CHAPTER XVIII. 



SEWAGE AND SEWAGE DISPOSAL. 



1. INTRODUCTION. 



FOR the purpose of this chapter, sewage may be described as a liquid 

 which holds in solution certain organic matter obtained both from ani- 

 mals and plants ; containing also animal excreta, vegetable remains, and 

 solid debris of animals and plants. Further, as decomposition takes place 

 very rapidly, numerous other bodies will be formed as products of this 

 decomposition. There will also be inorganic matter in suspension, such 

 as grit, gravel, street- washings, etc., and various elements in solution, 

 such as phosphates. The whole constitutes a medium which is ex- 

 ceptionally favourable to the growth of certain kinds of bacteria. 

 Unfortunately, as the numerous epidemics arising from sewage- 

 contaminated drinking water have shown, many disease-bacteria lurk 

 in sewage, so that its disposal is a matter of prime importance to the 

 community. The immense number of bacteria which sewage can 

 support is shown by the following table : 



These bacteria are engaged in the work of breaking down the complex 

 organic products. Speaking broadly, they effect the same changes as 



