The Western Treatment Plant (formerly the Metropolitan Sewage Farm or, more commonly, the Werribee Sewage Farm) is a 110 km2 (42 sq mi) sewage treatment plant in Cocoroc, Victoria, Australia, 30 km (19 mi) west of Melbourne's central business district, on the coast of Port Phillip Bay.
It was completed in 1897 by the Melbourne and Metropolitan Board of Works (MMBW), and is currently operated by Melbourne Water. The plant's land is bordered by the Werribee River to the east, the Princes Freeway to the north, and Avalon Airport to the west.
The Western Treatment Plant treats around 50% of Melbourne's sewage – about 485 ML per day – and generates almost 40,000 ML of recycled water a year. It is currently the largest wastewater treatment plant in the world by area.