rate of flow is reduced where larger solids can sink to the bottom or rise to the surface.
These solids are retained in the tank and the clarified effluent is discharged.
(2) A septic tank combines two processes. Sedimentation takes place in
one portion of the tank and the accumulated solids are digested by anaerobic
decomposition in the lower portion. This decomposition or treatment of wastewater
under anaerobic conditions is termed "septic," hence the name of the tank. The heavier
residues settle to the bottom of the tank forming a blanket of sludge. The lighter solids,
including fats and greases, rise to the surface and form a layer of scum. A considerable
portion of the sludge and scum are liquefied through decomposition or digestion.
During this process, gas is liberated from sludge. This gas carries portions of the solids
to the surface where they accumulate with the scum. Ordinarily, they undergo further
digestion in the scum layer and parts settle again to the sludge blanket on the bottom.
This action is retarded if there is much grease in the scum layer. The settling is also
retarded because of gasification in the sludge blanket. Furthermore, there are relatively
wider fluctuations of flow in small tanks than in large units. This effect has been
recognized in Table 1-5, which shows the recommended minimum liquid capacities for
2 or less
Each additional, add 250
(Provides for use of garbage grinders, automatic clothes washers, and other
Table 1-5. Liquid capacity of tanks (gallons).
(3) Septic tanks do not accomplish a high degree of bacterial removal.
Although the wastewater undergoes treatment in passing through the tank, this does not
mean that the infectious agents will be removed. This means that septic-tank effluent
cannot be considered safe. The liquid that is discharged from the tank is, in some
respects, more objectionable than that which goes in; it is septic and malodorous. This,
however, does not detract from the value of the tank. Its primary purpose is to condition
the wastewater so that it will cause less clogging of the disposal fields (absorption
fields). Further treatment of the effluent, including the removal of pathogens, is effected
by percolation through the soil. Disease-producing bacteria will, in time, die out in the
unfavorable environment afforded by soil. In addition, certain physical forces during