44 MANAGEMENT OF DAIRY PLANTS 



SEPTIC TANK 



In sewage purification, the septic tank proves of greater serv- 

 ice and of less danger than the cesspool. In the latter, sewage 

 is purified by filtration. In the former, it is cleansed mainly 

 by a process of fermentation. This process involves the follow- 

 ing principles which in turn influence the size and form of con- 

 struction of the septic tank: 



1. Solid matter in the sewage which is subject to fermenta- 

 tion must be given time to be decomposed by bacterial action. 

 This calls for a tank large enough to hold several days' drainage. 



2. The bacteria are anaerobic and work best where sewage is 

 quiet and covered with air-excluding scum. To accomplish this, 

 the tank is divided into compartments which will automatically 

 retard the flow of sewage between intake and outlet and keep 

 fresh sewage separate from the older, thus hastening decom- 

 position in the latter. 



It is maintained by some that the septic tank alone will be 

 sufficient for the purification of sewage from the creamery. But 

 it is safer to connect the septic tank with a filter bed over which 

 the fermented sewage is discharged from the tank, either directly 

 or through an intervening dosing chamber. By this method 

 the purification of sewage is accomplished in two stages and 

 with greater certainty of result. 1 



1. The reduction process, which takes place in the tank and 

 under conditions of air exclusion. Here the solid matter held 

 in suspension in the sewage is decomposed by fermentation into 

 an easily burned substance. 



2. The oxidation process, occurring in the filter bed and with 

 greatest possible exposure to air. Here the products of reduc- 

 tion are spread over the surfaces of numerous soil particles and 

 thus purified through oxidation. 



Size of Tank. The tank must be so large that it will hold 

 sewage accumulations long enough for complete decomposition 

 to take place. In creamery sewage, the solid matter is largely 

 milk remnants in which decomposition is confined mainly to 



1 Wisconsin Exp. Sta. Bulletin 245, 1915, p. 54. 



