Ocean outfall

Treated wastewater from our major wastewater treatment plants is returned to the marine environment via ocean outfalls.

We have ocean outfalls in Alkimos, Bunbury, Ocean Reef, Point Peron and Swanbourne.

How does ocean outfall work?

Treated wastewater is diluted through a number of physical and chemical processes. It is lighter and lower in density, so it rises and mixes with seawater, reducing the concentration of wastewater.  

The mixing process is impacted by:  

  • Water depth – greater dilution is achieved in deeper water. 
  • Water conditions - dilution is increased by winds and storms.
  • Outlet pipe diffuser design – ours are designed to maximise dilution of treated wastewater through multiple small diameter ports. These increase the exit velocity of wastewater, which enhances the mixing process.  

Is ocean outfall sustainable?

Yes, it is a sustainable wastewater disposal method.  

We undertake all ocean outfall activities in line with Perth’s Coastal Waters Environmental Management Framework. Developed by the Environment Protection Authority the framework aims to protect our coastal waters from pollution.  

The framework focuses on the following environmental values:

  • ecosystem health
  • fishing and  aquaculture
  • recreation and aesthetics
  • industrial water supply

Who regulates our ocean outfall practices?

Our ocean outfall practices must comply with the conditions of our Operating Licence, which are set by the Department of Environment and Conservation (DEC). These conditions specify the quantity and quality of treated wastewater that can be released to the ocean, as well as our monitoring and reporting requirements.  

To minimise our impact on the ocean environment we conduct 4 ongoing ocean outfall monitoring programs:

Between 1991-1994 we conducted a study to determine the effects of nitrogen, contained in treated wastewater, which is most likely to be utilised by algae and lead to nutrient related environmental impacts, on the marine environment.  

The study found that ocean disposal is unlikely to compromise the environmental values at Ocean Reef and Swanbourne outlets of the Perth Monitoring Program. The effects of ocean disposal at Point Peron outlet were considered minor and not ecologically detrimental.  

Since then, we have also upgraded our Beenyup, Swanbourne and Woodman Point wastewater treatment plants to reduce the volume of nitrogen reaching the marine environment.

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