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Wastewater treatment involves a series of processes that remove pollutants such as solids, oil and grease, detergents, nutrients, heavy metals and bacteria. Across Western Australia wastewater is treated at a variety of treatment plants.

People living in the Perth metropolitan area are served by three major wastewater treatment plants, at Beenyup, Subiaco and Woodman Point, and six smaller ones in outlying areas. These deal with approximately 290 million litres of wastewater per day.  Treatment plants are planned for Alkimos in the northern suburbs and East Rockingham in the southern suburbs.

In the metropolitan area, the two main stages of the treatment process are primary and secondary treatment.

Primary treatment is the process in which wastewater settles, allowing solids to separate, while secondary treatment involves the aerobic, biological stabilisation of the wastewater. In secondary treatment the more soluble material is removed.

The treated wastes produced during the treatment process are liquid effluent and stabilised sludge (biosolids). All biosolids produced in the metropolitan area are composted, or reused directly in broadacre agriculture.

Treated effluent is returned to the environment in a way that is safe for our health and for the environment. Sometimes it can be reused; otherwise it is returned to the water cycle (e.g. by being discharged to the ocean, rivers, streams or land, or through evaporation).

Effluent from the three major wastewater treatment plants in the metropolitan area is disposed of into the ocean via three large ocean outfalls at Ocean Reef (for Beenyup Wastewater Treatment Plant), Swanbourne (for Subiaco Wastewater Treatment Plant) and Sepia Depression (for Woodman Point Wastewater Treatment Plant).

In country areas, wastewater stabilisation ponds are used extensively to treat wastewater. These ponds provide an inexpensive and reliable method of stabilising the pollutants in wastewater. To varying degrees of efficiency, ponds are capable of carrying out the normal treatment processes of sedimentation, biological stabilisation and disinfection. The treated wastewater is then safe to be released back into the environment.

	Wastewater treatment process diagram