Septic tank

A septic tank is a small scale sewage treatment system within a domestic water system, that consists of the tank itself and a 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 clear portion of the tank to the drain field where the remaining impurities naturally decompose and the water is discharged into the ground water. A piping network distributes the wastewater throughout the field with multiple drainage holes in the network. The size of the drain 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

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. 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 non-digestable solids and the ambient temperature as anaerobic digestion occurs more efficiently at higher temperatures. The tanker truck that receives the septic tank sludge is colloquially called a "honey wagon". Users of septic tank systems are very careful not to put excessive waste (e.g. through a kitchen food disposal unit) or nonbiodegradable waste through their sewers.

The expression "The grass is always greener over the septic tank" is technically incorrect; the grass is greener over the drain field which is better watered and has more nutrients than the surrounding land.