682 PROCEEDINGS OF SECTION I. 



deriving their oxygen from nitrates, reducing them in some cases to 

 nitrites, and others to ammonia. 



In a recent paper by Johnson and others {Journal of Infectious 

 Diseases, Chicago, February, 1906) on " Putrescibility in Sewage 

 Effluents," the authors deal exhaustively with the factors involved, 

 both in causing and arresting decomposition. They point out that 

 better indications can be secured by incubation at 37° for from 24 to 

 48 hours than for longer periods. They further show how probably 

 putrescibility of a sample can be computed from chemical data. They 

 recommend that the oxygen consumed, the nitrogen as nitrate and 

 nitrite, and the dissolved oxygen be estimated. They arrive at their 

 oxygen consumed figures by treating the sample with acid permanga- 

 nate at boiling point for five minutes, and by dividing the results by 

 five. The final figure obtained closely approximates the oxygen 

 consumed figure for three minutes' treatment in the cold, but has in 

 the authors' opinion the advantage that it is not so readily influenced 

 by the presence of easily oxidised compounds, such as nitrites, ferrous 

 salts, &c. With these data they interpret the putrescibility of an 

 effluent in the following manner : — First, when the consumed oxygen 

 value is equal to or in excess of the amount of dissolved oxygen in the 

 effluent, and no nitrates or nitrites are contained therein, the sample 

 will putrify. Secondly, when the consumed oxygen value is equal to 

 or slightly less than the amount of oxygen contained in the effluent 

 in the form of nitrates, nitrites, and dissolved oxygen, the sample may 

 or may not putrify ; and thirdly, when the consumed oxygen value 

 is less than the oxygen contained in the effluent in the form of nitrates 

 and nitrites, under ordinary circumstances the sample will not putrify. 



The truth of their third conclusions is borne out by our experi- 

 ence with the effluents at the North Sydney outfalls. 



It will be seen that when the oxygen in the nitrates and nitrites 

 is calculated ( to do this multiply the nitrite figure by 1-71 and the 

 nitrate by 2-86) the amounts are, in practically every case, consider- 

 ably in excess of the oxygen absorbed in three minutes. 



SUMMARY AND CONCLUSIONS. 

 On comparing the structural features of the method of working 

 and the results secured at the three outfalls (the essential items in this 

 connection are set out in Table XIII. below) it will be seen that at 

 North Sydney, with a very rapid flow through covered tanks and 

 subsequent prolonged treatment by intermittent downward filtration, 

 very excellent final effluents are obtained ; and at the same time at 

 Balmoral, with more extended treatment in the tanks and compara- 

 tively short time in contact filters, the final effluents are satisfactory, 

 and contain a good proportion of oxidised nitrogen. Chatswood, 

 with open tanks and contact filters, occupies an intermediate position 

 as regards the period of treatment, and also gives a good type of final 

 effluent. Other points of comparison present themselves, but scarcely 

 call for comment. 



