How Does a Septic Tank Work?


LRM is often asked what a septic tank is, and how it actually works. Below is a general description of the function of a septic tank and system.

A septic tank is simply a big concrete or steel tank that is buried in the yard. The tank might hold 1,000 gallons (4,000 liters) of water. Wastewater flows into the tank at one end and leaves the tank at the other.

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. Wastewater comes into the septic tank from the sewer pipes in the house.

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 drainfield. A drainfield is made of perforated pipes buried in trenches filled with gravel.

The water is slowly absorbed and filtered by the ground in the drainfield. 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 bigger.

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. It is a completely passive system.

Land Resource Management is always available to answer any questions you may have about septic tank systems, and much more!

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