Wanganui's wastewater treatment plant has breached its discharge consent levels and the Wanganui District Council is rushing to get remedial action in place to bring the system back on track.
The treatment process is not working as it should be and as a result the level of bacteria discharged into the sea off South Beach is higher than the Horizons Regional Council consent allows.
Yesterday Mayor Annette Main was quick to explain that the discharge levels did not pose any health or environmental risk and that council staff were working on a process of "bio-augmentation" that would introduce bacteria to accelerate the breakdown of waste in the holding ponds.
The treatment plant, behind the city's airport, was commissioned in 2007 and marked the final stage of the city's $120 million wastewater separation and treatment project. But from the start it has been dogged with mainly mechanical problems.
Soon after it started, some of the aerators used to stir up sludge in the ponds failed but were replaced under warranty. This year more aerators failed, and checks showed sediment at the bottom of the ponds was more than 4m deep. That amount of sludge was not expected to be reached for several more years yet.
Then, a few weeks ago, 12 more aerators failed, leaving only seven of the total of 23 in working order.
The council had budgeted for ongoing maintenance of the treatment plant in its District Plan and that included the bio-augmentation process.
Ms Main said that was planned to happen in 2015 and was aimed at stabilising the plant and reducing the levels of sludge, faecal coliforms and other suspended solids.
"Bio-augmentation basically involves growing 'good bugs' in a container on site and then putting them into the ponds," she said.
The problem for council is that consents for the treatment ponds demanded that suspended solids must not exceed 100 parts per million (ppm) but currently it was running at about 180 ppm.
Mark Hughes, council's infrastructure manager, said the amount of suspended solids meant the final treatment of the waste using ultraviolet lights could not work properly.
"Because the solids going through there are cloudy and thick, the light can't penetrate and do its job of sterilising the faecal coliforms being discharged in the ocean outfall," Mr Hughes said.
"The coliforms started increasing quite dramatically around September last year and at the same time some of the aerators started failing," he said.
He said the problem was getting the liquid thin enough to go through that final treatment phase under the UV lights.
Kevin Ross, council chief executive officer, said public health was not being compromised.
"We've been monitoring both Castlecliff and South Beach and there's no sign that we've got any problems there. It's not raw sewage going into the sea," Mr Ross said.
Water samples taken along the coast showed that levels of faecal coliforms were well below thresholds allowed.
He said it appeared sludge in the ponds had built up quicker than was expected, and the microbes had not been as effective as anticipated.
Mr Hughes said the bio-augmentation equipment was expected to be on site by the end of this month and microbes put into the treatment ponds soon after.
"We're expecting to see the level of suspended solids stabilise quickly and start declining before Christmas," he said.
It will mean intense dosing at first to get both ponds stabilised and it would probably mean on-going dosing at lower levels in future.
"We've gone through the different options, and this is preferable to chemical dosing.
"That's what we don't want to be doing, and we know iwi would not want that to be happening either."
Mr Ross said council would be able to manage extra costs of the bio-augmentation plan because it had already flagged the work in its District Plan, albeit bringing it forward from 2015.
Ms Main said the key was getting the plant back to total compliance.
"That's an area of supreme importance, and to ensure the public health and environment is never compromised," she said.
Craig Grant, Horizons acting planning and regulatory group manager, said staff had started their own investigation into the issue but could not say when that report would be completed.
Treatment ponds are commissioned in 2007, but aerators prove problematic from the start.
November 2011: Report shows the plant is not doing what is designed to do and the gap to achieving compliance is widening.
Early 2012: More aerators fail and sludge layer at bottom of the ponds is much greater than expected.
Several weeks ago more aerators fail, leaving only seven of 23 operational
Public health and environment is not threatened and shellfish monitoring shows no cause for concern
Council has commissioned a report from marine scientists at Cawthorn Institute (Nelson) to review the data.