Standard of treated sewage worsening
The quality of Christchurch’s treated sewage flowing into the ocean from the city’s fire-ravaged wastewater treatment plant is getting worse.
Since two large filters at the plant went up in smoke in November, sewage treatment capacity has been significantly hampered. Now, the onset of winter has dealt another blow.
Christchurch City Council breached consent limits in three out of five routine treated wastewater quality tests, sampling taken towards the end of June showed.
The samples were taken from the city’s last oxidation pond and the council said they were ‘‘representative of what is being discharged to the environment’’.
Two of the tests that breached the limits were for faecal coliforms and enterococci – bacteria commonly found in sewage. They were useful indicators of whether treatment is effective, the council said. Environment Canterbury said last month the worsening wastewater quality results were not unexpected after the fire.
It said while limits in the consent were exceeded, the limits were to ensure that if breached, appropriate sampling was carried out. ‘‘We are satisfied for now that Christchurch City Council is doing all it can to manage the situation.’’
The council recently doubled its beach sampling and started testing shellfish.
Councillors were told by staff last week that samples at the beach had not been affected by the worsened quality of the treated wastewater going into the ocean.
The worsening quality is linked to the putrid pong plaguing residents in eastern suburbs. The stench has caused nausea, headaches and irritation, Canterbury’s medical officer of health said last week.
Some affected households were able to claim $200 compensation offered by the city council.
While rotting debris inside the damaged filters has also created a stink, the dominant source of the stench is the oxidation ponds.
Because the plant’s treatment capacity is reduced, the wastewater in the ponds has not had as much treatment, making it smell worse. The volume of midges and bird life on the ponds is now very low as a consequence.
Once the wastewater goes through the ponds, which takes about a month, it goes into the ocean, 3km offshore.
The council’s water boss, Helen Beaumont, said the wastewater plant ‘‘lost significant processing capability’’ in the November fire.
Since then, the oxidation ponds had higher loads of organic material, she said.
Micro-organisms in the ponds could handle the higher loads during summer, when temperatures were higher and there was more daylight, Beaumont said.
‘‘However, as the temperature has dropped and the sunlight hours have decreased, the ponds are not performing as well,’’ she said.
In a bid to improve wastewater quality, the council is racing to finish a longoverdue conversion at the damaged treatment plant.
Beaumont said ‘‘the [consent] exceedance issues’’ were expected to resolve when the conversion was done.
Work on it began in December and was initially expected to be finished in March.
Beaumont said there had been a number of delays, with the work plagued by Covid lockdowns and international supply chain disruptions.
One of the issues was securing specialist pumps from Sweden.
The last part of the conversion work – the installation of a pipe to bypass the firedamaged filters – would finish next week.
From there, it would take another six to 12 weeks for the ponds to improve and the stench to reduce.
In that scenario, the smell would be gone by August 26 in the best case, or in the worst, October 7 (assuming the pump is installed next Friday).
The delay is because the biological treatment process needs time to kick in and the bad quality wastewater has to make its way out of the oxidation ponds.