Western Wastewater Treatment Plant

At the Western Treatment Plant, sewage travels through a series of screens, tanks, bioreactors, clarifiers and ultraviolet treatments before being discharged as liquid into Cook Strait in the vicinity of the Karori Stream Mouth.

Ultraviolet System Outage

On October 23, between 10.30am and 11am, the plant’s UV disinfection system went offline due to a fault with an electrical transformer. The transformer fault is not believed to be weather-related. 

Unfortunately, the transformer has sustained significant damage and cannot be repaired. It will need to be rebuilt before the UV system can return to full operation.

Rebuilding the transformer involves disassembling the unit, cleaning, and inspecting all components, and replacing or repairing any damaged parts. This work is a priority and is already underway.

We expect the transformer to be operational again by early next week. As a precaution, daily shoreline water quality sampling is being carried out. We are also using additional network capacity, including the Karori Storage Tunnel, to reduce flows to the plant. This is a short-term contingency measure while repairs are completed.

The plant is still maintaining operations while discharging partially treated wastewater (biologically treated but not disinfected) via the Karori outfall pipe out to the Cook Strait.  The UV system is the final stage of the plant’s treatment process. Its primary function is to disinfect wastewater before it is discharged into the ocean from the Karori outfall pipe - 1.8 km offshore.

Resource consents

The Western Treatment Plant was granted new consents on 28 July 2010. These consents will continue for 25 years (expire 28 July 2035).

In general, the consents allow Wellington City Council to:

• discharge of treated and partially treated effluent through an existing outfall,
• to discharge mixed disinfected, treated and milli-screened wastewater during and/or immediately after heavy rainfall,
• the discharge of contaminants to the air,
• and permission to occupy the coastal marine area with an outfall structure.