11.7 Stormwater ingress

Supporting information

a. Wastewater system overview

The wastewater system is one of three subcomponents of Sydney's urban water system. The three subcomponents are the:

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

The Sydney region has several urban utilities responsible for managing wastewater in the region. Sydney Water Corporation, Shoalhaven City Council, Goulburn Mulwaree Council, Lithgow City Council, and Wingecarribee Shire Council all collect wastewater from users (11.4 Wastewater collected) and treat this wastewater at a number of wastewater treatment plants throughout the region.

During the 2012–13 year, a total of 507,638 ML was collected by the wastewater system. The majority of the outflow from the wastewater system (around 77%) was discharged to the sea (19.7 Wastewater discharge outside of region); around 10% was discharged to inland surface water (19.5 Urban water discharge to surface water) and a further 10% of that wastewater was recycled and the remainder was losses from the system (see 19.1 Evaporation from urban water system and 19.20 Other urban water decreases). The following figure shows all the inflows and outflows associated with the wastewater system.

For more detail on a particular flow associated with the wastewater system, refer to the line item notes. Note that volumes 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 Sydney's wastewater system; line item numbers are provided in brackets
Schematic diagram of water inflows and outflows during the 2012–13 year for Sydney's wastewater system; line item numbers are provided in brackets

b. Stormwater ingress

In the 2011–12 year stormwater ingress was reported separately by Wingecarribee Shire Council as 1,717 ML. In the 2013 Account, stormwater ingress is included in 11.4 Wastewater collected and therefore 0 ML has been reported here.

Quantification approach

Wingecarribee Shire Council

Data source

Supervisory Control and Data Acquisition (SCADA) data from wastewater treatment plants.

Provided by

Wingecarribee Shire Council.


The value represents the average volume of wastewater measured at the inlet to the wastewater treatment plants on dry days, compared to the average volume on days with rainfall. The difference is assumed to be storm water ingress.

The flow of wastewater from the plants is measured using ABB Magmaster flow meters installed on-site. The data from the flow meters are continually monitored and recorded. External flow verification audits for the flow meters are conducted annually by Hydrometric Consulting Services.

Assumptions, limitations, caveats and approximations

Total wastewater received includes net ingress from stormwater and groundwater. A calculation has been made based on an assumption that the average increase in daily inflow on rainfall days is ingress into the sewerage system.

Uncertainty information

The volume is based on measured and calculated data. Estimated uncertainty based on meter accuracy and professional judgment is +/– 7%.