Western Wastewater Treatment Plant

Kia ora koutou,

Welcome to the first edition of our new bi-monthly newsletter for the Western Wastewater Treatment Plant.

This newsletter is designed to keep you informed about plant operations, ongoing initiatives to enhance our processes, and the steps we're taking to responsibly manage our impact on local communities and the environment.

You are receiving this email because you have previously expressed an interest in receiving information about the plant. Feel free to share with others who may be interested.

If you do not wish to receive this newsletter in the future, please unsubscribe using the links at the bottom of the email.

In this issue

• Community Meeting

• UV System Outage Update

• Getting to know the Western WWTP

• What’s happening at the plant

• Contact us

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Western Wastewater Treatment Plant Community Liaison Group Meeting

Come and learn more about the plant with a presentation on some of the recent upgrades and future projects. Representatives from Wellington Water and Veolia (plant operator) will be on hand to answer your questions.

Where : 251 Karori Road, Karori, Wellington
When: Wednesday, 26 November from 5.30 - 6.30pm

Please RSVP: Community@wellingtonwater.co.nz

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UPDATE - Ultraviolet (UV) System Outage

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

During this outage, the plant remains operational while discharging partially treated wastewater (biologically treated but not disinfected) via the Karori Outfall Pipe out to the Cook Strait.

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, 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.

As works progress, we will provide further updates across our Wellington Water channels.

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Getting to know the Western WWTP

The Western Wastewater Treatment Plant is managed by Wellington Water on behalf of Wellington City Council and serves approximately 13,000 people.

The facility operated by Veolia is situated in Karori on Wellington’s south coast near the Long Gully Bush Reserve, and Mākara Peak Mountain Bike Park. The plant's discharge point for treated wastewater is in the coastal marine area of Cook Strait, near the Karori Stream mouth. This discharge point has been in operation since 1934.

The Western WWTP began operating in 1997 and was fully commissioned in September 1998. The plant was granted current consents in July 2010, and they expire 28 July 2035. In general, the consents permit:

• discharge of treated (fully treated and disinfected) and partially treated effluent through an existing outfall,

• discharge of 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.
  
  The Western Wastewater Treatment Plant is currently compliant for effluent quality. If you would like more details please check out our dedicated website page .  

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What’s happening at the plant

Ultraviolet System (UV) Replacement Project
From late November, we’re scheduled to start work on upgrading the UV disinfection system. The current equipment is old and unreliable. The plant’s UV treatment facility will be maintained while the upgrade takes place, with no impacts on the plant’s daily operations.

Electrical, Instrumentation and Controls (EIC) Renewal

The recent transformer outage at the plant has highlighted the ongoing challenges of maintaining assets across the region’s metropolitan wastewater treatment facilities.

A priority for Western is the renewal of outdated mechanical and electrical components; SCADA (Supervisory Control and Data Acquisition), PLC (Programmable Logic Controller) and MCC (Motor Control Centre).

These components are essentially the brain and nervous system of the plant. They monitor, control, and automate processes to ensure safe and compliant operation. Their age and condition makes them vulnerable to unplanned outages.

The good news is we do intend to undertake this key upgrade in early 2026. The EIC Renewal Project does require careful planning so that the plant’s operations are not affected during the change-over period. We’ll keep you informed about the timing of these physical works.

Karori Outfall Pipe

We are committed to minimising plant related impacts on the community and environment where possible. The Karori Outfall Pipe is the main channel for the transfer of wastewater from the plant to the south coast.

It became fully operational again in 2024, after repairs to a section of the pipeline were completed following severe weather. As a result, effluent is discharged to the Coastal Marine Area (CMA) and no longer to the Karori Stream during extreme weather events.

Karori Tunnel Storage Upgrade

Throughout the Wellington region there is increasing pressure on treatment plants to efficiently manage wastewater due to aging infrastructure, population growth and climate change.

One of our key priorities has been to expand network storage capacity to better manage flows into the Western Plant. In 2024, we installed a new outlet pipe and valve, increasing wet weather storage by an additional 1.1 million litres.

This has helped mitigate the impact of high wet weather overflows reducing the risk and frequency of discharges to the Karori Stream. It also acted as a contingency measure during the recent UV system outage helping to manage flows going into the plant.

Karori Storage Tunnel

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Thank you for taking the time to read this update.

Blair Johnson

Head of Wastewater Contracts

Linda Fairbrother
Project Director Major Projects

Asli Crawford

Project Director Major Projects