A septic tank is considered to be a part of a small scale sewage treatment system often referred to as a septic system, which consists of the tank itself and a septic drain field. Wastewater and refuse from toilets enter the tank where solids can settle and scum floats. If things work properly, such a sewage-disposal tank has a continuous flow of waste material that is decomposed by anaerobic bacteria.

Leach fields are an important component of successful waste disposal

Excess liquid drains from the relatively clear portion of the tank to a leach field (also referred to as a drain field, or seepage field, depending on the terms used in certain geographic localities) where the remaining impurities naturally decompose and the water is eliminated by percolating into the soil, and eventually taken up through the root system of plants or added to the groundwater.

A piping network, often constructed in a stone filled trench, distributes the wastewater throughout the field with multiple drainage holes in the network. The size of the leach field should be in proportion to the volume of the wastewater and how porous the drainage field is. The entire septic system can operate by gravity alone, or where topographic considerations require it, with the inclusion of a pump.

There are different kinds of septic systems

There are also two stage septic systems 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 otherwise the septic tank will overflow and the waste materials will discharge directly into the drainage field. Such actions can have a bad result for the environment and may even damage the leach field pipes at great expense for repairs.

How often the septic tank should 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 because anaerobic digestion occurs faster at higher temperatures.

Here is a list of "official" septic tank descriptions that provide a greater understanding of what septic tanks are

1. A tank, usually underground, into which sewage flows, the deposited matter being wholly, or partially broken down through anaerobic action. The final effluent may be allowed to soak into the ground through a system of agricultural drains, if the soil is suitable. Alternatively, the tank must be emptied at regular intervals by a special road-tanker.
—Source: European Environment Agency


2. An underground storage tank for wastes from homes not connected to a sewer line. Waste goes directly from the home to the tank.
—Source: Office of Communications, Education, and Media Relations


3. An underground tank to collect wastes from homes that are not connected to a municipal sewer system. Waste goes from the home to the tank and is decomposed by bacteria. Solids and dead bacteria settle to the bottom as sludge while the liquid portion flows into the ground through drains. While properly placed and maintained septic systems can effectively treat domestic wastewater, others are a major source of ground water and surface water pollution.
—Source: Office of Enforcement and Compliance Assurance


4. An underground tank used for the deposition of domestic wastes. Bacteria in the wastes decompose the organic matter, and the sludge settles to the bottom. The effluent flows through drains into the ground. Sludge is pumped out at regular intervals.
—Source: State of Texas, Local Government Guide


5. Underground tank for treatment of wastewater from a home in rural and suburban areas. Bacteria in the tank decompose organic wastes and the sludge settles to the bottom of the tank. The effluent flows out of the tank into the ground through a field of drainpipes.
—Source: Office of Policy: Inventory of U.S. Greenhouse Gas Emissions and Sinks

The sepsi-, -sepsis word unit is available here.