Secondary Treatment and Aerated Wastewater Treatment Systems

Secondary Treatment Systems (STS) and Aerated Wastewater Treatment Systems (AWTS) are self-contained wastewater (sewage) treatment systems. 

Secondary treatment system is a wastewater treatment system which produces treated wastewater of secondary standard, i.e. 20 mg/L of Biochemical Oxygen Demand (BOD), 30 mg/L of Total suspended solids (TSS) and 10 cfu/100 mL of Escherichia (E) coli. In order to be used in WA the system must be certified to AS/NZ 1546.3:2017.

Aerated Wastewater Treatment System is a system which uses the processes of aeration followed by clarification to achieve biological treatment of wastewater.

They are used on properties that cannot be connected to the reticulated sewerage. In some instances the Government Sewerage Policy 2019 requires installation of STS but not AWTS. For more details please refer to the Government Sewerage Policy 2019.

These systems are designed to receive and treat all household wastewater from the:

  • toilet
  • bathroom
  • kitchen
  • laundry.

There are several STS and AWTS designs approved for use in Western Australia. An Application to Construct or Install an Apparatus for the Treatment of Sewage (Word 303KB) in accordance with the Health (Treatment of Sewage and Disposal of Effluent and Liquid Waste) Regulations 1974 (external site) must be lodged every time you intend to construct and install an apparatus for on-site wastewater disposal. For more details go here.

They can also be scaled up in size and used in multi-residential and non-residential developments. Larger systems require individual assessment and approval of their technical design. 

The systems

STS and AWTS use biological processes to treat the wastewater.

STS and AWTS typically consist of a series of treatment chambers combined with an irrigation or subsoil disposal systems.

The first chamber is similar to a primary treatment system in that the wastewater enters the chamber and the solids settle to the bottom.

In this chamber they undergo anaerobic digestion by bacteria and form a layer of sludge on the chamber floor.

Scum, consisting of oils, grease, etc. floats to the surface of the liquid and the partially clarified wastewater flows into a second chamber.

Here the liquid is mixed with air to assist bacteria to break down more of the finer suspended organic material.

A third chamber allows additional clarification through the settling of suspended solids which are returned to the first chamber.

The clarified effluent in the third chamber is of a high enough standard for it to be successfully disinfected, usually by chlorination, in a fourth chamber.

Disinfection reduces the number of bacteria in the final effluent. 

The above described treatment process is an example only, the treatment designs are differ from system to system and achieve different water quality.

Therefore, it is important not to dispose of any chemicals, medicines such as antibiotics or household disinfectants down the toilet or sink. These may be detrimental to the biological performance of the system.

STS and AWTS servicing requirements

STS and AWTS are electrically driven mechanical systems.

They require regular servicing to keep them running at their optimum performance.

When STS and AWTS fail to perform, inadequately treated sewage may be discharged from the system and be:

  • offensive
  • a risk to public health.

Under health legislation, STS and AWTS are therefore required to be serviced at every 3 months or as approved by Chief Health Officer (CHO).

Servicing can only be carried out by a person who has approval from the CHO to service STS and AWTS. The list of authorised service person is located here.To obtain authorisation from the CHO to service STSs, refer to the Becoming an authorised service technician for domestic on-site wastewater treatment systems document.


Disposing of the effluent

In WA, STS and AWTS that incorporate a disinfection mechanism are permitted to irrigate treated effluent into a dedicated irrigation area.

Irrigation systems can use either above ground spray heads or subsurface or subsoil dripper lines.

While STS and AWTS effluent most of the time has undergone disinfection and is treated to a higher standard than primary treated effluent, it can still contain pathogenic microorganisms.

Therefore STS and AWTS effluent:

  • is unsuitable for human contact or consumption
  • must only be used on a dedicated irrigation area
  • must not be surface irrigated on areas such as lawns or areas used for recreational purposes unless approved by the CHO
  • is unsuitable for irrigation onto vegetable gardens or food crops.

Surface irrigation of effluent may not be permitted in some developments, such as:

  • childcare centres
  • shopping centres.

This is because areas dedicated for the use of effluent may still be accessed.

In such cases dripper irrigation or below ground disposal may be alternative options. Permission to irrigate grassed areas is generally given when subsoil dripper irrigation systems are used.

The irrigation area

If the effluent is used for irrigation, the irrigation area (size and location) forms part of the approved wastewater system.

It is illegal for owners to modify the location or size of the approved irrigation area without approval from either the local government or the CHO.

Irrigation areas must:

  • have warning signs displayed advising that effluent is being used and is not suitable for human contact or consumption
  • achieve minimum setbacks from certain features such as buildings, boundaries and paved areas, to avoid human exposure to the effluent
  • not be altered or reduced in size without approval
  • be looked after so that irrigation lines are accessible for maintenance
  • be planted out with salt and nutrient resistant plants to avoid puddling or run off of effluent.

Nutrient removal capability of STS and AWTS

Due to environmental concerns associated with the degradation of environmental water bodies, some developments are required to use a wastewater system that is capable of removing nutrients.

STS and AWTS systems can be capable of removing nutrients, depends on the design of the system. Nutrient removal capability of the system must be stated on the system certification and approval.

How does an STS and AWTS remove nutrients?

STS and AWTS can achieve nutrient removal in 1 of 2 ways:

  • In-system nutrient removal is where nutrients within the system can be precipitated into the accumulated sludge.
  • Use of a specially approved amended soil mix which is incorporated in the disposal area.

It is important to know which nutrient removal method is used when selecting a system because that choice will generally limit the disposal options available for the particular system.

In some locations the natural soils are able to bind nutrients. In this case nutrient removal systems are not necessary.

Most analytical laboratories have tests available to provide information about the soil on your property.

Costs associated with using STS and AWTS

The costs associated with installation of these systems include:

  • application fee to local government or CHO
  • purchase and installation cost of the STS and AWTS
  • 3 monthly service (or equivalent) charges for the life of the system
  • cost of chlorine tablets and other parts
  • cost of periodically desludging the system
  • replacement cost of components such as air and effluent pumps in the event of a breakdown.

Approval to install STS and AWTS

Approval to install or modify a wastewater system must be obtained either from local government or the CHO, depending on the particulars of the proposal.

This includes modifications such as changing the location of the land application area (disposal area).

Application forms are available here.

Please note it is an offence under the Health Act to install or modify a wastewater system without the required approval.

More information

  • Water Unit, Environmental Health Directorate
    Department of Health
    Telephone: 9222 2000
Last reviewed: 01-11-2022
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