CHAPTEE XVII 



THE CHEMISTRY OF SEWAGE PURIFICATION 



Owing to the general adoption of the water carriage 

 system, together with the increasing scarcity of land in the 

 vicinity of towns, great developments have taken place during 

 recent years in so-called artificial methods for the purification of 

 sewage. In order that works for this purpose shall be designed 

 with due regard both for economy and efiiciency, it is necessary 

 that the changes, which it is intended to bring about by their 

 means, should be thoroughly understood by those concerned 

 in their construction. Although the actual construction of 

 the modern sewage works is largely a matter of engineering, 

 the design depends on careful adaptation of means to ends, 

 and the bacteriological chemist and the engineer should here 

 work in collaboration. The object of the works is to convert 

 objectionable waste products, which if left to themselves 

 would be a source of nuisance and danger, into other sub- 

 stances which are incapable of producing such objectionable 

 developments. In the course of the necessary transformation, 

 at one point or another, practically all the typical chemical 

 changes, which have been considered in the theoretical chapters 

 of this book, are met with, and the consideration of the pro- 

 cesses carried on in a modern sewage works forms therefore an 

 excellent illustration of the apphcation of bacteriological and 

 enzyme chemistry. 



It will be necessary, in the first place, carefully to consider 

 the composition of ordinary town sewage ; for this purpose, 



