Now, Cardno BTO has been asked to find out if it is viable continuing with the plant in its present form and, if it is, then what needs to be done to achieve satisfactory performance and have that work completed within two years.
If the consulting engineers find the plant is not viable then it has been asked to find out what sort of treatment plant is needed and what size it should be.
In his latest report to the WDC, Mr McCoy said short-term efforts to snuff out the stench were not permanent solutions, did not address the root cause or look at methods to reach compliance.
Since the end of January, hydrogen peroxide has been added to the raw wastewater at the Beach Rd pumping station which helped suppress some of the smell. Tonnes of hydrated lime have also been dumped directly into the treatment ponds, aimed at dampening the odour coming from sludge at the bottom of the ponds.
Mr McCoy said a medium-term solution needs the use of mechanical aerators to create an oxygen-rich level on the top of the ponds. To help this, a coagulant was added to the wastewater system to settle the sludge while the aerators are working. That treatment stopped on February 21 because while it did its job, it was a high cost option.
The council introduced a bio-augmentation programme, adding microbes to the pond water to help break down the sludge layer. But Mr McCoy said Cardno BTO recommended that be stopped because it actually increased the likelihood of odour being released. The use of other chemical additives was also being evaluated.
He said the original design of the ponds depended on the aerobic conditions in the top few metres of the ponds to minimise the odour coming up from the sludge.
"Creating an aerobic environment in the pond is essential to the elimination of odours," he said.
He said this was far more complex than simply adding oxygen but this would be looked at in detail when his report on long-term solutions was completed.
Mr McCoy said Cardno BTO and council were working hard to source and install aeration equipment that may be suitable for a medium-term operation "over the next year or more until more long-term solutions are implemented".
"We cannot stress enough the importance of the effective implementation of this stage of the strategy," he said. Bigger aerators were being installed in the main pond which would be expected to work until a long-term solution is decided on.
Meanwhile, five venturi aerators are working in the bigger of the two ponds and a sixth was being sourced.
There are a number of small-scale aerators that were installed when the treatment plant was commissioned in 2007 but Mr McCoy said the impact these would have on increasing oxygen levels was small.
Chemical dosing would continue medium-term "because they have had a positive impact".
He said finding a long-term solution would include a detailed evaluation of possible solutions which would be followed by detailed design and construction.
But Mr McCoy warned council these combined phases would take over a year to implement.
