Page 310 - Week 01 - Thursday, 10 February 2022

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(2) Toxicity is assessed by concentration. The partially treated volume discharged equated to approximately 0.3 percent of the flows in the Molonglo River, and 0.1 percent of the flows in the Murrumbidgee River at that point in time. This proportion of total river flow is an insufficient concentration to have a toxic effect.

However, Icon Water provided advice to downstream residents to avoid drawing water from the river for drinking, food preparation, caring for animals, swimming in or having other bodily contact water sports in the Murrumbidgee River for the days following.

For context, the three progressive stages of the treatment process remove solids, nutrients then microbes. All of the 20ML released had all coarse solid materials larger than 3mm removed by screening before entering the dam. Of the 20ML volume, 16ML had received full primary treatment removing the majority of particulate sewage components from the effluent.

(3) Icon Water’s Lower Molonglo Water Quality Control Centre (LMWQCC) is designed in such a way that it can accommodate surplus flows during a prolonged or intense wet weather event.

A key control is a bypass storage dam which is typically kept empty in case of a need to store excess flow, which can then be progressively recirculated within the treatment process. This storage facility operated as designed during the event and resulted in a controlled discharge once its capacity was exceeded and consistent with operating licence requirements. As such, partially treated sewage did not leak from LMWQCC.

In advance of the event, Icon Water planned and prepared in such a way as to maximise the treatment capacity of the Plant. All activities were managed and operated in accordance with the Icon Water Environmental Management Plan, a requirement of its licence. Additional management controls were applied through diversion and storage of some flows through the sewerage network (essentially delaying portions of the peak inflow to the Plant) in combination with using the onsite storage capacity of the bypass dam mentioned above.

(4) There are many safeguards and degrees of redundancy that are deployed both within Icon Water’s management of upstream network activities, and available treatment pathways within the plant itself (as described in the response to Question 3).

Icon Water successfully used existing controls to effectively minimise the release of partially treated effluent into the environment. The incident was exacerbated by the limited capacity of the catchment to absorb excess rainfall. Due to the sustained above average rainfall experienced during Autumn and Winter and increased soil moisture content, a greater than normal amount of stormwater entered the sewer network. This stormwater infiltration into the network exceeded the plant’s capacity.


a) Wildlife (terrestrial)

Only individual animals caught in the direct flow from the overflow pond are likely to have been impacted by the overflow event. However, it is likely that terrestrial animals would have already moved out of the spillway during the significant rain event occurring at that time.

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