Melbourne
3.2 Wastewater system

Supporting information

The volume presented in the water accounting statements (29,132 ML) represents the estimated volume of wastewater stored in Melbourne Water's western and eastern treatment plants. 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 2013 Account.

The breakdown of the estimated volume of wastewater storage at the western and eastern treatment plants is shown in the following table.

Wastewater storage

Treatment plant

Component

Volume at 30 June 2013 (ML)

Volume at 30 June 2012 (ML)

 

115 east lagoon

4,373

4,055

 

55 east lagoon

6,138

5,805

 

25 west lagoon

6,201

5,935

 

85 west C lagoon

973

931

 

85 west A lagoon

483

459

 

85 west B lagoon

845

736

Western Treatment Plant

145 west A lagoon

1,133

829

 

145 west B lagoon

1,430

1,065

 

Walsh's lagoon

1,429

1,140

 

Lake Borrie north lagoon

1,782

1,139

 

Lake Borrie south lagoon

1,875

1,153

 

Western lagoon

540

166

 

T-section lagoon

523

163

Western Treatment Plant subtotal

 

27,725

23,576

Eastern Treatment Plant

Volume in storage undergoing treatment

409

580

 

Volume stored in effluent holding basins

998

1046

Eastern Treatment Plant subtotal

 

1,407

1626

Total

 

29,132

25,202

Quantification approach

Data source

Melbourne Water Western Treatment Plant and Melbourne Water Eastern Treatment Plant records.

Provided by

Melbourne Water.

Method

Wastewater system storage was calculated using the following methods:

Melbourne Water Western Treatment Plant

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 this plant was the sum of the individual lagoon volumes.

Melbourne Water Eastern Treatment Plant

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

Melbourne Water Western Treatment Plant
  • 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.
Melbourne Water Eastern Treatment Plant
  • 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.

Uncertainty information

The uncertainty estimate for the Melbourne Water Western Treatment Plant volumes was +/– 20%.
The uncertainty estimate for the Melbourne Water Eastern Treatment Plant volumes was not quantified.