Flooding has significantly damaged the Omakau wastewater treatment site and a clean up that could take several weeks begins today (Friday 22 July) according to Central Otago District Council (CODC).

On 19 July the site was flooded by the Manuherekia River for around a day during the heavy rain that hit Central.

The security fencing was pushed down by an accumulation of tree debris. One side of the pond embankments has been undermined and will require reinstatement to protect it from further damage. While still structurally intact, a large amount of tree debris, silt, gravel, and flood water has entered two treatment ponds.

While treatment is still occurring, the performance of the ponds has been impacted, resulting in a reduction in the quality of the discharge to the Manuherekia River.

The Otago Regional Council (ORC) is monitoring the situation and the CODC response.

The ORC advises that the highest previous flow recorded for the Manuherekia River at Ophir since 1971, was 602 cumecs in December 1995, and Tuesday’s event is likely to be around the third highest flow, below 600 cumecs.

CODC is carrying out an assessment of the repairs to the treatment site with associated cost estimates, along with a review of options to improve the level of treatment while the ponds are being cleaned.  

During the clean up and once rivers have dropped to safe levels, CODC will also be monitoring the water quality upstream and downstream of the site.

People are advised to stay out of the Manuherekia River between Omakau and the Clutha River until further notice. Any water that is taken downstream from Omakau for drinking should be boiled.

The first Omakau wastewater treatment pond was constructed in 1965, with a second pond added in 1976.

CODC has been upgrading the site to improve the discharge quality over the past two years.

CODC Three Waters Director Julie Muir, said it was distressing to see the damage that had occurred to the new pond liner, and the equipment that had recently been installed on the site.

“This is the second time the site has flooded in five years. The first flood prompted a review of the long-term future of the site, given that increased frequency of flooding is expected in the future as a result of climate change.

“A business case is currently being prepared to consider alternative options for treatment of wastewater from both Omakau and Alexandra. This is considering options for treatment and for discharge to land instead of waterways.”

Included in the analysis is an option to combine the Omakau and Alexandra treatment onto one site, as well as options for continuing with two separate sites. 

CODC has committed to identifying a preferred option by 2025 as a condition of its resource consents for Omakau, and by 2026 for Alexandra.

These changes at both Omakau and Alexandra are required due to ageing infrastructure, increased rainfall from climate change, population growth in the area, and changes to societies’ views regarding wastewater treatment and discharges.