664 PROCEEDINGS OF SECTION I. 



The first and essential tenet of the belief to which we subscribe 

 is that sewage contains elements capable of effecting its reduction to 

 innocuous matter. 



The other articles are subsidiary to and included in the above, 

 and outline the stages in the purifying process. It has been shown 

 that during the first stage there is a breaking up of complex organic 

 molecules with solution of solid matter, and that during the second 

 there is more or less oxidation of the nitrogen, carbon, and hydrogen 

 previously existing in organic combination. The two stages are 

 distinct, and the second cannot be accompHshed without the comple- 

 tion of the first. 



I propose to consider the practical application of these principles 

 as illustrated by the results of the experience we have gathered from 

 the working of several installations in Sydney, in which the sewage 

 is treated on purely biological lines. Before, however, discussing 

 any details I wish most emphatically to draw attention to one point 

 that I hold can and should never be forgotten, namely, that while the 

 above principles hold good under a wide variety of circumstances, 

 the methods of their application must be varied to suit the peculiar 

 requirements of the different sewages calling for treatment. These 

 requisites are incapable of definition, and, further, cannot be decided 

 by any at present known test or series of tests. They can only be 

 determined by experience gained at outfall works, so that in fact each 

 installation must be regarded to some extent as an experiment, in which 

 various forces are manipulated and directed from time to time as the 

 demand arises. 



As part of the metropolitan system of sewerage in Sydney there 

 have been constructed three biological installations, each serving a 

 separate but neighboring locality, and each dealing with ordinary 

 domestic sewage. Since November, 1905, regular monthly analyses 

 of samples collected under strict conditions have been carried out. 

 The results contain some points of interest, and appear to me worthy 

 of record. 



NORTH SYDNEY OUTFALL WORKS. 

 The works are situated at the head of Willoughby Bay, near Folly 

 Point in Middle Harbor. They were constructed by the Department 

 of Public Works and handed over to the Board in 1898. Originally 

 they were designed for treating sewage by lime-precipitation, and were 

 fully equipped with the necessary machinery for sludge-pressing. In 

 1901 the Board decided to alter the system by converting the precipi- 

 tation tanks into open septic tanks. In 1905 the tanks were provided 

 with substantial covers. No change was made in the use of the filter 

 beds below described. 



The installation consists of (1) two silt pits through which sewage 

 is run alternately to facilitate the operations incidental to removing 



