculations that omitted the extra high 

 M.P.N, figure for each tank. 



The individual M.P.N, values from the three tanks were scored for each 

 sampling date, allowing three points for the highest M.P.N., two for the middle 

 value, and one for the lowest. On that basis, for the first operating season, the 

 north tank scored 27, the middle tank 25, and the south tank 14 for eleyen tests. 

 The numbers of high, middle, and low scores for the three tanks were as follows: 



For the second collection period, the scores for eighteen tests were: middle 

 tank, 41; north tank, 38.5; south tank, 28.5. The numbers were larger than for 

 the first period because there were more tests. The numbers of high, middle, 

 and low values were: 



In one instance all three values were the same and were counted as a tie for 

 high. The results from the scoring procedure show that the south tank had the 

 lowest M.P.N, values for both seasons, which agrees with the averages shown 

 in table 3. 



When all the data are considered, the evidence favors the south tank as being 

 more efficient than the other two in eliminating coliform bacteria" from sewage. 

 It must be remembered, however, that the north tank received three times as 

 much sewage as the south tank, and the middle tank twice as much. This 

 gives a ratio of 3:2:1. The M.P.N, averages for the three tanks for either operat- 

 ing period are closer than that, which could be taken as an indication that the 

 8-hour and 12-hour retention periods were relatively as efficient as the normal of 

 24 hours. Even if that were true, however, the net result would be the liberation 

 in 24 hours of fewer coliform bacteria in the effluent from the south tank than 

 from either of the other two tanks. 



Differential Distribution of Coliform Bacteria Isolated 

 from Positive Fermentation Tests 



The cultures included in this part of the study were isolated, as was mentioned 

 previously, from eosin-methylene blue plates, five cultures from the highest 

 dilutions of each sample that had given positive gas tests in lactose broth. The 

 cultures were purified according to accepted methods (3) and then were put 

 through the so-called "IMViC" tests (4). These are indol production (I), the 

 methyl-red test (M), the Voges-Proskauer test (V) and growth in sodium citrate 

 C). The letter "i" is included to complete the word. 



