Melbourne
19.1 Evaporation from urban water system

Supporting information

a. Evaporation from urban water system

This volume (17,031 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 344
Melbourne Water Melbourne Water Western Treatment Plant 15,774
Melbourne Water Eastern Treatment Plant
Western Water 913
Total 17,031

b. Wastewater system overview

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

  • urban water supply system
  • wastewater system
  • recycled water system.

In the Melbourne region, wastewater is collected (line item 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 delivered to one of the major wastewater treatment plants: Melbourne Water Eastern Treatment Plant or Melbourne Water Western Treatment Plant. During the 2012–13 year, Melbourne Water received and treated almost 90% of the wastewater collected within the region.

During the 2012–13 year the majority 75% (line item 19.7) of wastewater system outflows was treated wastewater discharged to sea; approximately 17% was recycled (an increase from 11% in 2011–12); discharge to surface water was < 3% (line item 19.5 ), evaporative losses and other accounted for about 5% (line item 19.1).

The following figure shows all the inflows and outflows associated with the wastewater system. The small misbalance shown (< 1%) 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 an asterick (*) indicate flows between subcomponents 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 2012–13 year; line item numbers are provided in brackets
Schematic diagram of water inflows and outflows for Melbourne's wastewater system during the 2012–13 year; line item numbers are provided in brackets

Quantification approach

City West Water

Data source

Supervisory Control and Data Acquisition (SCADA) database.

Provided by

City West Water.

Method

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. Meters located at the Altona WTP are read on a daily basis.
  • The volume of water discharged to evaporation lagoons for maintenance.

Assumptions, limitations, caveats and approximations

Nil.

Uncertainty information

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

Melbourne Water

Data source

Supervisory Control and Data Acquisition (SCADA) databases.

Provided by

Melbourne Water.

Method

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

Assumptions, limitations, caveats and approximations

The Melbourne Water Western Treatment Plant logs data daily and reports quarterly. The Melbourne Water Eastern Treatment Plant did not log the data in the 2012–13 year.

Uncertainty information

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

Western Water

Data source

Information Technology database–Annual plant performance data.

Provided by

Western Water.

Method

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.

Assumptions, limitations, caveats and approximations

The volume reported is an estimate only.

Uncertainty information

+/– 5% in accordance with manufacturer's annual bulk meter test.