There are questions over the capacity of the ACT's wastewater treatment system, after partially treated sewage was repeatedly released into the Murray Darling catchment during recent heavy rainfall.
On three occasions in the past three months, Icon Water had to release wastewater from a holding dam at the Lower Molonglo Water Quality Control Centre as heavy rain pushed it beyond capacity.
The waste was released first in September, then twice in November, into the Molonglo River, which joins the Murrumbidgee River — a main tributary of the Murray Darling system.
Icon Water described it as a "controlled release" of partially treated effluent, to avoid risking the dam's "integrity".
It also prompted multiple warnings to residents downstream to avoid primary contact with the water.
Treatment plant 'overwhelmed'
Icon Water's general manager of Infrastructure Services, Gerard Brierley, said the company was acting within its licence, which allowed it to release some waste during extreme weather events.
"When we saw these discharges in November, they were out of the 'bypass dam' when the rest of the treatment plant couldn't cope.
"2021 has been an incredibly wet year and November of 2021 was the wettest on record.
Amount discharged 'not actually measurable'
Icon said that rainwater was infiltrating the sewerage system through manhole covers, leaking pipes, and illegal plumbing connections — and heavy rain meant a dramatic increase in inflows to the city's treatment plants.
But there was only so much capacity to fully treat the effluent before it had to be released.
The ACT's Environment Protection Authority confirmed that Icon Water's actions were permitted under an "environmental authorisation", which set the conditions for its effluent discharge and monitoring.
It also confirmed that Icon had to notify the EPA and health authorities when wastewater pollution exceeded prescribed limits.
But the EPA was satisfied that "the effect of partially treated effluent is insignificant and not actually measurable in the composition of the river during the high flood flows".
Mr Brierley said the risk was low, but Icon was required by law to notify downstream users.
Climate change and the Canberra population
Professor Fiona Dyer, from the University of Canberra's Centre for Applied Water Science, said there was nevertheless still some potential risk to human health.
"There is the small risk of waterborne pathogens making their way downstream, and so people need to be aware of that," Professor Dyer said.
She said it would be impractical and hugely expensive to build a wastewater treatment works that could cater for extreme weather events.
However, she said a changing climate meant that heavy rainfall from storms would become more frequent, leading to the potential of more wastewater discharges.
Icon Water admitted a growing population was also putting pressure on the capital's main wastewater treatment plant, which was completed in 1978 when Canberra was half its current size.
"We've seen pretty significant population growth in Canberra over the last ten years and we anticipate that that's going to continue," Mr Brierley said.
And Professor Dyer said if Canberrans were concerned about spilling wastewater into the Molonglo, they would need to be prepared to "stump up the cash" to pay for a treatment plant upgrade.
