Desludging maintenance at Taumarunui WWTP


2021-01-12T08:43:00 Pacific/Auckland

Mon 12 Jan 21


Preparation for desludging of the Taumarunui (Hikumutu) Wastewater Treatment Plant is well underway with work on a lined pit for holding three large bio-bags to contain the settling pond sludge almost complete.

Acting Environmental Manager Stuart Watson said that the desludging is part of the scheduled maintenance of the settling ponds required to keep the bugs in the ponds working efficiently and producing water clean enough to release into the environment.

“The settling ponds were last desludged around eight years ago with the sludge spread directly onto surrounding fields however new National Environmental Standards for Freshwater means that this method is no longer appropriate,” he said.

“The process this time involves dredging the sludge and pumping it into three custom built bio-bags each around 13m wide by 30m long held in the purpose built pit above the settling ponds.

As desludging of the settling ponds is an on-going maintenance requirement the bio-bag pit has been made as large as possible to future proof it for future desludging for years to come.

Once the construction of the holding pit is complete the bio-bags will be installed prior to the filling process which will take around a week per bag.

A special polymer agent that acts as a coagulant is mixed with the sludge so that when it is pumped into the bio-bags the solids settle and form into larger clusters.

The polymer binds the solids together locking up any potential hazards including odour within the bio-bag.

From there the sludge is left to dry out and breakdown within the bio-bags. Water seeping out of the bio-bags as the sludge dries comes out crystal clear but is piped back to the settling ponds for treatment.

The breakdown of the sludge can take between two to five years when it can then be utilised as a high quality soil. As the sludge breaks down it consolidates allowing more sludge to be added to the existing bags.

An alternative to leaving the bio-bags to dry out is to plant directly into the top of them and let the plants use up the nutrients and reduce the sludge volume as they grow.”

Mr Watson added that while our specialist contractor Hydra-Care states local residents will not be impacted by offensive odours or other hazards we will be improving the look of the site by installing fencing and plantings.

“Hydra-Care have undertaken desludging work for numerous councils all around NZ and I would encourage anyone interested in their work to look at their website

If local residents in the vicinity of the Treatment Plant have any concerns they are welcome to contact me directly at Council on 07 895 8188,” he said.


A purpose built pit above the settling ponds will hold three massive bio-bags which the pond sludge will be pumped into and left to breakdown over a period of two-five years.

Page reviewed: 12 Jan 2021 8:43am