Christchurch City Council staff are backing away from a controversial plan to pump millions of litres of sewage out to sea as councillors prepare to vote on proposals it is hoped will take pressure off the city's struggling treatment plant.
A plan put forward by the mayor to send partially treated sewage out to sea, which prompted concern and condemnation, has not been recommended by council staff.
Instead, staff backed a plan to increase aeration in the oxidation ponds, which would be more cost effective, less ecologically damaging, had mana whenua support and was easier to build and operate than the plan the mayor mooted.
The odour mitigation options report from staff will be considered by councillors on Thursday, April 2. The meeting will be livestreamed on the city council's website.
Putrid stench plagues city
Residents in the east have been plagued by the stench since a blaze destroyed parts of the Bromley wastewater treatment plant in 2021.
Complaints began to surge in October, with the regional council receiving more than 7600 reports from the east and city centre over summer.
The city council said heavy rain reducing oxygen and algae in the ponds worsened the smell.
In late February, Environment Canterbury issued an abatement notice to the city council requiring it to provide a comprehensive plan to comply with its resource consent or face prosecution.
Days later, Mayor Phil Mauger proposed pumping around a third of the city's sewage - between 45 to 55 million litres a day - into Pegasus Bay via the existing outfall pipe.
At the time, the regional council's director of operations Brett Aldridge said the council was "surprised and concerned" by the comments.
On Tuesday, Aldridge said he was confident the council had now provided all the information required.
The regional council would do "a little bit of due diligence" and have its experts look at the council's plans, Aldridge said.
"We will leave it to the city engineers to really get into the nitty gritty of what that design is and how it will be implemented."
Aldridge confirmed the council's two recommended options were not included in the response to the abatement notice.
The council had signalled longer term options were under development with a wastewater specialist, but did not set out specific options or proposed pathways, he said.
Two recommended options
In its report to councillors, council staff offered six options but said only two were viable and cost effective - increasing aeration to the ponds, or increasing aeration and diverting some partially treated wastewater around the ponds and out to sea.
Staff warned neither option addressed odours caused by excessive sewage loads or chemicals, equipment failures, those caused by extreme rain events or by things other than biological oxygen demand (BOD) - a measure of how much oxygen was needed to break down sewage into CO2 and sludge.
High BOD levels in the plant's ponds were just one reason for the stench, but were the most significant cause, according to the report.
Increasing electrical supply to power the additional aerators could take four months, and staff proposed using diesel generators in the interim.
Staff costed the recommendations between $7.7 million and $11.2m to add differing levels of aeration, or between $12.2m and $16.6m to add aeration and then divert 400 litres a second of partially treated sewage to sea, either as needed (18 to 60 days a year) or year-round.
The most expensive option, to divert almost 2000 litres a second of partially treated sewage to sea for 243 days a year would cost $18.3m, had a very high risk of failure, and would take five months to implement.
Staff noted the partially treated wastewater would include BOD, enterococci, TSS (total suspended solids) and chlorine which could have effects on the ocean and public health, but that there had not been time to assess the ecological and health impacts.
If councillors backed aeration, the only resource consent needed would be for the temporary diesel generators.
If they chose one of the diversion options, they would need a new consent, which would be processed with priority and under the new wastewater regulations that came into effect in December, Aldridge said.
Two abatement notices in less than a month
In March, the regional council issued another notice over a series of illegal sewage discharges into Whakaraupō Lyttelton and Akaroa harbours.
The breaches prompted Banks Peninsula hapū Ngati Wheke to consider legal action over the failures and lack of communication from the council.
Ecologist Dr Mike Joy said discharging sewage to the ocean, rivers and estuaries was "Victorian".
"We should be way past this kind of attitude that it's all right just to dump the waste."
Terms such as "treated wastewater" needed clearer definitions, he said.
".. the word treatment can mean anything from just taking the lumps out to completely taking it back to drinking water."
Assurances about the safety of chlorine did not relate to environmental impact.
"They mean safe in that the chlorine will kill bacteria that are harmful to humans - that doesn't mean it's safe for the environment or safe for the ecology of the near shore environments."
Sewage discharge caused an influx of nutrients which drove algal blooms and potentially cyanobacteria blooms, resulting in "dead zones where the water becomes deoxygenated, and no life in any form can survive without oxygen".
The idea the sheer quantity of water would disperse sewage was outdated, Joy said.
"This is this old saying when there were a hell of a lot less people on the planet that the solution to pollution is dilution, but it's not that at all - the solution to pollution is assimilation."
Nor did he think the council's altered proposal was much of an improvement.
"It's just another ambulance at the bottom of the cliff thing. To treat this discharge properly we need to create an industrial-type wetland where you grow flax and raupō and you harvest and compost it."
Concern over new national wastewater rules
The proposal underscored the increased nutrients and pathogens that could be discharged under the new wastewater standards, Joy said.
"It was a completely backward step, and I think the city council's trying to take advantage of the weakening of national regulations to allow more stuff to be dumped into the ocean."
Joy disputed claims the standards would improve performance.
"The only performance that will improve might be the economic bottom line for these councils because they'll be able to get away with dumping more of the stuff without treating it. It's quite clear analysis ... that much more of these contaminants will be allowed under this new legislation."
Some bypass events could go on for weeks or months and could go completely unnoticed.
"The actual wastewater treatment plants in many cases are OK, but the infrastructure that feeds into it is old and worn out and has illegal connections so when you get a heavy rainfall event you get a massive increase and they don't have anywhere to store it."
Sewerage infrastructure would come under more strain as climate change caused more extreme weather events, he said.
"It's a massive and growing problem in New Zealand and it's just another one of these [issues] of lack of spending on infrastructure that's now coming back to bite us."
Upgrades urgently needed
Taumata Arowai spokesperson Sara McFall said more than 20 percent of the country's wastewater plants were operating on expired consents and around half of underground wastewater networks were rated as in poor or very poor condition, so it was important to make the urgently needed upgrades affordable.
The council's wastewater treatment plant operations manager Adam Twose presented to the Waitai Coastal-Burwood-Linwood community board in mid-March, and was clear the council was only able to consider the diversion plan because the regulations were "significantly more relaxed" than current consent conditions.
"Under the new wastewater standards, there's the option to go a lot looser, so you're allowed to discharge more contaminants to the environment," he told the meeting.
He told the board there was a "high level" risk of the stench increasing if the council did not act, as well as risks to the plant itself, which had been operating "at maximum capacity and minimum redundancy" since the fire.
Yani Johanson.
More oversight, more support
City councillor Yani Johanson said it was obvious the plan to put partially treated wastewater into the outfall pipe was not realistic.
"There are too many risks around it, too much concern around environmental impact, too many unknowns."
He was frustrated aeration had not been suggested earlier.
"Many of us around the council table have asked for options to address what was going on and what was causing it for years. I welcome the idea that we can do more, but it's frustrating it's taken this long to get to that point."
Johanson wanted better council oversight of the activated sludge plant project, due to be completed in 2028.
"While there's some things being done, there's no clear plan that the community can look at to hold us accountable for mitigating the impact on their health and well-being."
He also wanted staff to prepare a plan to reduce the impact on the community.
This could include free GP visits, a register of vulnerable residents or distributing air purifiers, he said.
