Central Otago approves $3m Cromwell sludge disposal plan using Winton landfill

Central Otago District Council has approved a $3.023 million programme to tackle a major build-up of sludge at the Cromwell wastewater treatment plant, with existing dried material to be transported to a landfill in Winton.

A survey earlier this year identified about 20,000 cubic metres of sludge accumulated in Cromwell’s treatment ponds. According to the council, the material is now breaking the surface of Pond One, reducing available pond capacity and affecting treatment performance.

The council says leaving the build-up unresolved could also increase the risk of the plant reaching stricter nitrogen limits under its resource consent, potentially bringing forward the need for more expensive treatment upgrades.

Under the approved approach, the Cromwell ponds will be desludged and newly removed material dried and stored in geobags on site. Council says this method has previously been used at Cromwell and other Central Otago wastewater treatment locations.

Existing geobagged material will then be transported to landfill in Winton following a competitive tender process. A disposal site at Lauder had also been considered, but the operator advised it might not have enough capacity for all of the Cromwell material after also committing capacity to Queenstown Lakes District Council.

Council also identified uncertainty over whether all of the material would meet the Lauder site’s acceptance requirements once more detailed testing was carried out during removal. It concluded that landfill disposal provided greater certainty and would allow the wider desludging programme to remain on schedule.

A smaller, ongoing stream of screened sludge from the Alexandra Wastewater Treatment Plant, around 10 tonnes each week, will continue to be transported to the same Winton landfill.

The $3.023 million expected cost covers removal and transport of the existing Cromwell material as well as pond desludging. That figure is substantially below the council’s pre-tender estimate of $4.4 million.

General Manager Waters Julie Muir said certainty was the priority, with the council focused on getting the Cromwell ponds addressed before winter rather than pursuing a potential saving that might not ultimately be available.

The physical desludging work needs to be completed over summer ahead of planned upgrades to the ponds’ aerators. Council says this sequencing is intended to help the plant continue meeting its consent requirements through winter. Nearby residents will be notified before work begins because the programme is expected to increase truck movements around the wastewater site.

Council’s current wastewater information says it operates seven wastewater schemes across the district. Its long-term planning documents identify treatment compliance and ageing infrastructure as major investment pressures, while earlier council material notes that the Cromwell network serves Cromwell as well as Bannockburn, Lowburn and Pisa.

The council has separately carried out desludging work at Ranfurly, where it says removing accumulated material restores pond capacity and improves the biological treatment process.

Source: Central Otago District Council. Webfit News reviewed the complete council media release and current council wastewater information.