408 APPLIED PHYSIOLOGY 



one thousand people continuously and in safety without 

 needing attention except the occasional removal of the 

 accumulated solid matter. This method is still used with 

 success by some large European cities. 



A modern plan is to provide a water-tight cesspool or 

 septic tank in which the solid matter rots away, leaving 

 only liquid to flow out upon the land. Where this method 

 is used, an acre of land can soak up the sewage of several 

 thousand people without the accumulation of offensive 

 solid matter. 



A septic tank should be large enough to contain at least as much 

 sewage as the town produces in a day. In the course of the twenty-four 

 hours which it takes a given specimen of sewage to pass through the 

 tank, it undergoes decay by which over half of its solid matter is liquefied ; 

 the larger particles fall to the bottom or float on the surface, and re- 

 main in the tank till they are liquefied also. The bacteria which pro- 

 duce the greater part of the decomposition, especially of the fats, flourish 

 only in the absence of air ; and this condition is secured by the thick 

 scum which accumulates in the tank. As the sewage flows out of the 

 tank, it is made to absorb oxygen in some manner, as by letting it fall in 

 small jets or thin sheets through the air, or by making it flow over a bed 

 of broken stone. Then it is usually allowed to soak into the soil of 

 large sandy beds set aside for the purpose. In the soil the remaining 

 decaying matters are oxidized to harmless products by bacteria which 

 flourish in an abundance of oxygen. 



This system closely imitates nature's method of returning decompos- 

 ing substances to the soil, and is one of the simplest and cheapest of 

 all systems for the disposal of domestic sewage. In a septic tank that 

 is working properly no solid matter accumulates, but the system does 

 not work well if the sewage contains much chemical or factory waste, 

 as such substances prevent the bacteria from flourishing and destroying 

 the solid matter. Street drainage should not be mixed with the sewage. 



712. Water supply. An open well is not a safe source 

 of drinking water, for under the best conditions germ-laden 

 dust and dirt blow into it. In a thickly settled village or 

 city sewage cannot be prevented from filtering into wells, 



