3.2 Wastewater system
The volume presented in the water accounting statements (25,202 ML) represents the volume of wastewater stored in Melbourne Water's Western Treatment Plant and Eastern Treatment Plant. There are a number of other smaller wastewater treatment plants operated by the urban retailers in the Melbourne region; however, their volumes were not included as they were not considered significant or material to the 2012 Account.
The breakdown of the volume of wastewater storage at the Western Treatment Plant and Eastern Treatment Plant is shown in the following table.
|Treatment plant||Component||Volume at 30 June 2011 (ML)
||Volume at 30 June 2012 (ML)
|Western Treatment Plant||115 east lagoon||4,055||4,055|
|55 east lagoon||5,805||5,805|
|25 west lagoon||5,935||5,935|
|85 west C lagoon||931||931|
|85 west A lagoon||459||459|
|85 west B lagoon||736||736|
|145 west A lagoon||829||829|
|145 west B lagoon||1,065||1,065|
|Lake Borrie north lagoon||1,139||1,139|
|Lake Borrie south lagoon||1,153||1,153|
|Western Treatment Plant subtotal||23,576||23,576|
|Eastern Treatment Plant||Volume in storage undergoing treatment||310||580|
|Volume stored in effluent holding basins||333||1,046|
|Eastern Treatment Plant subtotal||643||1,626|
Wastewater system storage was calculated using the following methods:
An estimate of wastewater storage volume was based on surveys conducted by Melbourne Water between 1980 and 2000 to determine the approximate volume of each lagoon at the Western Treatment Plant. The total storage volume at the Western Treatment Plant was the sum of the individual lagoon volumes.
An estimate of wastewater storage volume was determined using level gauging in effluent-holding basins. Levels were interpreted as volumes using stage–storage relationships. Stage–storage relationships were determined from surveys of the effluent holding basins. Effluent-holding basins at the Eastern Treatment Plant that held wastewater at the time of measurement were Forebay, SEHD and EHB6.
Assumptions, limitations, caveats and approximations
- The total storage volume for the Western Treatment Plant is assumed to be the same for 1 July 2011 and 30 June 2012.
- The calculation of wastewater storage was based on surveys conducted between 1980 and 2000. Since the surveys were completed, the volume of sludge in the process has varied considerably, and there have been significant changes in the operation of the lagoons that undertake the bulk of the wastewater treatment.
- The content of liquid in aeration tanks was approximately 50% primary effluent and 50% activated sludge.
- Raw sewage, and primary and secondary effluent channel volumes have not been included as the volumes are insignificant.
The uncertainty estimate for the Western Treatment Plant volumes was +/– 20%.
The uncertainty estimate for the Eastern Treatment Plant volumes was not quantified.