Melbourne Water / During the primary treatment of sewage, sludge is separated from the sewage before it goes to secondary treatment. This sludge is then treated in an anaerobic environment, where there is no free oxygen.
In this anaerobic environment, micro-organisms such as bacteria break down organic materials in the sludge. This creates biogas including methane and carbon dioxide. At both the Eastern Treatment Plant (ETP) and the Western Treatment Plant (WTP), the biogas is captured to produce electricity for use on the site. By capturing these gases, we reduce the greenhouse gases and odours released into our air. At the ETP, the sludge is anaerobically treated in large tanks called digesters. At the WTP, this treatment occurs in large covered lagoons.
Secondary treatment of sewage / During the secondary treatment of sewage, bacteria are used to reduce nutrients in the wastewater.
This occurs in the activated sludge plant, where different types of bacteria exist in aerobic (with added oxygen) and anoxic (without added oxygen) environments, which are side by side. These micro-organisms break down organic materials and remove ammonia at both the Eastern Treatment Plant and the Western Treatment Plant. Ammonia is first converted to nitrates and finally to nitrogen gas that is released into our air. The water then passes through sedimentation tanks where more sludge is removed and treated secondary effluent is produced.
Tertiary treatment of sewage at the Western Treatment Plant / In the maturation lagoons at the Western Treatment Plant, effluent is disinfected naturally by sunlight.
These lagoons are shallow, allowing UV light from the sun to penetrate the water and remove bacteria and pathogens. Some of the micro-organisms are also reduced as they are eaten by larger organisms. After 30 to 35 days in the lagoon system, the treated effluent is either recycled, sent on for further disinfection or is discharged to Port Phillip Bay, under strict EPA Victoria licence requirements.