Melbourne
19.7 Wastewater discharge outside of region

The volume presented in the water accounting statements (299,614 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 2011–12 year.

Wastewater discharged outside of region

Authority	Discharge location	Volume (ML)
Melbourne Water	Western Treatment Plant discharge to Port Phillip Bay	158,297
	Eastern Treatment Plant discharge to Bass Strait	128,658
	Melbourne Water subtotal	286,955
City West Water	Altona Sewage Treatment Plant (STP) discharge to Port Phillip Bay	3,523
	City West Water subtotal	3,523
South East Water	Somers STP discharge to South East Outfall	1,370
	Mt Martha STP discharge to South East Outfall	5,011
	Boneo STP discharge to South East Outfall	2,755
	South East Water subtotal	9,136
Total		299,614

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.

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.

Nil.

The uncertainty estimate was +/– 5%.

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

Melbourne Water.

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

Nil.

The uncertainty estimate was not quantified.

Internal database.

South East Water.

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.

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

The uncertainty estimate is +/– 5% in accordance with the Water Industry Regulatory Audits 2012, independent annual audit report.