Canberra
11.4 Wastewater collected

The volume presented in the water accounting statements is 35,708 ML that represents the volume of wastewater collected within the Canberra region by ACTEW Water and QCC (Queanbeyan City Council) during the 2012–13 year.

The value represents the metered wastewater volume collected in the region.  This volume includes wastewater discharged to the sewer after usage and any variation due to (a) ingress of stormwater; (b) infiltration of groundwater; (c) wastewater overflows to stormwater; and (d) exfiltration of wastewater to groundwater.

Wastewater is treated at five sewage treatment plants (STPs) in the Canberra region: the Lower Molonglo Water Quality Control Centre (LMWQCC), Fyshwick, Urriara, Outward Bound operated by ACTEW Water, and the Queanbeyan STP operated by QCC. Some wastewater from Fyshwick and surrounding suburbs industrial area is treated at Fyshwick STP and returned to sewer for further treatment at LMWQCC.

The treated wastewater volume discharged to surface water after treatment was 31,636 ML, approximately 77% of total treated wastewater volume, 41,114 ML.

During the 2012–13 year, approximately 12% of treated wastewater was recycled; the rest was the losses. 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. Note that volumes annotated with an * indicate flows between sub-components 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 Canberra's wastewater system. Line item numbers are provided in brackets.

This volume 32,743 ML is calculated from the measured inflows to LMWQCC and Fyshwick sewerage treatment plants (STPs), minus the volume returned from Fyshwick STP to the sewer system for further treatment at LMWQCC, plus the volume of inflows to Uriarra and Outward Bound STPs.

For the 2012 - 13 year, ACTEW Water estimated that the metered wastewater volume at LMWQCC included 1,033 ML of net ingress of stormwater (and groundwater) to the sewer system.

This volume is calculated by comparing calculated annual dry weather inflow to measured annual inflows, with the difference assumed to be ingress to the sewer system. The dry weather inflow volume is calculated by multiplying the volume of inflows to LMWQCC during a dry week by 365/7 to give an annual volume. The dry week used this year was 6-12 March 2013, when inflows were 589.6 ML (last year was 562.8 ML in 1-7 Nov). The larger value this year is due to the drier climate.

The dry weather inflow would be 30,744 ML.

• The estimation of volume of stormwater ingress is based on the assumption that in the dry reference week, there is no net gain or loss of stormwater and/or groundwater into or out of the sewer system.
• Stormwater ingress was estimated only for the inflow to the LMWQCC as the data for other STPs were not available.

• The uncertainty of the volume of wastewater collected is estimated to be of the order of +/–20%.
• The uncertainty in the estimated volume of storm water and / or groundwater ingress is in the order of +/- 50%

The volume of wastewater collected is the metered volume at the Queanbeyan STP.