If 



684 PROCEEDINGS OF SECTION I. 



Conclusions in matters concerning tlie biological purification of 

 sewage must, like the principles enunciated above, be of a broad and 

 comprehensive character if submitted as available for general applica- 

 tion. The only conclusions of a positive nature that I feel justified 

 in drawing from the experience gathered during the present investiga- 

 tion is that the natural processes at work can carry on under a fairly 

 wide range of circumstances, and that, if asked an opinion, one would 

 more probably fall into error in venturing to prophesy a failure than 

 in submitting a favorable view, provided the suggested conditions 

 Avere not altogether beyond a reasonable limitation. In addition, it 

 Inust be evident that 24 hours — as generally laid down— is not in- 

 flexible minimum for treatment in a septic tank, and that double this 

 time dqes not always give rise to the train of symptoms described as 

 resulting from " over-septicisation," but, on the contrary, appears 

 favorable to active nitrification. 



..,,, But, although I regret I cannot bring forward a series of final' 

 ppimons on the many vexed points that are associated with sewage 

 treatment, still there were certain incidents of such interest as to 

 warrant special notice, and from which one may draw certain provisional 

 conclusions. Reference has been made to them above, but a short 

 enumeration may not be amiss. They are as follows : — 



NORTH SYDNEY OUTFALL. 



1. The scum formation and changes therein. 



2. The rapid rate of flow through the tanks. 



3. The method of filtration. 



4. The high degree of purification secured. 



CHATSWOOD OUTFALL. 



1. The absence of heavy scum formation. 



2. The growth of water hyacinths in the tanks. 



3. The small amount of silt in the tanks after four years' working. 



4. The degradation of the coke in filter beds. 



BALMORAL OUTFALL. 



1. The effects of lengthy treatment in the tanks on the " oxygen 

 absorbed " figures. 



2. The high nitrification in the filters. 



3. The cessation of nitrification after the contents of the tanks 

 had been suddenly washed through the beds. 



4. The slow recovery of the nitrifying agencies in the filters. 



Finally, I take this opportunity of expressing my indebtedness 

 to Mr. W. E. Cook, District Engineer for the Metropolitan Board of 

 Water Supply and Sewerage, who has direct supervision over the 

 outfall works at North Sydney, for his ready kindness in supplying 

 me with the engineering details and other like information necessary 

 for completion of this communication. 



