ON THE TKEATMENT AND UTILIZATION OF SEWAGE. 



227 



It is remarkable how little the true average composition of the sewage 

 differs from the results obtained in previous years ; and the Committee con- 

 sider that this circumstance affords considerable proof of the accuracy of their 

 methods of sampling, the principle of which has always been that the samples 

 should be taken in proportion to the amount of flow at the time ; thus the 

 amount of nitrogen in parts per 100,000 in the sewage has been, according 

 to the calculations from the results of the gaugings and analyses, as follows :— 



With regard to these figures your Committee would observe that the rain- 

 fall in the year 1872-73 was excessive, and this no doubt accounts for the 

 sewage containing a smaller proportion of nitrogen in that year ; and that 

 with regard to the year 1874-75, the number given was the result of a single 

 analysis of a mixture made of all the monthly samples taken in quantities 

 proportionate to the amounts of sewage distributed each month. 



The Committee have thought it desirable to make some observations on the 

 changes which occur in sewage and effluent water when kept for some time, 

 with the ^iew of ascertaining how far this result for 1874-75 is reliable. 



Bottles were filled with portions of the samples of sewage and of effluent 

 water collected during November 1875, and put aside in a cool cellar ; they 

 were analyzed in May 1876, and the results of these analyses compared with 

 the previous ones of the same samples were as follows : — 



