Melbourne
19.1 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

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

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

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 between the treated wastewater volume and treated wastewater outflow is losses, mostly due to evaporation; losses are estimated by back calculation.

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.

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

Information Technology 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.

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