Perth
9.9 Discharge from urban water system

This line item volume (62,504 ML) represents the total volume of water discharged from the urban water system to surface water within the Perth region during the 2011–12 year.

This volume is comprised of two components:

• Discharge to surface water from the urban water supply system (62,232 ML)
• Discharge to surface water from the wastewater system (272 ML).

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

• internal water transfers (60,232 ML)
• riparian releases (2,000 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 2011–12 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 water storages and water mains within Perth's urban water supply system into rivers. For more information on riparian releases, refer to the Environmental benefit note.

Total volume of riparian releases made during the 2011–12 year at each environmental release point are given in the following table.

The volume of treated wastewater discharged from wastewater treatment plants into rivers during the 2011–12 year is given in the following table.

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, Waroona 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 (Guidelines for water meter installation 2009).

In the 2011 Account, riparian releases from the urban water system were not included in the total volume of water discharged to surface water. These releases should have been included; this was an error in the 2011 Account.

In the 2012 Account, riparian releases are included in the total volume of water discharged from the urban water system to the surface water store. Consequently, the comparative year value has been restated in the water accounting statements from 72,278 ML (as published in the 2011 Account) to 73,764 ML.