Perth
19.5 Discharge from urban water system to surface water

This line item volume (72,508 ML) represents the total volume of water discharged from the urban water system to surface water within the Perth region during the 2012–13 year.

This volume is comprised of two components:

• discharge to surface water from the urban water supply system (72,256 ML)
• discharge to surface water from the wastewater system (252 ML).

Perth's urban water system comprises three subcomponents:

• urban water supply system
• wastewater system
• recycled water system.

Approximately 99.7% of the urban water discharged to surface water during the 2012–13 year came from the urban water supply system (72,256 ML).

During the 2012–13 year, approximately 69% of the urban water supply system outflows was water supply to urban users (part of 19.4 Delivery to urban water system users). Approximately 22% of the urban water supply system outflows was transferred back to surface water storages (72,256 ML). Less than 1% of the urban water supply system outflows was transferred to Western Australia's southwest region (19.11 Transfer of water outside of region); the remaining outflows was lost through leakage to groundwater (19.3 Urban water system leakage to groundwater) and operational losses (19.20 Other urban water system decreases).

The following figure shows all the inflows and outflows associated with the urban water supply system. For more detail on a particular flow associated with the urban water supply system, refer to the line item notes.

Schematic diagram of water inflows and outflows during the 2012–13 year for Perth's urban water supply system; line item numbers are provided in brackets

There are two types of discharges to surface water from the urban water supply system:

• internal water transfers (70,877 ML)
• riparian releases (1,379 ML). 

Internal water transfers from the urban water supply system are a combination of surface water, groundwater and desalinated water. These water transfers occur in order to store winter production of desalination water and groundwater and to balance the storages to prevent overflow and enable summer peak supply capacity.

Volume of water transferred to surface water storages from the urban water supply system during the 2012–13 year is given in the following table.

¹ Water transfer is direct to Victoria Reservoir and via Bickley Pumpback Reservoir.

Riparian releases are made from Perth's urban water supply system into rivers for environmental purposes. For more information on riparian releases, refer to the Environmental benefit note.

Total volume of riparian releases made during the 2012–13 year at each environmental release point are given in the following table.

Approximately 0.3% of the urban water discharged to surface water during the 2012–13 year came from the wastewater system (252 ML).

During the 2012–13 year, almost 8% of treated wastewater was recycled; 88% discharged to the sea (19.7 Wastewater discharge outside of region); and the rest discharged to rivers (252 ML) or infiltration lagoons (19.8 Other wastewater discharge). The following figure shows all the inflows and outflows associated with the wastewater system.

For more detail on a particular flow associated with the recycled water system, refer to the line item notes. Volumes in the figure annotated with an asterisk (*) 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 during the 2012–13 year for Perth's wastewater system; line item numbers are provided in brackets

The volume of treated wastewater discharged from wastewater treatment plants into rivers during the 2012–13 year is given in the following table.

Operational Data Storage System database and the Water Corporation Annual Report 2013.

The volume of water transferred from the urban water system to surface water storages is based on measured inflow data at the storages.

The volume of riparian releases from the urban water system are based on measured discharge data at the environmental releases points.

The volume of water discharged to rivers from the Williams and Harvey treatment plants are based on measured discharge data collected by flow meters installed at the sites.

Expected error for water meters is +/– 5%.

The Department of Water requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c).