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Sludge smell in Carterton

After 10 years without maintenance, Carterton District Council emptied its wastewater clarifier at the weekend. PHOTO/SUPPLIED

Overdue maintenance has led to high sludge levels in Carterton’s wastewater ponds, throwing off their biological balance and contributing to a nasty smell in the town.

At a Carterton District Council infrastructure and services committee meeting last week, councillors heard that maintenance on the treatment plant’s clarifier had not happened for 10 years.

“The last record that I have of the clarifier being emptied and work being done on it was about 10 years ago, so yes, it’s well overdue,” council consents, compliance, and operations manager Rachel Round said

A clarifier was used to reduce the amount of sludge piped into a treatment plant’s oxidation ponds.

Carterton’s clarifier was constructed in 1976.

After last week’s meeting, council workers emptied and cleaned the clarifier during the weekend.

Round said that the cleaning had produced an unpleasant smell due to the movement of wastewater that had been sitting still for months.

She said that this smell had dissipated within two days. However, in the past six months, Carterton had experienced other smells from the ponds, with water stirred up to provide oxygen for bacteria to survive.

Round said that good practice for a clarifier as old as Carterton’s would be to empty and inspect the clarifier every three to five years.

“However, this is also dependent on other factors such as the loads and volumes going through the plant.”

Round started working at the council in January and could not comment on why there had been a 10-year gap in maintenance.

She said that in the previous maintenance, workers had drained the clarifier, removed and sandblasted its scrapers, and installed new rubbers.

Since this last maintenance, the clarifier’s wiring had disintegrated in several places underground.

“At 45 years old, and the wiring being 3.2 metres underground, it had done extremely well,” Round said.

She said the original wiring was laid straight into the ground, whereas the council would now place all wiring in ducting for protection from the elements.

With no power, the clarifier had been out of action since mid-August.

Covid-19 had then delayed the shipping of new cabling to New Zealand.

Corroded pipework within the clarifier would also need replacing before rewiring could begin.

At last week’s meeting, councillor Steve Cretney asked if the clarifier’s inefficiency was the cause of excess sludge building up in the oxidation ponds.

“It’s a likely contributor to that,” Round said.

She said the ponds needed a layer of “activated sludge,” which carried out a biological process to break down solids.

Any excess sludge, known as “surplus activated sludge,” needed to be removed to keep the biological processes of the ponds in balance.

In June 2021, the council had contracted sewage disposal service Parklink to survey the levels of sludge within the oxidation ponds.

Parklink completed the survey on Friday.

Round said the survey would help the council understand the impact of sludge on the storage capacity of the ponds.

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