The Supply and Uses of Water 



215 



GROUND SURFACE 



FIG. 64. Lengthwise section of a 

 septic tank. 



faction of sewage. It may have any external form, but should 

 have two or, better, three divisions of the interior. The design 

 which is given in Fig. 64 is simple, effective, and not expensive 

 to construct. The dimensions of a tank must vary with the 

 amount of sewage it is to contain or with the average size of 

 the household. The dimensions given will give a tank large 

 enough to meet the require- 

 ments of a family of six to 

 eight. 



The tank should be lo- 

 cated at least twenty feet 

 away from the house and 

 where the drainage of the 

 surface and subsoil is away 

 from the water supply. It 

 may be located so that the 

 drainage can be conducted to a field or to the garden. As the 

 upper surface is at least one foot below the surface of the 

 ground and usually covered with soil, no question of appear- 

 ance is involved. 



The excavation should be made large enough to allow for the 

 framework and molds for the walls. It may be five feet wide 

 by nine feet long by six feet deep. The molds are to be made 

 of ordinary lumber with one side surfaced ; the inside surface 

 of the cement should be smooth. The cement floor should be 

 five inches thick, and the walls and partitions about four inches 

 thick. The entrance and discharge pipes are put in place while 

 the cement work is being done so that they will be solid and of 

 a piece with the whole tank. 



86. Kinds of bacteria in sewage. Such a tank is septic be- 

 cause it provides the conditions in which putrefaction occurs 

 rapidly and thoroughly. It provides such conditions perma- 

 nently and effectively. Putrefaction is brought about by 

 the activities of certain bacteria. Bacteria of many different 

 varieties are already in sewage in countless numbers, millions 



