FARM SANITATION 



503 



The reservoir provided for this purification by bacterial 

 action is known as the septic tank. To secure the best 

 results, this septic tank should be designed to exclude light 

 and air and to bring the sewage to rest and hold it so for a 

 time. 



The purification of % the sewage, however, is not completed 

 in the septic tank. To complete the process, means must 

 be provided to permit another class of bacteria to act upon 



Orf/ef 



Fig. 308. A general view of a septic tank arranged to be connected with 

 an underground irrigation or filter system without a siphon. 

 (After Stewart.) 



the sewage. These must have air and light or they cannot 

 live. To supply the proper conditions for this second bac- 

 terial action, two plans are followed : the first is to provide a 

 filter bed of coarse material, usually gravel, over which the 

 sewage from the septic tank is discharged at intervals; and 

 the second is to provide a shallow tile system from which per- 

 colation will take place. These tile are usually placed within 

 ten to twelve inches of the surface, and, if the soil is retentive, 

 a second and deeper system is laid to carry away the purified 

 sewage. In some places this filter system of drain tile is used 



