Unfortunately, too many homeowners don't know their septic tank. The tank is buried and forgotten, along with the rest of the system, until sewage is ankle deep in the basement or bubbles up in the yard. Then another septic tank system is unfairly criticized for failing to do its job. When the car breaks down or the stove is so full of ashes that no more wood can be put in, we don't blame the car or the stove.
Raw sewage flows into the tank from the house sewer.
The solids separate from the liquid and stay in the tank. The liquid flowing out is called septic tank effluent. Bacteria that do not use oxygen from the air grow in the tank. These bacteria are called anaerobic and the by-products of their activity are methane and hydrogen sulfide gas, plus other substances having odor. But the septic tank might also be described as a settling tank where the sewage solids are stored while the bacteria decompose them and reduce their volume.