Melbourne
19.7 Wastewater discharge outside of region

Supporting information

a. Wastewater discharge outside of region

The volume presented in the water accounting statements (268,134 ML) represents the volume of wastewater discharged from water authorities in the Melbourne region to Port Phillip Bay and Bass Strait.

The following table shows the breakdown of wastewater discharged from each treatment plant by authority in the Melbourne region during the 2012–13 year.

Wastewater discharged outside of region
Authority Discharge location 

 Volume (ML)

City West Water Altona sewage treatment plant (STP) discharge to Port Phillip Bay

3,488

City West Water subtotal

3,488

Melbourne Water Melbourne Water Western Treatment Plant discharge to Port Phillip Bay

132,526

Melbourne Water Eastern Treatment Plant discharge to Bass Strait

123,132

Melbourne Water subtotal

255,658

South East Water South East outfall

8,988

South East Water subtotal

8988

Total

268,134

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

Meters are located at the Altona STP and are read on a daily basis. This line item is calculated by deducting the metered volume of water used in-process within the treatment plant from the metered wastewater discharge value.

Assumptions, limitations, caveats and approximations

Nil.

Uncertainty information

The uncertainty estimate was +/– 5%.

Melbourne Water

Data source

ASPEN and Supervisory Control and Data Acquisition (SCADA) databases.

Provided by

Melbourne Water.

Method

The volume is determined using a mass-balance calculation of all wastewater collected, treated, stored, and recycled at the Melbourne Water Western Treatment Plant and the Melbourne Water Eastern Treatment Plant.

Assumptions, limitations, caveats and approximations

Nil.

Uncertainty information

The uncertainty estimate was not quantified.

South East Water

Data source

Internal database.

Provided by

South East Water.

Method

This item represents the daily metered volume of treated wastewater discharged to the sea via the South East outfall.

The volume of treated wastewater discharged to the sea via the South East outfall is metered and recorded on a daily basis.

Assumptions, limitations, caveats and approximations

  • Assumes meters and readings are accurate.
  • This item only refers to wastewater volumes directly managed by South East Water.

Uncertainty information

The uncertainty estimate is +/– 5% in accordance with the Water Industry Regulatory Audits 2013, independent annual audit report.