3.2 Wastewater collection system

Supporting Information

The volume presented in the water accounting statements (24,219 ML) represents the volume of wastewater held in Melbourne Water's Western Treatment Plant and Eastern Treatment Plant. There are a number of other smaller wastewater treatment plants that operate in the Melbourne region, and their volumes were not included as they were not considered significant or material to the 2011 Account.

The breakdown of the volume of wastewater storage at the Western Treatment Plant and Eastern Treatment Plant are shown in the following table.

Wastewater storage
Treatment plant


Voluem at 30 June 2010 Volume at 30 June 2011
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
Walsh's lagoon 1,140 1,140
Lake Borrie north lagoon 1,139 1,139
Lake Borrie south lagoon 1,153 1,153
Western lagoon 166 166
T-section lagoon 163 163
Western Treatment Plant subtotal 23,576 23,576
Eastern Treatment Plant Volume in storage undergoing treatment 357 310
Volume stored in effluent holding basins 804 333
Eastern Treatment Plant subtotal 1,161 643
Total 24,737 24,219

Quantification Approach

Data Source

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

Provided by

Melbourne Water.


Wastewater collection system storage was calculated using the following methods:

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 Western Treatment Plant. The total storage volume at the Western Treatment Plant was the sum of the individual lagoon volumes.

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

Western Treatment Plant


  • The total storage volume for the Western Treatment Plant is assumed to be the same for 1 July 2010 and 30 June 2011. 
  •  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 magnitude of uncertainty as a result of these changes is approximately 20%.


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

Western Treatment Plant volumes uncertainty +/– 20%
Eastern Treatment Plant volumes are ungraded.

Comparative year

This item corresponds to line item 5.2 Urban wastewater system reported in the 2010 Account. Following publication, an error in the volume reported in the 2010 was identified. Although the change to the volume reported was small, the volume was restated to correct this error and provide comparability between years.

This change and the respective volumes are detailed in the following table. The restated comparative year volume is 24,737 ML.

Restated comparative year information for line item 3.2 Wastewater collection system

2010 Account line item name

Volume at 30 June 2010 reported in 2010 Account (ML)

Value difference to volume reported due to prior period error correction (ML)

Volume at 30 June 2010 reported in 2011 Account (ML)

5.2 Urban wastewater system