212 Introduction to the Study of Science 



The particular way by which any community disposes of its 

 sewage depends upon the geographical features of the region, 

 as well as upon the sanitary wisdom and efficiency of the com- 

 munity. A town or city situated near an ocean, a lake, or 

 running stream, probably makes use of such an easy and in- 

 expensive means of disposal. One may find that the chief 

 municipal problem is to avoid a nuisance or the possible pollu- 

 tion of its own water supply. The danger to other communi- 

 ties is not always considered. The connection of any recent 

 epidemic and the contamination of the water supply by sewage 

 should be investigated. One should observe how waste is re- 

 moved and destroyed or allowed to pollute the water supply. 



Need of sewage disposal plant. Where sewage is discharged 

 into the collecting area of a water supply, it should be thoroughly 

 treated in a modern plant for sewage disposal. Such a plant 

 is usually elaborate and costly in construction and operation. 

 A coarse screen is used to remove large materials, such as gravel, 

 pieces of wood, and so on. Septic tanks (see page 215) of 

 large dimensions are used for the decomposition or putrefac- 

 tion of the organic matter. The final product, called the efflu- 

 ent, may for greater safety be treated where it is discharged 

 with some chemical substance as hypochlorite of lime. Soil 

 filtration or stream action may be counted on to remove any 

 possible remaining elements of danger. 



The house system of drainage is an important part of a sewage 

 system. All branches of waste pipes should be as short as 

 possible and lead directly from fixtures to the main waste pipes. 

 Open, easily accessible, and thoroughly ventilated pipes are 

 now considered the most sanitary. To prevent the entrance 

 into the house of sewer gas, which is not only unpleasant but 

 dangerous, traps are provided. These in general are U-shaped 

 (Fig. 63) so as to retain enough water to block the passage 

 of gas. 



A practical point in the care of such devices may be mentioned. 

 Grease and other clogging materials accumulate in the traps 



