19.6 Discharge from urban water system to irrigation scheme
a. Discharge from urban water system to irrigation scheme
The volume presented in the water accounting statements (3,089 ML) represents the metered volume of recycled water provided by Melbourne Water's Eastern Treatment Plant to the Eastern Irrigation Scheme.
b. Recycled water system overview
The recycled water 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 treatment plants located within the region produce the majority of recycled water (see 'wastewater system overview' for further information). A very small volume of recycled water (237 ML : 11.21) is received from external organisations to supplement Melbourne's recycled water system.
During the 2011–12 year, approximately 70% of the recycled water produced (including the recycled water received from external organisations) was used on-site at wastewater treatment plants throughout the Melbourne region. Of the recycled water used on-site, approximately half that volume (14,876 ML) was recirculated back into the wastewater system (and therefore not reported in the water accounting statements). The remaining recycled water used on-site was for irrigation/outdoor purposes or in-process and reported at line item 19.4.
The remaining recycled water produced in Melbourne was supplied for use; 8,415 ML was supplied to urban water system users [19.4]; and the remaining volume (3,089 ML) was supplied to either the Eastern Irrigation Scheme or Werribee Irrigation District [19.6].
The following figure presents the inflows and outflows associated with the recycled water system. A small misbalance (1.1%) is shown, due to the difference between the volume of recycled water available for supply and the volume of recycled water supplied for use. 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 recycled water system, refer to the line item notes. Note that volumes annotated with a * indicate flows between urban sub-components and are not reported in the 2012 Account. As such, these flows do not have an associated line item number or note.
Schematic diagram of water inflows and outflows for Melbourne's recycled system during the 2011–12 year. Line item numbers are provided in brackets.
Assumptions, limitations, caveats and approximations
In the 2011 Account, urban water supply to the irrigation scheme was reported as part of line item 19.4 Delivery to urban water system users.
In the 2012 Account, urban water supply to irrigation districts/schemes within the Melbourne region was reported at line item 19.6 'Urban water discharge to irrigation scheme', in order to be consistent with the classification of urban water supply to irrigation schemes applied to the other regions of the National Water Account.
Consequently, the comparative year volume has been restated as 3,457 ML. Notably, this volume includes recycled water provided from Melbourne Water's Western Treatment Plant to Werribee and Bacchus Marsh irrigation districts as well as recycled water provided from Eastern Treatment Plant to the Eastern Irrigation Scheme.