Filtration
How was sewage treated at the plant?
In the past the plant used three sewage treatment methods: land filtration, grass filtration and lagoon treatment.
Land filtration
The land filtration method dates back to 1897, and was the main sewage treatment method used during summer. In this method, an open paddock was flooded with sewage, up to a depth of 10cm. The land acted as a filter - rubbish and other solids were filtered by the land, while the filtered sewage seeped through the soil, and flowed out at the lower end of the paddock into an earthen drain. The earthen drain then carried treated effluent to Port Phillip Bay.
Nutrients from organic waste were used by the grass and pollutants were broken down by the bacteria in the soil.
The treatment process normally took about three weeks, and happened in cycles. It took about one to two days to flood the paddocks, and a further five days for the paddocks to dry out and for sewage to seep through the soil. Then, sheep and cattle grazed on the paddocks for about two weeks, before the land was flooded with sewage again.
Grass filtration
The grass filtration method was adopted in the 1930s as the main winter treatment method. There were two stages in this process:
- Pre-treatment, and
- Filtration in grass paddocks.
Large rubbish was first removed from the water in large concrete tanks through the process of sedimentation. Lighter rubbish floated to the top of the tank, while heavier rubbish sank to the bottom of the tank, leaving a middle layer of water called primary treated sewage.
The primary treated sewage then slowly flowed over sloping bays planted with a type of grass that was tolerant to continuous flooding. As sewage trickled through the grass, all other solids were filtered out. Pollutants in the water were removed by a film of bacteria which were present on the grass and in the soil. At the end of the bay, the filtered sewage flowed out into earthern drains which took the treated effluent to Port Phillip Bay.
Lagoon treatment
The first treatment lagoon was constructed in 1936. Lagoon treatment facilities have been continuously upgraded to meet the needs of Melbourne’s growing population. The first large, modern lagoon was installed in 1986.
Today all sewage at the Western Treatment Plant is treated in modern lagoons, replacing old lagoons and traditional land and grass filtration methods. The new methods remove large amounts of nitrogen, which would otherwise enter the bay, and generate high quality recycled water, which is a valuable resource for onsite and offsite use.