How a Septic System Works


Figure 1. A typical septic system.
Figure 1. A typical septic system.

The septic system consists of three components (Figure 1): the tank, the drain lines or discharge lines, and the soil treatment area (sometimes called a drainfield or leach field).

Tank sizes vary depending on the size of the building. The average home (three bedrooms, two bathrooms) will often have a 1,000-gallon tank. New tanks must have two chambers, while older tanks may have only one. The tank is often made from concrete, but other materials are also used.

The tank works by settling and microbial digestion of waste. As wastewater flows into the septic tank, three layers are formed (Figure 2): a bottom sludge layer, the top scum layer and a “clear” zone in the middle. Heavy solids settle to the bottom, forming the sludge layer, and grease and light solids float to the top to form the scum layer.

Figure 2. A typical septic tank.
Figure 2. A typical septic tank.

Bacteria and other microbes are responsible for partial decomposition of the sludge. Fortunately, many of the microbes involved are present in large numbers in the human intestine. A new supply of these is added to the septic tank with each flush (no additives are needed). However, the microbes cannot break down all the material in the sludge, which is why septic tanks need to be pumped out every two to five years.

Baffles on the discharge side of the septic tank only allow water from the middle layer to go out to the field lines. As new water comes into the septic tank, an equal water volume is pushed out the discharge lines and into the drainfield. There may be a distribution box located between the tank and drain lines, sending wastewater to several lines in the drainfield. The water trickles out the perforated drain lines, through a layer of gravel and into the soil (Figure 3).

Figure 3. A typical drainfield.
Figure 3. A typical drainfield.


The soil is teeming with life. Plants, bacteria, fungi and protozoa, along with larger creatures such as mites, earthworms and insects, thrive in the soil. All these creatures help release nutrients and other compounds from the wastewater, using and reusing them in the soil and cleaning the water. Minerals and metals bind to soil particles, removing them from the waste water. The treated water eventually moves into ground water.


Any issues with any septic components or maintenance servicing (pump outs),

please call Cape Cod Septic Services for assistance.



Like us on Facebook!

Leave a Comment

Cape Cod Septic Services