DISPOSAL OF SEWAGE 



515 



in solution. The methods followed and apparatus used to 

 accomplish this final and complete oxidation of the sewage 

 depends upon surrounding conditions and the amount of 

 sewage to be handled. If small amounts of sewage only are to 

 be disposed of, and the character of the soil permits, the final 

 oxidation may be accomplished by means of subsoil drains and 

 underdrains only (Fig. 318). If large amounts of sewage are 

 to be disposed of, or if the surrounding soil is not reasonably 

 open, porous soil, CONTACT FILTER BEDS are generally provided 

 (Figs. 319 and 321). In either case the same general principles 



FIG. 321. Contact filters of stone. (Julietta, Ind.) Sewage is discharged 

 from tank, Fig. 320, to these beds. (Courtesy of Chas. Brossman.) 



are applied; namely, suitable conditions are provided whereby 

 aerobic bacteria may work upon the sewage, completing its 

 oxidation to mineral matter. 



606. The Subsurface Drain. After passing through the 

 siphon the sewage enters the SUBSURFACE DRAIN (see Fig. 318). 

 This is merely a line of drain tile laid a few inches beneath the 

 surface of the soil. The septic tank must evidently be con- 

 structed on higher land than the plot used for the drainage. 

 All joints between the tile are left slightly open, from Y to 

 Yl in . To prevent dirt from entering through these open j oints, 



