Septic Tank FAQs

The purpose of a septic tank is to treat waste water from your property which is generally not connected to the public waste water system. It is usually either a large rectangular box made of brick, stone or concrete, or a bottle-shaped plastic tank buried underground not far from the property it serves.

A septic tank works like a simple waste water treatment works and the treated waste water drains from the septic tank's outlet pipe to a soakaway.

Waste material (sludge) is allowed to settle in the tank and is digested by natural bacteria breeding in the tank. Over time this sludge builds up on the bottom of the tank. This sludge has to be removed regularly to ensure that the tank continues to work properly and to prevent the soakaway becoming choked.

If you own or use a septic tank, the quality of the discharge and its impact on the environment are your responsibility. You may be held accountable for any pollution caused by your septic tank. This area is regulated by SEPA who will give you advice if required.

You are also responsible for ensuring that:

  • Your septic tank is properly maintained
  • Your septic tank access lids are secure and in good working order.
  • Any drains to and from the septic tank, including the soakaway, are free-flowing and free from blockages.

If your septic tank isn't maintained properly they can fail and cause flooding and foul odours as well as health risks and pollution of water supplies - such as nearby wells.

For more information see our Septic Tank pages

De-sludging should normally take place every twelve months. However, experience has shown that depending on the tank size and usage, this period may be extended, but it is not normally beyond two years.

If you know where the septic tank outlet drains to, check that the discharge is a light grey colour. If the liquid includes dark solid material or recognisable sewage solids, this shows that the septic tank needs de-sludging. For more information go to our Septic Tank pages


If you dispose of certain substances and materials down the toilet, or down sinks, you risk either blocking a drain or upsetting the natural balance within the septic tank. This will stop it working properly and could lead to serious problems of pollution or choking of the soakaway disposal system.

For more information see our Looking after your septic tank page and our Cycle Campaign pages.