A septic tank system is a method of disposing of domestic sewage safely and effectively when a municipal sewage system is not available. For most isolated dwellings, the only alternative to a septic tank system is to construct a cesspool on the property, a cesspool being simply a pit dug into the ground into which the raw sewage is discharged. Within the cesspool, the organic wastes are partially broken down by bacteria, and the liquid portion of the sewage is gradually absorbed into the surrounding soil, the bacteria along with it. Cesspools will badly pollute any nearby bodies of water, they stink, and they breed disease organisms; for these reasons their construction or use is universally forbidden by local health codes. In the absence of a municipal sewage system, therefore, a septic tank system is the homeowner’s only possible method of getting rid of sewage, although there are now coming on the market alternative systems that may eventually replace septic tank systems.
There are two main parts to any septic tank system (see Fig. 1): (1) the septic tank itself, which is a large, watertight, sealed container buried in the soil, and (2) an underground dispersal, or distribution, or absorption system (all three terms are synonymous and in widespread use, though absorption is the most common term) of some kind that enables the liquid discharged from the septic tank to be safely dispersed (or distributed, or absorbed) into the soil.
Fig. 1. Septic tank systems and their components.
The overall design of any septic tank system depends mainly on two factors: (1) the amount of raw sewage that must be treated in the septic tank within a 24-hr period, and (2) the ability of the soil to absorb the treated liquid that is discharged from the septic tank within the same 24-hr period. Some soils are more porous than other soils, some soils are only semi permeable, and still other soils are completely impermeable and will not absorb any liquid at all. The nature of the soil thus has a very important influence on the overall design of a dispersal system. In comparison, the design of a septic tank is relatively straightforward.
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