The aerial bombardment of Wanganui's ailing wastewater treatment ponds will continue today in an effort to stop the stink.
A helicopter equipped with a monsoon bucket showered about 14 tonnes of hydrated lime over the two ponds on Wednesday with a final 36 tonnes being delivered the same way today.
The treatment was suggested by engineering consultants Cardno BTO, the Wellington firm Wanganui District Council has called on to try to sort out problems at the plant.
The slurry will sink to the bottom of the ponds and cover the thick layer of sludge which has created the strong stench. And more aerators have been brought on site in a bid to boost the oxygen levels in the top water layer, again helping nullify the odours.
Odour emissions have been a problem virtually since the plant started up five years ago but since late last year the stench has spread across most of the city almost every day.
Havelock North company Webster Lime has been brought in to spread the lime slurry.
The company specialises in dosing irrigation and farm effluent ponds with lime but company spokesman Nick Webster said they had never tackled anything on this scale.
Mr Webster said his company was using an out-of-town helicopter company for the job because they had worked together many times before.
"They've got the truck, the slurry mixer and a helicopter and are familiar with this sort of operation," he said.
Mike McCoy, of Cardno BTO, said the lime would have a positive effect in helping to neutralise the odour that continues to plague the city.
"You've got to remember this is only a temporary solution to trying to control the odour," Mr McCoy said.
He said his company had worked with many local authorities experiencing similar problems with their wastewater systems.
"Those other ponds may be significantly different in design to Wanganui's but the biological issues they had are similar," he said.
Mr McCoy said he was confident the temporary fix would work. As to the short-term and long-term solutions, he said Cardno BTO expected to have those results in front of the council within three months.
"How much work that might entail and how much it might cost we don't know at this stage."
He said compounding the problems in Wanganui's system was the sheer size of the ponds.
"Surface area and size does make a difference and in this case these ponds are huge. So even small amounts of odour coming from different areas can create odours coming off a much larger area," Mr McCoy said.
He said the accumulation of sludge at the bottom of the main pond was significant. "There's still waste coming in here every day."
Meanwhile, council infrastructure manager Mark Hughes said three more aerators were on site, brought over from Central Hawke's Bay Regional Council, and three more were expected in a few days from the same place.
Another three unused blower-aerators were brought in from Kapiti Coast District Council.
Mr Hughes said all the aerators were expected to be operating within the next couple of days, helping to intensify the oxygen layers in the top level of the ponds and improving the treatment process.