How Septic Tanks Work

A septic tank will treat wastewater from premises which are not connected to the mains drainage system. It has usually been a large rectangular structure made of brick, stone, concrete or steel buried underground. More modern systems come in the form of a bottle-shaped plastic tank.  A septic tank works like a simple sewage treatment plant, except that it is a completely passive system powered entirely by gravity.

Wastewater flows into the tank at one end. Waste material is allowed to settle in the tank and is digested by natural bacteria which must be allowed to breed within the tank.  As new water enters the tank, it displaces the water that's already there. This treated effluent drains from the tank’s outlet pipe, normally to a soakaway field drainage system.

Over time partially-decomposed solids build up on the bottom of the septic tank. This sludge has to be removed regularly to make sure the tank continues to work properly and to prevent the soakaway becoming choked.  De-sludging should normally take place every twelve months.

In the diagram of a traditionally constructed septic tank below, you can see the 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 water contains bacteria and chemicals like nitrogen and phosphorous that act as fertilizers, but it is largely free of solids.

septic tank diagram
  1. Sewage enters via the inlet T pipe and discharges to the lower of the tank.
  2. Gravity pulls the solids in the sewage to the base and via an aerobic biological action, a scum layer can form on the surface
  3. Effluent (with a very low solids content) leaves via the oulet T pipe. This can then enter a second or third chamber before leading to a soakaway field drainage system.

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58-64 Station Street
PE11 4UJ

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