Treatment pond sludge removal leaves bad smell

Wanganui wastewater ponds PHOTO/FILE

After a period with barely any smell from Wanganui's wastewater treatment ponds, the stench was back with a vengeance yesterday.

Mark Hughes, Wanganui District Council infrastructure manager, said the latest incident was caused by the transfer of sludge from the settling pond back into the main pond which is part of the process of cleaning out the settling pond.

The clean out is integral to the $24 million upgrade of the treatment plant.

Mr Hughes said the community needs to be aware that odour will come from the plant from time to time as first the sludge is removed and work starts on the major contract.

"Work was suspended after about three hours today because of the smell and then because the strong winds made it too risky for people working on site," he said. He said work is expected to resume today.

Contractors started work last month on a installing a bypass system at the ponds. Pipework around the lower settling ponds will divert the city's waste directly from the upper main pond to ultraviolet light sanitisers before the waste is pumped into the sea.

Sludge from the settling pond is currently being pumped back into the main pond. Eventually the sludge will be put through augurs to wring as much water out of the sludge as possible.

It will be mixed with lime and polymers to stabilise it before it is put back into the smaller pond, covered with green waste followed by a layer of clay, and capped off.

The treatment plant has been troubled from the time it was commissioned in 2007 and problems reached a head last December, with odour from the ponds blowing over the city.

The new plant, expected to be operating by early 2015, will see any sludge removed daily.

Residential and industrial waste will be pumped into a covered anaerobic pond, then shifted into stabilisation and clarifying ponds, before undergoing UV treatment and being pumped to sea through the South Beach outfall.

Mr Hughes said once building of the new plant started in April next year, there would be no smell because any sludge would be contained within the covered pond.

The council wants to have the main contract out to tender before Christmas, with major work starting in March, 2014.