1 88 SCIENCE PROGRESS 



maximum growth of vegetation (consistently with due purifi- 

 cation) for the amount of sewage supplied." The sewage should, 

 if possible, reach the land by gravitation (to avoid the expense 

 of pumping). The great difficulty, as a rule, is the cost of the 

 land. The extent acquired varies with its nature and other 

 circumstances, but on an average about one acre to every 

 hundred persons is sufficient. The most suitable land is, of 

 course, a friable loam, and the least suitable heavy clay or peat. 

 Where it is not porous the land should be underdrained. The 

 sewage is distributed over the land by channels, the ridge and 

 furrow system being the best. The crops grown are Italian 

 rye-grass, mangolds, cabbage, etc. These sewage farms may, 

 with careful management, just pay their way ; but for many 

 reasons, which cannot be set forth here, one cannot expect them 

 to yield any large profit. They are not unhealthy, and should 

 not produce any nuisance. No facts have ever been brought 

 forward to prove that diseases are spread or produced by these 

 farms. The great difficulty, as a rule, is that land suitable for 

 these farms in the immediate neighbourhood of towns is much 

 too expensive to be put to the purpose of sewage purification. 

 Hence our next method — artificial bacterial filters. 



Bacterial Treatment of Sewage 



This system depends upon the presence of certain organisms, 

 which may be classified thus : 



(a) Anaerobes — organisms which work in the absence of 



air — (oxygen). 



(b) Aerobes — working in the presence of air — (oxygen). 



(c) Facultative aerobes — capable of working either in 



the presence or absence of air — (oxygen). 

 Ever since cesspools were employed for the reception of 

 sewage, it has been noticed that the material which is periodically 

 emptied out, or overflows from them, contains very little 

 suspended matter. Solid matter is, in fact, generally only found 

 at the bottom of the cesspools. What happens, then, to all the 

 solid material which enters with the sewage ? Its disappearance 

 is undoubtedly due to the presence of myriads of micro- 

 organisms, which are constantly feeding upon it, and ultimately 

 convert it into products which become dissolved in the liquid 

 part of the sewage. The anaerobic organisms are probably 



