Winton landfill deemed appropriate for wastewater sludge disposal

Central Otago District Council has approved a plan to address a growing sludge management issue at its Cromwell wastewater treatment plant, and to manage ongoing sludge disposal from its Alexandra plant.

A survey earlier this year found 20,000m³ of sludge had built up in the Cromwell treatment ponds – now breaking the surface of Pond One. Left unaddressed, this reduces pond capacity and treatment performance, and risks triggering stricter nitrogen limits under the plant's resource consent, which would require significantly more costly treatment upgrades.

Council has approved desludging of the Cromwell ponds, with the dried material to be stored in geobags on site, as has been done at this and other Central Otago treatment sites in recent years.

Following a competitive tender process, the existing geobag material from Cromwell will be transported to landfill in Winton. While a site in Lauder was being considered for disposal, the site's operator advised it may not have capacity to take all the Cromwell material, having also committed capacity to Queenstown Lakes District Council. There was also a risk that some of the material, once tested more thoroughly as it's removed from the geobags, may not meet the site's requirements. Taking the material to landfill avoids that uncertainty and keeps the wider desludging programme on schedule.

A separate, much smaller volume of screened sludge from the Alexandra Wastewater Treatment Plant – around 10 tonnes a week – will continue to be transported to the same landfill.

The total cost of removing and transporting the Cromwell material to Winton, including pond desludging, is expected to be $3.023 million – well below the pre-tender estimate of $4.4 million.

Council's General Manager Waters, Julie Muir, said having a clear, reliable plan in place was the priority.

"At this time, we'd rather have certainty than chase a saving that might not hold up. Getting the Cromwell ponds sorted before winter is the focus, and landfill is the option we can rely on to get that done."

Desludging work at Cromwell needs to be completed over summer, ahead of planned upgrades to the ponds' aerators, to ensure the site continues to meet its consent requirements through winter. Residents near the Cromwell wastewater site will be notified ahead of the desludging work, which is expected to increase truck movements in the area for its duration.