682 PROCEEDINGS OF SECTION I. 



that the increase in oxygen absorbed could be accounted for by an 

 increase in the amount of nitrites which occurred during incubat'on. 

 I was, therefore, compelled to put this test aside, and I subsequently 

 employed the methylene blue test. This test has proved satis- 

 factory, and, for the past three years, with me has given results 

 concordant with the smell test. In carrying out the oxygen 

 absorbed test, I make use of a weak permanganate solution (1 c.c. 

 contains .1 milligram of oxygen) and permit the sample to remain 

 at room temperature. The results so obtained do not vary sub- 

 stantially from those which would follow if the test were made as 

 recommended at 80°F. 



In the following table I give the results of 108 samples of final 

 effluents which have been tested in the routine way in my labora- 

 tory. Each is a composite sample made up of nine separate 

 samples taken over a period of 24 hours. Series A and C are from 

 installations comprising open septic tanks and contact beds. Series 

 B are from closed septic tanks and sand filter beds worked on an 

 intermittent downward filtration system. In the table I have 

 shown (i.) the amount of ox5'-gen in nitrites and nitrates ; (ii.) the 

 oxygen absorbed in four hours ; (iii.), the figures obtained by 

 dividing the former by the latter, which figures I have termed, for 

 convenience sake, " decomposition factors," and also (iv.) the result 

 of the incubation test. The figures in this and other tables repre- 

 sent parts per hundred thousand. 



Of the 108 samples, 11 were found to have decomposed, and in 

 all of these the decomposition factor was fess than -7. Seven 

 samples gave a factor of less than ■?, and of those three decomposed, 

 two less than -6 and one decomposed, six less than -5 and three 

 decomposed, four less than -4 and all decomposed. Sample C34, 

 which gave a decomposition factor of -28 and yet failed to decom- 

 pose, calls for some comment. This sample was taken immediately 

 after very heavy rain, and was found to contain an undue amount 

 of finely- divided suspended matter, which had apparently been 

 washed through from the tanks. This matter, which is the organic 

 residue resulting from decomposition in a tank and is somewhat of 

 the nature of humus, does not undergo offensive decomposition, but 

 still has the power of absorbing oxygen from permanganate. Under 

 these circumstances I feel justified in classifying this sample in 

 Table II. as having a factor between -6 and -7, which in all proba- 

 bility would represent the actual truth were the above matter not 

 contained in it. It will be noted that I have presented no figures 

 for dissolved oxygen in these samples, the conditions under which 

 they were collected precluding the possibility of making any 

 significant estimation of this element. 



