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Swimming is now safe again on most of Hobart's beaches — in time for Christmas — after the sewage treatment plant at Cameron Bay was brought back online.
Over the weekend, TasWater "trucked in" new sewage treatment bacteria, which had been killed off by "high strength trade waste" from the Cadbury chocolate factory last week.
The new bacteria came from Blackmans Bay and is used to chemically break down and treat the sewage to make it safe before it is released into the River Derwent.
Two investigations have been launched into Cadbury owner Mondelez International over the incident.
A public health warning was issued on December 19 for the River Derwent. (ABC News: Ebony ten Broeke)
The newly functional treatment plant means warnings against swimming due to health risks of improperly treated waste have been lifted for most parts of the River Derwent.
Samples taken by local councils on Friday have returned their results.
They indicate the water is safe for recreational use at Sandy Bay's Nutgrove Beach and Long Beach, as well as Howrah Beach, Little Howrah Beach, and Glenorchy's Windermere Beach.
But further testing is needed at the western end of Bellerive Beach, where a "localised water quality issue" remains, according to Public Health Services.
Public Health advice remains to avoid swimming at the western end of Bellerive Beach's until further notice.
Public Health director Mark Veitch said beach-goers should stay cautious.
Dr Veitch said people should wait at least two days after heavy rain before swimming at urban beaches, and avoid swimming near stormwater outlets.
This is because rainfall can affect water quality by washing elements into it from the land.
Bacteria reseeded after sewage treatment failure
TasWater had been working with Mondelez International over the last three weeks as the company's factory disposed of trade waste whilst its pre-treatment plant was being upgraded.
TasWater said it had sent warnings to Mondelez that their trade waste was "above the acceptable limits".
However, last week "a big spike of sugar" from the factory killed off necessary bacteria in the Cameron Bay Sewage Treatment Plant.
Public Health Services issued a warning against swimming at many of Hobart's beaches on December 19 after the plant could no longer fully treat the sewage before discharging it.
TasWater's general operations manager Brendan Windmeyer said due to the "high-strength trade waste" the bacteria at the plant "were wiped out, effectively taking the plant offline".
TasWater said about 12 megalitres of only partially treated wastewater was discharged into the river between Thursday and midday on Sunday.
Mondelez International is one of the world's major snack companies. (Supplied: Mondelez International website)
Mr Windmeyer said TasWater stopped accepting trade waste from the company and the team "worked around the clock last week and through the weekend to get the plant back up and running".
This involved "reseeding" the plant with the necessary bacteria, which had been lost.
"The plant is now treating wastewater again and will continue to improve in the coming days," Mr Windmeyer said.
"We are confident there is no longer a public health risk to downstream users of the river," he said.
Bacteria was reseeded at the TasWater plant, meaning it could treat wastewater again. (ABC News: Ebony ten Broeke)
When the matter arose, a spokesperson for Mondelez International said it was working "vigorously and collaboratively with TasWater to fully investigate and resolve the matter".
"We have recently undertaken a $3 million upgrade of Mondelez International's wastewater treatment plant which prepares milk solids and other manufacturing by-products for safe disposal," the spokesperson said.
"The health and safety of our community and protecting the environment are our upmost (sic) priorities."
Mondelez under investigation
Mondelez International is under investigation by both the Environmental Protection Authority (EPA) and TasWater for the incident.
Speaking on ABC Radio Hobart, the EPA director Wes Ford did not comment on whether charges would be laid, saying "it would be inappropriate" to do so while the investigation was ongoing.
EPA director Wes Ford said the pollution was out of TasWater's control, but it was investigating Mondelez International over compliance. (ABC News: Maren Preuss)
Mr Ford said TasWater would not be investigated for breaching the waste discharge limits.
"This was something that was completely outside their control, and therefore it's not appropriate to pursue an investigation against TasWater," Mr Ford said.
TasWater is conducting its own investigation but said it did not have the power to charge Mondelez, however it could issue infringement notices.
It is not the first time Mondelez International's operations have caused issues, with right-to-information documents revealing the chocolate factory released a noxious gas at 10 to 15 times above the high-risk exposure limit into the Claremont area in July.
The gas was caused by a build-up of dairy product in pipes, generating toxic waste.
Loading..."The whole hill reeks of the gasses and this is likely substantially impacting local residents," an internal email written by a technical services lead at TasWater read.
They described the results of a report about the incident as "shocking, to be blunt".
TasWater received contractual guarantees from Mondelez that such an incident would not occur again.
TasWater to continue refusing Mondelez waste
Following last week's issue, Mr Windmeyer said trade waste from Mondelez's Claremont site would not be accepted "until we can be confident in it's ability to discharge compliant waste".
He said TasWater was continuing to have "collaborative discussions" with Mondelez.
An investigation into Mondelez operations will take some weeks.
"We have to get very detailed information from the companies in order to be able to determine what actually happened, and and then we need to look at it from a point of view of legal non-compliance issues."