The plant's primary pond was retrofitted on existing infrastructure. Photo / NZME
Sludge storage at Whanganui’s wastewater treatment plant is expected to reach capacity within three years and getting rid of it will be “quite a challenge”.
A report from Whanganui District Council senior stormwater engineer Kritzo Venter said a removal programme had to be integrated into future planning at the plant.
“Historical industrial discharges and ineffective trade waste controls have resulted in heavy metal contamination of the sludge in the primary pond and [in] storage,” the report said.
“This necessitates capex [capital expenditure] provisions for remediation or off-site disposal.”
Sludge settles on the bottom of the plant’s primary pond, with the rest of the wastewater separated.
From there, it is pumped into a decanter and then to a thermal dryer.
Speaking to the council’s operations and performance committee last month, Venter said options for removing sludge could be brought back to the council.
Desludging the primary pond would be “quite a challenge”.
“Do we try and stretch and provide more storage in the interim?” he said.
“Some of the initial assumptions and information may come to this table, but I would anticipate the new CCO [council-controlled organisation] would take over and govern it really aggressively.”
A new water services CCO covering Whanganui and Ruapehu will begin operations on July 1 next year.
The Chronicle reported in 2024 that the estimated cost of transporting sludge to Hampton Downs, north of Hamilton, was $2 million a year.
However, council chief infrastructure officer Lance Kennedy told the Chronicle this week there was no “definite estimated cost to quote at this time” and the council would need to reconfirm available options.
He said dried sludge was stored in a lined pond at the plant, with about two to three years of capacity remaining.
“We extract as much as we can [from the primary pond] under normal operational protocols,” he said.
“We know a portion breaks down through anaerobic digestion but the amount cannot be calculated accurately.”
The plant off Airport Rd was completed in 2017 for $39m after the previous facility on the same site failed.
In March, Venter said the primary pond, which holds 57 million litres, made him uneasy at times because it was retrofitted on existing infrastructure and not accessible.
When a diver went into the pond, they became disoriented and had to be pulled out with a safety rope due to zero visibility, he said.
“There may be a day of reckoning on what we need to do. It would be expensive, and it would be disruptive.”
Councillor Peter Oskam said the plant “reeks of risk”.
“I will be recommending to the risk and assurance committee to do a deep dive on the wastewater plant,” he said.
“I would like to be assured that going into the future, it is covered well.”
The council set aside $3.4m in its 2024-34 long-term plan for sludge disposal, with the possibility of recycling it into fuel to power the thermal dryer.
Kennedy told the Chronicle the council was working to confirm a specialist adviser to assist in evaluating options and identifying the most appropriate long-term solution for biosolids management.
Councillor Charlotte Melser asked if the council’s trade waste controls were sufficient to deal with contamination.
Trade waste users, or wet industries, were major contributors to the wastewater scheme but also significant funders, Venter said.
“We are in the process now of reviewing their permits, in line with the extension that’s been granted for seeking an updated consent for wastewater discharges.
“We’ve also reviewed the trade waste bylaw and identified some improvements.”
Most existing consents have been extended to about 2031, while the Resource Management Act is replaced with the Planning Act and Natural Environment Act.
Melser said there was a huge opportunity to create a usable biosolid product from the plant’s waste.
“We must set strong expectations to the new CCO to ensure we can get there in a timely manner,” she said.
Mike Tweed is a multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is local government, primarily Whanganui District Council.