Tendering is underway for the desludging of Ranfurly’s wastewater ponds and wetlands, with physical works expected to begin around mid-November.

The project is part of Central Otago District Council’s ongoing effort to bring all its wastewater treatment plants into compliance following abatement notices issued after a 2023 audit by the Otago Regional Council.

That audit identified several maintenance issues and non-compliance across five of the district’s seven wastewater plants.

Since then, abatement notices have been lifted for Alexandra, Cromwell and Naseby but remain in place at Roxburgh and Ranfurly.

In a report received at Wednesday’s council meeting, sampling and compliance monitoring team leader Joyce Thomas told elected members that once Ranfurly’s desludging is complete, the constructed wetlands will need to be restored before the abatement notice can be lifted.

She said the recent example at Roxburgh was “encouraging”.

Additional monthly effluent monitoring there has shown the plant’s effluent was compliant in both August and September, suggesting earlier desludging and aeration upgrades are improving nitrogen removal.

However, future compliance could become more challenging.

The council is still waiting for Taumata Arowai, the national water regulator, to release new National Performance Standards for wastewater discharges, now expected in December.

“Early engagement on the proposed standards indicated that the discharge at Ranfurly may not meet the new performance standards due to the way in which dilution is assessed,” the report said.

Before restoring the wetland, the council will review its discharge options to find the most cost-effective way to meet those future standards, which could include abandoning the wetland and moving to land discharge instead.

Consultants Pattle Delamore Partners are being considered to carry out that review, with a report due to councillors mid next year.

Wednesday’s council meeting was the first for several new councillors, and three waters boss Julie Muir presented Joyce’s report in person.

She said the regular wastewater compliance update has “got way more simpler than when we first started doing them”.

“We’ve come from five sites to two, and we’re in the last stages with Roxburgh,” Julie said.

She highlighted the positive results from Roxburgh, saying it showed “the value of doing that (desludging) work. It’s a way more cost-effective way to improve performance than doing expensive plant upgrades.”