A septic tank is simply a large concrete or plastic tank that is located underground in the yard. The tank will hold a minimum of 1,500 gallons of water in a new system. Wastewater flows into the tank at one end and leaves the tank at the other. The tank looks something like this in cross-section:
In this picture, you can see three layers. Anything that floats rises to the top and forms a layer known as the scum layer. Anything heavier than water sinks to form the sludge layer. In the middle is a fairly clear water layer. This body of water contains bacteria and chemicals like nitrogen and phosphorous that act as fertilizers, but it is largely free of solids.
A septic tank naturally produces gases (caused by bacteria breaking down the organic material in the wastewater), and these gases don’t smell good. Sinks therefore have loops of pipe called P-traps that hold water in the lower loop and block the gases from flowing back into the house. The gases flow up a vent pipe instead — if you look at the roof of any house, you will see one or more vent pipes poking through.
As new water enters the tank, it displaces the water that’s already there. This water flows out of the septic tank and into a drain field. A drain field is made of perforated pipes buried in trenches filled with gravel. The following diagram shows an overhead view of a house, septic tank, distribution box and drain field:
A typical drain field pipe is 4 inches (10 centimeters) in diameter and is buried in a trench that has a maximum of 3 feet of cover and 2 feet (0.6 m) wide. The gravel fills the bottom 2 to 3 feet of the trench and dirt covers the gravel, like this:
There are also innovative alternative systems that are approved by Massachusetts DEP for general use and remedial use. This is an expample of a Infiltrator System:
The water is slowly absorbed and filtered by the ground in the drain field. The size of the drain field is determined by how well the ground absorbs water. In places where the ground is hard clay that absorbs water very slowly, the drain field has to be much larger.
A septic system is normally powered by nothing but gravity. Water flows down from the house to the tank, and down from the tank to the drain field. You can also have a pump system that lifts the effluent from the pump chamber to the d-box for distribution.