Work to repair the flood damaged Omakau wastewater treatment plant is under way.
Heavy rain on July 19 flooded the site for about 24 hours, extensively damaging the embankments and plant with large amount of tree debris, silt, gravel and floodwater entering the two treatment ponds and causing wastewater to spill into the Manuherikia River.
However, damage to the site is less than anticipated.
Central Otago District Council (CODC) water services manager Ian Evans said council staff and contractors visited the site as soon as safe access was available on July 20 and repairs and a clean-up had been ongoing since then.
A full assessment of the site revealed the damage was less than initially thought, he said.
Despite tree debris pushing over security fencing and accumulating in and around the ponds, there was no significant erosion of the ponds or the surrounds and they remained structurally intact, he said.
"There was minor damage to one side of the pond embankment which required reinstatement to protect it from further damage. The pond liner, initially feared to have sustained significant damage, after further investigation and inspection by the liner specialist has received minimal impact, only needing some relatively minor patch repairs."
It appeared the liner protected the pond embankments from more significant damage, he said.
While damage occurred to mechanical and electrical components, the impact was reduced by quick action taken to turn off power at the site.
The access track required reinstatement after the flood event.
The full cost to repair the damage was not "fully understood" yet as work at the site was ongoing, but an early estimate had come in at $110,000.
That figure had not been finalised, he said.
Since the flooding, debate has arisen online regarding the location of the treatment ponds and whether they should be relocated.
Mr Evans said a business case was being prepared to consider alternative options for treatment of wastewater from both Omakau and Alexandra, exploring options for treatment and discharge to land instead of waterways.
"This decision is being considered carefully, as potential alternatives range in the vicinity of $30 million to $50 million," he said.
Otago Regional Council (ORC) general manager regulatory and communications Richard Saunders said last week that any discharge of contaminants into water was a concern and the situation was being "closely monitored" with staff making "multiple visits" to the site.
"A full investigation into the incident will occur, but the immediate priority has been to allow CODC staff to focus their attention of remedial work to stop the discharge of the partially treated wastewater to the Manuherikia River."
Any event that might result in a discharge to freshwater was a concern and the ORC would continue to work with the district council to prevent this, Mr Saunders said.
The CODC had resource consent for the operation of the treatment ponds in their current location, he said. However, he was aware the district council was preparing a business case considering alternative options for treatment of wastewater from both Omakau and Alexandra.
