There is an opportunity to recycle the dried sludge from the wastewater treatment plant as fuel to power the thermal dryer. Photo / Bevan Conley
There is an opportunity to recycle the dried sludge from the wastewater treatment plant as fuel to power the thermal dryer. Photo / Bevan Conley
Dried sludge from Whanganui District Council’s wastewater treatment plant is piling up, but a $3.4 million plan could turn it into fuel in the future.
Hampton Downs, north of Hamilton, is currently the only option for disposal once the storage pond is full. That is expected to happen in2026 or 2027.
The council’s senior wastewater engineer, Tony Hooper, said sludge - a byproduct of the wastewater treatment process - settled in the plant’s primary pond and was pumped into a decanter to remove 19 per cent of the water.
Next, it went to the thermal dryer, where a further 70 per cent of the water was removed.
“From start to finish, this process reduces the sludge volume by about six times,” Hooper said.
If it gets the green light, $400,000 will be spent in the 2024-25 financial year, followed by $3.4m in 2025-26 and $500,000 in 2026-27.
The annual cost of taking the sludge to Hampton Downs once Whanganui is at capacity is estimated at $2m.
While the council would need to stump up the cash, there was potential for the project to receive central government funding, the report said.
The plant could potentially take sludge from other sites as well.
Hooper said if the proposal went ahead, the basic process for wastewater treatment would remain the same, but there were two advantages of converting the end product to fuel.
“It would reduce energy costs - making the drying part of the process self-sufficient - and would significantly reduce the volume of biosolids to find an end use for.”
Sneijder’s report said the heat generated to run the dryer would reduce or eliminate the need for natural gas, with no increase in contaminants to the air.
Once complete, plant maintenance and additional personnel costs over and above normal plant operations are estimated at $200,000 a year.
The ash left over from the burning process could also find a home, but chromium contamination of the sludge from sources such as Tasman Tanning will need to be eliminated first.
Tony Hooper at the Whanganui wastewater treatment plant in 2020. Photo / Bevan Conley
Hooper said Tasman Tanning was partway through the process of upgrading its effluent treatment and it was expected to be completed by May this year.
“We have been informed that this should significantly reduce the amount of chromium discharged,” he said.
“As the effluent with lower chromium levels comes through, the sludge in our pond contaminated with high levels of chromium will be diluted to acceptable levels.
“Once this occurs, there will be the potential to use biosolids as a fertiliser for agriculture.”
Hooper said using sludge as fuel to power the dryer was a relatively new idea and “wasn’t on our radar in the early days” of the plant.
No New Zealand wastewater treatment plants currently burned dried sludge for fuel but there were several in Australia, he said.
Another report from Sneijders said methane gas generated from the plant’s anaerobic pond could be harnessed, either as a supplemental energy source to the dryer or to generate electricity.
The gas is currently flared off.
“Palmerston North City Council currently has an electricity generator attached to their wastewater treatment plant utilising gas from their digesters and landfill,” the report said.
Whanganui’s plant consumed $844,000 in electricity and $171,000 in gas in 2022.
Generating electricity would require a consistent quality of methane gas greater than 50 per cent by volume.
Hooper said going by the analysis of biogas samples collected to date, that was unlikely to go ahead.
Mike Tweed is an assistant news director and multi-media 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.
