Melbourne
19.1 Evaporation from urban water system

This volume (18,622 ML) reported represents the estimated losses through evaporation at wastewater treatment plants. Evaporation from the wastewater system is itemised by water authorities responsible for wastewater treatment in the following table.

Evaporative losses from the wastewater system

Water authority		Volume (ML)
City West Water		699
Melbourne Water	Western Treatment Plant	10,943
	Eastern Treatment Plant	6,760
Western Water		220
Total		18,622

The wastewater system is one of three sub-components of Melbourne's urban water system. The three sub-components are the:

• Urban water supply system
• Wastewater system
• Recycled water system.

In the Melbourne region, wastewater is collected (11.4) from users by the four retail water authorities within the region:

• City West Water
• South East Water
• Western Water
• Yarra Valley Water.

Following collection, wastewater is either delivered to smaller, local wastewater treatment plants operated by the authorities, or, more commonly, delivered to one of Melbourne Water's major wastewater treatment plants: the Eastern Treatment Plant and Western Treatment Plant. During the 2011–12 year Melbourne Water received and treated almost 90% of the wastewater collected within the region. The breakdown of the volumes of wastewater received by Melbourne Water's treatment plants and the authority's smaller treatment plants is presented in the following table.

Wastewater inflow for treatment received by Melbourne Water and the retail water authority's treatment plants

Water authority treatment plants		Volume (ML)
Melbourne Water	Western Treatment Plant	180,840
	Eastern Treatment Plant1	139,226
City West Water		5,474
South East Water		13,126
Western Water		7,966
Yarra Valley Water		10,560
Total		357,192

¹ Includes 1,623 ML of treated wastewater received from Pakenham Sewage Treatment Plant.

During the 2011–12 year the majority (85%: 19.7) of treated wastewater was discharged to sea; approximately 11% was recycled and the remaining portion was discharged to surface water (3%: 19.5 ) and the landscape (<1%: 19.8).

The following figure shows all the inflows and outflows associated with the wastewater system. A small misbalance (> 1%) is shown. The misbalance can be attributed to metering inaccuracies, unaccounted losses and inaccuracies in the estimation of the recycled water on-site in process use.

For more detail on a particular flow associated with the wastewater system, refer to the line item notes. Note that volumes annotated with a * indicate flows between sub-components and are not reported in the account. As such, these flows do not have an associated line item number.

Schematic diagram of water inflows and outflows for Melbourne's wastewater system during the 2011–12 year. Line item numbers are provided in brackets.

Supervisory Control and Data Acquisition (SCADA) database.

City West Water.

The volume of water lost through evaporation is estimated as the total of the following components:

• evaporative losses were calculated for the Altona Wastewater Treatment Plant (WTP) by deducting the metered volume of water at the inflow point to the metered volume immediately preceding UV treatment (226 ML). Meters located at the Altona WTP are read on a daily basis.
  
• the volume of water discharged to evaporation lagoons for maintenance (473 ML).

Nil.

The uncertainty is estimated to be +/– 5%.

Supervisory Control and Data Acquisition (SCADA) databases.

Melbourne Water.

This value is estimated by deducting the volume accounted for at outlets from the inlet volume. The difference in the volume between the inlet and outlet is losses, mostly due to evaporation; losses are estimated by back calculation.

Both Western Treatment Plant and Eastern Treatment Plant log data daily and reports quarterly.

The accuracy for this form of data is assumed to be +/– 10%.

IT database (Annual Plant Performance Data).

Western Water.

This value is estimated by calculation of the evaporation in comparison with rainfall against the surface area of all lagoons. Additionally a seepage factor is also applied.

The volume reported is an estimate only.

The uncertainty estimate was not quantified.

In the 2011 Account evaporative losses from the urban water system were not reported.

In the 2012 Account, due an improved understanding of the urban water system balance, evaporative losses from the urban water system were reported at line item 19.1 'Evaporation from urban water system'.

Consequently, the volume has been restated as 4,141 ML. Notably, this volume only includes evaporative losses from Melbourne Water's Western Treatment Plant. Comparative year data for evaporative losses was not available for Melbourne Water's Eastern Treatment Plant, City West Water and Western Water.