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Septic Tank Emptying Explained

We have produced the following diagrams to help clarify the operation of a septic tank in addition to the reason for and result of emptying a septic tank. For a more detailed description please see our septic tank emptying video above. For frequently asked questions relating to septic tank emptying please see our FAQs.

There are many variations of Septic tank, the traditional two chamber septic tanks are quite common, as well as the more modern baffled/fibreglass tanks. The principals are the same in both, these diagrams show a traditional system.

The liquid waste from the house drains into the tank, bacteria in the tank then break down the solids and the remaining liquid flows out into a soak-away. Solids that cannot be broken down must be removed regularly, in order to ensure efficient operation of the tank. This also helps to prevent the system from clogging the soak-away, which would otherwise lead to a sewerage backup.

The primary chamber of the tank is where the main solids are broken down, any non-dissolvable residue remains at the bottom as sludge (indicated by the pink arrow). It is important to empty the tank 
regularly, otherwise the solids build up so much that they can enter the clean water chamber and even the soak-away leading to blockages.

The second chamber is known as the liquids chamber where any finer remaining solids are removed from the liquid. It is important that all outlet and inlet T-pieces are intact, otherwise raw sewage can enter the liquids chamber and potentially the soak-away.

Some systems, such as this one, have an inspection chamber before and/or after the septic tank, where you can check the flow to the tank or out to the soakaway.

It is usual for a bacterial crust to form on top of the liquid waste in the solids (primary) chamber. This is beneficial and will often be left after the emptying process is complete. The entire emptying process usually takes under an hour and for a short time after the tank has been emptied, the liquid level will be very low. However, new waste entering the tank will raise this level back to the normal working level within 5 to 10 days. This can be a confusing aspect of the process and some customers expect the level to remain low. It is the sludge that builds up in the bottom of the tank that must be removed, the high liquid level and dryer top crust are normal in a operational tank.