Water Supply and Sewage Disposal. 221 



days exposure is a turbid liquid with a disagreeable odor. 

 The bacterial action is so complete that the solid organic 

 matter is wholly destroyed and scarcely any sediment 

 collects in the tank ; hence it need be emptied and cleaned 

 only at intervals of several years. The tank must be so 

 arranged that a quantity of sewage can be discharged 

 from it at intervals rather than constantly as is the case 

 with the sewage flowing into the tank. 



The sewage should remain in the tank for a sufficient 

 length of time so that it will no longer putrefy. If this 

 is accomplished, the farther steps in purification and dis- 

 posal can often be very simple. If the septic tank is so 

 situated that a drain can be laid which shall discharge 

 onto the surface of a field that has some fall at a distance 

 from the house, the sewage can be discharged onto the 

 surface of the soil. Since the sewage contains no solid 

 matter it sinks rapidly into the soil, where the purifica- 

 tion is completed. This arrangement can best be used 

 on a sandy or loose soil. The drain should empty into 

 open furrows. Since the tank is arranged so that the 

 sewage can be discharged at intervals the soil has an op- 

 portunity to become aerated between the periods of dis- 

 charge. This avoids keeping the soil saturated with 

 moisture which of course would be detrimental to nitrifi- 

 cation. A preferable process is to discharge the sewage 

 from the tank into drains of ordinary tile laid with open 

 joints. The sewage as it flows from the tank at intervals 

 fills the tile and gradually percolates into the soil 

 through the open joints. On account of the intermittent 

 discharge it has time to drain away and thus favorable 

 conditions for nitrification are maintained. If the sew- 

 age flows into the drains constantly, it would all seep 

 out through the first few joints and keep the soil water- 



