Septic Tank Systems

Oct 10, 2022, 15:53 PM
Title : Septic Tank Systems

What are septic tank systems and how do they work?

Most unsewered developments in Western Australia use septic tank systems to treat and dispose of sewage. These generally consist of two water tight tanks (or one large unit) and two sets of drainage receptacle e.g. leach drains or soak wells.

When wastewater passes through the septic tanks, heavier solids sink to the bottom and undergo bacterial digestion. This reduces the quantity of solids and also changes its composition to sludge, which builds up in the bottom of the tank.

Materials such as grease and oil float to the surface in the tanks to form a crust over the liquid. The remaining liquid, called effluent, flows from the tanks into the drainage receptacles to soak into the surrounding soil where it may undergo further natural treatment processes.

What is the process for installing a septic tank system?

System installations are only permitted with local government or Department of Health approval. The Department of Health assesses larger development proposals.

 

If you wish to install septic tank systems with leach drains/soakwells or an Aerobic Treatment Unit you will need to lodge an Application to Construct or Install an Apparatus for the Treatment of Sewage with the relevant Local Government Authority.

This is to ensure that the system is of an approved design and manufacture, is properly sized and is suitably located.

It is an offence to install a wastewater system without an approval and it is also an offence to commission a system prior to a final inspection and approval to use from the local government.

Onsite treatment and disposal of effluent systems will not normally be approved if a property can be reasonably connected to sewer. As they are designed to retain the solids in household wastewater, septic tank systems require routine maintenance.

For more information about septic tank systems please read the Department of Health's information about Understanding Septic Tank Systems.

What is the process for decommissioning septic tank?

When a developed property is connected to sewer, usually under the infill sewerage program, any onsite effluent disposal systems such as septic tank and leach drain/soakwell systems will eventually require decommissioning.

This means that the contents of the tank are pumped out (by an approved contractor) and the system is either removed or backfilled.

If the system can’t be removed, the tanks should still be pumped out and the bottom of the tanks can be broken up and the system backfilled with sand.

As there are costs involved in both decommissioning a septic tank and connecting to sewer, householders are not automatically required to decommission tanks when they connect to the sewer.

Under the Health (Treatment of Sewage and Disposal of Effluent and Liquid Waste) Regulations 1974

decommissioning is required when:

  • The property is sold
  • The use of the development changes e.g. from a residence to a child care centre
  • Building extensions may encroach on minimums setback requirements from the system.

Septic tanks which have not been decommissioned should not be paved or built over without approval from the relevant local government.

Under the Code of Practice for the Reuse of Greywater in Western Australia, disused septic tank systems can be converted into the primary treatment tank for a greywater system.

The existing drainage receptacle (e.g. soakwells or leach drains) must be replaced with an approved greywater system.

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