Types of Onsite Sewage Management Systems

There are a number of different on-site sewage management systems available. Each user and site will have different factors which will determine the type of system most suitable for installation. For more information on installation of OSSMs click here.


These are the most common types of systems. Effluent is settled in a septic tank and then disposed of into trenches or beds. The septic tank pre-treats the wastewater before it goes to the land application system. Three main things happen in the septic tank:

  • Solids settle to the bottom of the tank and form a layer of sludge.
  • Lighter wastes such as fat and grease float to the surface and form a scum layer.
  • Bacteria, which live in the septic tank, help break down the solid wastes and reduce the volume of sludge collecting in the bottom of the tank.

It is important to remember that:

  • Septic tanks do not remove nutrients.
  • The wastewater is not disinfected and because it is highly infectious it must be applied to land below ground level where bacteria can digest the pollutants and pathogens.
  • A poorly operated and maintained septic tank will allow excessive solids to pass to the land application system causing it to clog. This will result in health hazards and a need for expensive reinstatement of the absorption area.
  • De-sludging of the septic tank is required every 3-5 years depending on use.


Aerated wastewater treatment systems (AWTS) increase the level of treatment of the effluent by aeration and disinfection, so it is suitable for irrigation onto lawns or gardens.

It is important to remember that:

  • Although effluent produce is of a higher quality than septic tank effluent, most systems do not significantly reduce nutrient levels. Therefore an appropriately designed and maintained effluent disposal site is necessary.
  • AWTS must be operated continuously; power to the system must not be turned off. If AWTS are used at irregular intervals the system might need to be serviced before use.
  • Servicing is required every 3-4 months by a Council approved service provider. This cost must be considered when choosing this system.


Sand filters further treat effluent from a septic tank, effluent quality is comparable with that from AWTS. Effluent is pumped through a bed of coarse sand, collected and disposed in shallow subsurface irrigation in lawns or gardens. Sand filters are generally low maintenance systems and can handle surge loads. They may be used to extend the life of the land application system.


Effluent is treated in a septic tank prior to a reed bed. The reed bed further treats the effluent through settlement and nutrient removal. Treated effluent is then distributed into subsurface irrigation a minimum of 300mm below the ground surface. Reed beds can be incorporated into landscaping. Effluent is contained beneath the rock surface of the reed bed. Reeds require some annual pruning / removal.

The Office of Local Government (OLG) has produced the Easy Septic Guide which provides information about the different types of OSSM systems available and gives advice and tips on how best to operate an OSSM system.