A septic tank is a large tank meant to store sewage waste while it settles. The septic tank design is used by nearly a fourth of the population in the United States, and is also widely used throughout the world.

The septic tank is one part of a standard rural septic system, the other part being the leach field. Sewage enters the septic tank from the house where it is left to separate. Heavy solids settle to the bottom of the septic tank, while the relatively clear water rises to the top where it is allowed to leach off into the earth. Once in the leach field, any remaining solids are taken care of, and the water rejoins the greater water table.

Solids left in a septic tank mostly break down through the digestion of anaerobic processes. Not all the solids will work their way out, however, and occasionally a septic tank must be drained before it reaches capacity. The speed at which this filling occurs depends on the size of the septic tank, the amount of waste being pumped into it, the temperature the tank is kept at, and whether a large amount of non-biodegradable solids have been added to the tank.

The truck that comes to remove solid waste from a septic tank is commonly referred to as a honey truck or honey wagon. Most rural areas have only one septic truck working their region, as the rate of removal tends to be fairly low. In some high-density areas, a number of septic companies may be needed, but it tends to be the case that as populations grow to levels where multiple trucks would be required, environmental factors force the transition to a centralized waste treatment system anyway.

A typical septic tank is constructed of either some sort of metal treated against corrosion or, more commonly, a sturdy plastic. Some septic tanks may be fitted with an additional heating apparatus as well, to aid in the anaerobic breakdown of solid waste and lessen the frequency with which the septic tank needs to be drained. An average-sized septic tank has a capacity of anywhere from 1000 to 1500 gallons (3785-5678 liters), though for high-impact areas large tanks with 5000 to 10000 gallon (18927-37854 liters) capacities exist. Installation of a septic tank requires no small amount of work, with a percolation test required to locate an ideal place for a leach field, and heavy machinery required to dig a sufficient pit in which to place the septic tank.