6o 



SEWAGE AND ITS PURIFICATION 



sewage entering and the effluent leaving a works. As ammonia 

 must be considered a preparatory or transition, and not a 

 finished product, it must be counted as part of the residual 

 unoxidized sewage, and only indicates progress towards com- 

 plete purification, and gives a criterion as to whether a process 

 is working satisfactorily. A few examples to show how the 

 percentage of oxidation reveals the purification effected by 

 different agencies may be quoted here : 



I may remark that " percentages of purification," the only 

 figure given in some statements, indicate the work done by 

 the sewage installation, but not necessarily the quality of 

 the effluent. 



