Biological Sewage Disposal 241 



SEPTIC TANKS 



These are closed, underground, metal or concrete tanks in 

 which sewage is subjected to anaerobic decomposition for periods 

 varying from twenty-four hours upwards. This principle of uti- 

 lizing bacteria to liquefy sewage solids was first proposed by 

 Donald Cameron in England. The process involves a breakdown 

 of sewage solids similar to that found when organic matter is at- 

 tacked in the soil. A minimum capacity of thirty gallons of sewage 

 per person per day may be used in calculating septic tank require- 

 ments, but forty or even fifty gallons is more realistic. Tempera- 

 ture, among other factors, has a direct bearing upon the speed with 

 which digestion takes place, and when the temperature drops too 

 low, the rate of decomposition of sewage may slow down to a point 

 where solids might build up more quickly than biological digestion 

 is able to liquefy them. 



After the preliminary anaerobic decomposition in the septic 

 tank, the sewage is carried out into the soil by means of tile piping 

 embedded in gravel or coarse sand. By leaving a space between 

 the sections of the tile or by employing porous tile, the partially 

 decomposed sewage can seep into the sand or gravel and undergo 

 aerobic breakdown. Another common practice is to empty the 

 sewage into cesspools after the material has undergone decom- 

 position in a septic tank. This leaves a relatively stable, odorless 

 end product. 



Some septic tanks are reported to have been in operation for 

 over fifty years with no cleaning, but in general some non-digestible 

 sludge accumulates in the tank and needs to be removed periodi- 

 cally. It is helpful, however, after cleaning a septic tank to put 

 back some of the active material (sludge) to serve as a culture 

 "starter." Unless this is done, the proper bacterial flora may re- 

 quire an extended period to become established, and the efficiency 

 of the tank is thereby reduced. When septic tanks demand fre- 

 quent attention, the difficulty can usually be attributed to one or 

 more of the following: 



