How Does A Septic System Work?

Sep­tic Tank

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:

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.