Proposed Beenyup wastewater ocean outlet duplication into Marmion Marine Park

The proposal, by the Water Authority of Western Australia, consists of a preferred option of constructing a 1420 mm diameter pipeline from the Water Authority of Western Australia's property adjacent to the Ocean Reef Boat Harbour, out to sea for a distance of 1800 m. This proposal will increase the capacity to dispose of treated wastewater from the current level of 54 Megalitres per day (ML/day) to a maximum of 150 ML/day. The secondary treated effluent will be discharged into the ocean via some 50 individual ports (~125 mm diameter) located laterally along the last 200 m of the pipe. This section of the pipeline is known as the diffuser. At this point the water depth is about 10 m and as the effluent rises to the surface from each port, it mixes with the surrounding seawater. The initial dilution is estimated to be approximately 50 times and further dilution is achieved by dispersion through wind driven currents. Bacterial concentrations are also reduced by die-off due mainly to the action of ultraviolet light. The proposed outlet is required by the winter of 1992 and this implies a launch date of March 1992, on the basis of a bottom-pull method of construction.