Melbourne
11.4 Wastewater collected

The volume presented in the water accounting statements (356,462 ML) represents the volume of wastewater collected within the Melbourne region from retail water authorities City West Water, South East Water, Yarra Valley Water and Western Water during the 2011–12 year.

The value represents the metered wastewater volume collected in the region.  This volume includes wastewater discharged to the sewer after usage and any variation due to (a) ingress of stormwater; (b) infiltration of groundwater; (c) wastewater overflows to stormwater; and (d) exfiltration of wastewater to groundwater.  

Yarra Valley Water estimated that 21,854 ML of stormwater and groundwater entered the wastewater system—components (a) and (b), 15 ML of wastewater overflowed to stormwater from the emergency relief structures of the wastewater system—component (c).

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.

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.

The volume of wastewater collected by City West Water was calculated from the following components:

• wastewater collected from the City West Water service area and transferred to Melbourne Water's Western Treatment Plant, metered by Melbourne Water
• wastewater collected at the Altona Treatment Plant, metered by City West Water
• wastewater taken at the Sunshine Golf Club sewer mining plant, metered by City West Water.

Melbourne Water bulk meters are read daily. The location of the meters and details of the network are contained in the bulk supply agreements for each water authority.  

City West Water meter flows are read daily at the Altona Treatment Plant and Sunshine Golf Club.

Melbourne Water has a suite of bulk meters and formulas in order to attribute volumes to the three retailers. The meter network and locations are contained in a bulk supply agreement with City West Water.

The uncertainty is estimated to be +/– 10% based on meter specifications.

The volume of wastewater collected at Melbourne Water's Western Treatment Plant and Eastern Treatment Plant represents the total metered volume of wastewater received by Melbourne Water. Wastewater is metered at the influent pump stations of both the Eastern and Western treatment plants. These plants have primary inflow metering, as well as additional metering to record the inflows should the primary metering be offline.

• The total volume includes 57 ML/year of wastewater, which is an estimated volume based on historical averages from City West Water that enters the Western Treatment Plant downstream of the billing meter.
• The Western Treatment Plant logs data daily and reports quarterly. The Eastern Treatment Plant logs data daily and reports monthly. Thus, metering periods align with the reporting period.

At the Eastern Treatment Plant, wastewater inflows are measured using magnetic flow meters with an associated uncertainty of +/– 5%.

At the Western Treatment Plant, wastewater inflows are measured using magnetic flow meters with an associated uncertainty of +/– 15%.

The volume of wastewater collected by South East Water was calculated from the following components:

• Wastewater collected from the South East Water service area and transferred to Melbourne Water's Eastern and Western treatment plants, metered by Melbourne Water
  
• Wastewater collected at South East Water's sewage treatment plants, metered by South East Water.

Melbourne Water bulk meters are read daily. The location of the meters and details of the network are contained in the bulk supply agreements for each water authority.

South East Water meters record flows daily at their sewage treatment plants.

Melbourne Water has a suite of bulk meters and formulas in order to attribute volumes to the three retailers. The meter network and locations are contained in a bulk supply agreement with South East Water.

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

The volume of wastewater collected was calculated from metered daily flows at Western Water's recycled treatment plants. Meters are located at the entrance to the plant and record all incoming water. Meters are read daily by the treatment plant operators.

The volume does not include wastewater collected from Woodend.

The volume of wastewater collected by Yarra Valley Water was calculated from the following components:

• Wastewater collected from the Yarra Valley Water service area and transferred to Melbourne Water's Eastern and Western treatment plants, metered by Melbourne Water
• Wastewater collected at Yarra Valley Water's local treatment plants, metered by Yarra Valley Water.

Melbourne Water bulk meters are read daily. The location of the meters and details of the network are contained in the bulk supply agreements for each water authority.

Melbourne Water has a suite of bulk meters and formulas in order to attribute volumes to the three retailers. The meter network and locations are contained in a bulk supply agreement with Yarra Valley Water.

The uncertainty is estimated to be +/– 10% based on meter specifications.