How septic tanks work

A septic tank will take the wastewater from your property’s toilets, sinks, baths and showers, dishwashers and washing machines.

The waste water leaves your property through the drainage pipes to your septic tank, and will usually pass through a number of inspection chambers or manholes.

There are a number of different types of septic tanks, but the most common types are constructed of brick or Glass Reinforced Plastic (GRP).

Brick-built septic tanks and are typically comprised of two chambers. Once the waste water reaches the level of the outlet pipe in the first chamber, it will overflow into the second chamber.

Within the septic tank are dip pipes or T-pipes (depending upon the age of the tank) which are attached to the inlet and outlet pipes of the tank. The dip pipes play a crucial role in retaining the solids from the wastewater within the tank, and preventing them entering into the drainage field.

A septic tank does not provide any treatment of the waste water, instead the solids in the water break down and separate into 3 distinct zones or layers:

• The top layer is made up of less dense matter – fats, oils and solids that have not yet broken down, also known as the crust.
• The second layer is mostly wastewater without any solids remaining, and it is this water which passes out of the septic tank and into the soakaway system.
• The bottom layer is known as sludge, and consists largely of more dense waste that builds up slowly over time. This and the top layer of crust is what must be removed during a routine septic tank emptying.

This separation process will repeat itself depending on how many chambers your septic tank has. The liquid leaves the tank and makes its way into the soakaway system, where it should safely percolate.

GRP septic tanks work in much the same way as brick-built tanks, although they are designed differently. In the same way as a brick-built tank, a GRP tank does not provide treatment of the wastewater, instead it allows it to separate into the same distinct layers. Whereas within a brick-built septic tank, the dip pipes (or T-pipes) prevent solid waste particles from leaving the tank, in a GRP septic tank this role is performed by a baffle.

A septic tank doesn’t work in isolation. In order to perform its function, it must connect to a drainage field or soakaway system.

A drainage field is a network of pipes which follow on from a septic tank or sewage treatment plant. A soakaway system can be a rubble pit or deep bore soakaway - but please be aware that these types of soakaways are no longer compliant with current regulations, which set down that a septic tank must discharge to a drainage field.

You can find out more about septic tank regulations in our guide here.

The purpose of a drainage field is to allow the liquid waste that has left the septic tank to percolate through the soils surrounding the pipework, and the pipework is typically perforated or slotted to enable this to happen.

The process of the liquid waste percolating through the particles in the soil provides a form of treatment of the waste, and it can then safely disperse into the surrounding sub soils.