8 MASS. EXPERIMENT STATION BULLETIN 446 



growth response was obtained in plates incubated at 30° C. for 48 hours, so this 

 procedure was employed. All counts were made in duplicate from duplicate 

 samples, and the related counts were averaged. 



The counts represented the two collection periods: I — October 10 to Novem- 

 ber 29, 1944, and March 30 to April 24, 1945, a total of ten collections; II — 

 October 10 to December 19, 1945, a total of seventeen collections (see table 1). 

 The counts are summarized in table 2. 



Table 2. — Summary of Plate Counts from Sewage Effluents. 

 Counts as numbers per milliliter of effluent. 



The author is of the opinion that the counts would have little significance in 

 comparing the efficiency of the tanks unless there were marked differences in the 

 results. No such differences appeared in either collection period. One low 

 count from the north tank in Period I lowered the average for that tank for the 

 season. In Period II there were three high counts for the north tank and one 

 for the middle tank. Except for this one low count and the four high counts, the 

 spread of the counts was similar for all three tanks. The high and low deviations 

 just mentioned did not occur a sufficient number of times among the samples to 

 justify drawing any conclusions from them. The results of the plate counts did 

 not appear to afford any evidence to the advantage or disadvantage of any of 

 the three tanks of the installation. 



Most Probable Number of Coliform Bacteria in Septic Tank Effluents 



Testing for Escherichia coli and related species of bacteria, collectively termed 

 coliform bacteria, is the recognized procedure for detecting sewage pollution m 

 water supplies. For that reason the procedure was applied to the effluents from 

 the three tanks to determine the effect, if any, from the three different retention 

 periods on the numbers and types of coliform bacteria present in the effluents. 



