A septic tank is part of a small scale sewage treatment system often referred to as a septic system, which consists of the tank itself and a "leach" (drain) field. Wastewater enters the tank where solids can settle and scum floats. Anaerobic digestion occurs on the settled solids, reducing the volume of solids. The term "septic" comes from the anaerobic bacterial activity. Excess liquid drains from the relatively clear portion of the tank to the leach field (also referred to as a drain field, or seepage field, depending upon locality) where the remaining impurities naturally decompose and the water is eliminated through percolation into the soil, and eventually taken up through the root system of plants or added to the groundwater. A piping network, often constucted in a stone filled trench, distributes the wastewater throughout the field with multiple drainage holes in the network. The size of the leach field is proportional to the volume of wastewater and inversely proportional to the porosity of the drainage field. The entire septic system can operate by gravity alone,or where topographic considerations require, with inclusion of a lift pump.

An Imhoff tank is a two stage septic system where the sludge is digested in a separate tank. This avoids mixing digested sludge with incoming sewage. Also some septic tank designs have a second stage where the effluent from the anaerobic first stage is aerated, before it drains into the seepage field.

Waste that is not decomposed by the anaerobic digestion eventually has to be removed from the septic tank or else the septic tank fills up and wastewater discharges directly to the drainage field. Not only is this bad for the environment, but if the sludge overflows the septic tank into the leach field, it may damage the leach field piping requiring expensive repairs. How often the septic tank has to be pumped out depends on the volume of the tank relative to the input of solids, the amount of indigestible solids and the ambient temperature (as anaerobic digestion occurs more efficiently at higher temperatures).

The tanker truck that pumps out the septic tank sludge is colloquially called a "honey wagon". Users of septic tank systems need to be very careful not to put excessive waste (e.g. through a kitchen food disposal unit) or nonbiodegradable waste through their sewers, thus overloading the system design capacity. Likewise, users of septic systems should limit water usage to the maximum extent practical in order to avoid exceeding the capacity of the leach field to disperse the water portion of the waste.

Erma Bombeck's book The Grass is Always Greener Over the Septic Tank (ISBN 0070064504) and the common expression that is the same as the title is technically incorrect; the grass is greener over the leach field which is better watered and has more nutrients than the surrounding land.

In Cockney and Australian slang "septic tank" or, more commonly, simply "septic" can be a pejorative word for an American, based on rhyming slang for "Yank".

See also

• Bioreactor
• Cesspit
• Latrine
• Outhouse
• Pit toilet
• Sewage treatment
• Sewer
• Waste disposal
• Backwater