The Operation of a Septic Tank

A modern Alpha Septic Tank (as produced by Marsh)

A Septic Tank is a form of holding chamber utilising a form of internal construction to slow down the flow through the tank unit. The outlet from the septic tank leads to a soakaway system below the ground surface.

The diagram shows a traditional single septic tank having just one chamber and using inlet and outlet T pipes. This is the simplest form of septic tank and works as follows:

  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 anaerobic 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 and then leads to a soakaway field drainage system.


The basic principle is to remove as much of the solids content from the final effluent as possible.

Here is a modern septic tank formed via a resin mould. This type of tank is very efficient and has internal baffles which reduce the flow and allow solids to settle to the base.

By understanding the basics of a septic tank you may be able to appreciate the problems that can be encountered.

Please return to Septic Tank Help Page.

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