Remedial work at Wanganui's ailing wastewater treatment ponds is gaining some headway, albeit gradual.
In the latest development, a dive team sent into the murky ponds found the sludge layer in the main pond is breaking down.
According to Mark Hughes, Wanganui District Council infrastructure manager, this is the result of dumping tonnes of lime slurry into the ponds, as well as dosing the water with hydrogen peroxide.
Mr Hughes said commercial divers found the sludge had dispersed into the smaller, lower pond where it was forming a dense layer about 2m deep. The sludge layer was creating the hydrogen sulphide gas that has wafted over the city since December.
Yesterday, he said the bad odours coming from the ponds were still occurring and would for some time yet, but they were happening less frequently.
He said the movement of sludge from the large pond to the smaller one did not pose any immediate problem for the treatment system.
"This is a positive sign and will make it easier for any longer-term remedial or modification work to be done on the main pond," he said.
He said the strong smell of hydrogen sulphide had gone and that indicated treatment efforts to stop that gas bubbling up from the sludge layer had worked. However, the smell coming from the ponds now was that of raw sewage, so there was still work to be done.
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Mr Hughes said the sludge layer in the main pond has been replaced with a less dense, suspended, solid-type flock, which was about 4m deep.
He said Cardno BTO wastewater treatment consultants said the improvement was mostly because of the improved hydraulics of the new aerators working in the top pond.
There are 14 aerators of various sizes working in the top pond, aimed at intensifying oxygen levels to assist the treatment process, but he could not say how many aerators would eventually be used in the plant.
"These aerators are more effective than the original ones.
"They're a different design and with improved hydraulics."
Mr Hughes said another new aerator went into operation about a week before Easter, one designed and built by council staff "and it's working well".
"We've been working on modifying another aerator and new turbo jet aerators will begin arriving at the end of this week," he said.
"We are now starting to see a gradual increase in dissolved oxygen in the main pond." He said the bio-augmentation programme that ran for some weeks has stopped, but applications of lime slurry and doses of hydrogen peroxide continued, all aimed at lifting oxygen levels.
A new high pressure atomiser system will be installed soon, with pipes being laid this week.
"The higher pressure means a finer mist will be sprayed over the treatment plant to help neutralise the odour.
"This mist will also disperse in the atmosphere more quickly and provide more coverage than the previous low-pressure system," Mr Hughes said.