How a Conventional System Works
The Tank:The typical septic tank is a large buried cylindrical container made of concrete. A septic tank's purpose is to separate solids from the wastewater, store and partially decompose as much solid material as possible, while allowing the liquid (or effluent) to go to the drainfield.
Wastewater from your toilet, bath, kitchen, and laundry flows into the tank and remains there for up to 24 hours (known as the retention time) before it passes to the drainfield. This helps prevent clogging of the drainfield, which can lead to failure and costly repairs. Retention time is necessary to allow the solids to properly separate from the liquids—heavy solids settle to the bottom as sludge and the lighter particles rise to the top, forming a scum layer. Although bacterial action partially decomposes some of the solids, up to 50 percent remain in the tank.
As wastewater flows into the tank, a tee (or baffle) at the tank's inlet pipe slows the incoming wastes and reduces the disturbance of the settled sludge. The outlet tee keeps the solids or scum in the tank.
Consequently, it is important that solids be removed by periodic pumping, so they do not overflow into the drainfield. Most septic tanks need to be pumped every 2 to 3 years, depending on the tank size and the amount and type of solids entering the tank. (see chart below)
The Drainfield:After solids settle in the septic tank, the liquid wastewater (or effluent) is discharged to the drainfield, also known as an absorption or leach field. The final treatment takes place in the soil below the drainfield.
The drainfield has a network of perforated pipes laid in gravel-filled trenches (2-3 feet wide) or beds (over 3 feet wide) in the soil. Wastewater trickles out of the pipes, through the gravel layer, and into the soil. The size and type of drainfield depends on the estimated daily wastewater flow and soil conditions.