11.7 Stormwater ingress
a. Sydney wastewater system overview
The wastewater system is one of three sub-components of Sydney's urban water system. The three sub-components are the:
- Urban water supply system
- Wastewater system
- Recycled water system.
In the Sydney region, wastewater is collected from users by the Sydney Water Corporation, Shoalhaven City Council or Wingecarribee Shire Council (11.4) and transferred to wastewater treatment plants throughout the region.There is also some stormwater ingress reported by Wingecarribee Shire Council which is considered a wastewater inflow (11.7).
During the 2011–12 year, 7% of treated wastewater was recycled; 78% discharged to the sea (19.7); and the rest discharged to inland surface water (19.5). 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 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.
The Sydney region's wastewater system using data retrieved from Sydney Water Corporation, Shoalhaven City Council and Wingecarribee Shire Council
b. Stormwater ingress
Stormwater ingress has been reported by Wingecarribee Shire Council as 1,717 ML.
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.
The volume is based on measured and calculated data. Estimated uncertainty based on meter accuracy and professional judgment is +/– 7%.