Western Wastewater Treatment Plant Refurbishment Projects

The Wellington region’s wastewater treatment plant assets are vulnerable and require regular maintenance to continue operations.  

For example, mechanical and electrical assets are at an age which make them more challenging to operate and maintain. These risks are currently being addressed through a programme of work to renew assets. This activity requires careful planning and implementation to avoid disruption to daily plant operations.

A priority at the Western Wastewater Treatment Plant is to renew and upgrade the outdated and unsupported SCADA (Supervisory Control and Data Acquisition), PLC (Programmable Logic Controller) and MCC (Motor Control Centre).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 and are key to ensuring plant compliance with current standards.

A carefully planned programme is underway to upgrade these components with completion expected by 2027.

Why does the Plant need upgrades?

Much of the plant’s equipment and parts are nearing the end of their service life, which means its more prone to service interruptions and needs more maintenance.

A multi-million-dollar programme of work is planned over the next seven years to renew and upgrade equipment at the plant to improve its operation, subject to funding being confirmed. This programme includes improving the reliability and effectiveness of the treatment, increasing capacity to handle population growth, and improving safety.

Ultraviolet (UV) System Renewal

Wellington Water is replacing old Ultra-Violet (UV) equipment at the Western Wastewater Treatment Plant in Karori, Wellington. Ultra-Violet (UV) treatment uses lamps to emit ultraviolet light to deactivate and disinfect bacteria, viruses and protozoa. It’s the final stage of the wastewater treatment plant and an important part in putting safe, healthy water back into the environment.

The Western WWTP project is in addition to two other, similar projects at Moa Point and Seaview Wwastewater Treatment Plants.  

The existing UV disinfection system was installed when the plant was originally commissioned in 1998, approximately 25 years ago. For this type of electro-mechanical equipment, 25 years is considered to be at the end of its useful life.  

The new UV equipment will deliver: 

• Greater dose efficiency and control (and therefore improved compliance) 
• Better design for access for maintenance 
• Improved cleaning system (reduced need for operator input). 

Works start in January 2026 and are expected to be completed by April 2026. During this period, the plant’s UV treatment facility will continue with no other impacts on Plant operations. 

Sludge Management Facility

Wellington City Council is managing a project to build a new Sludge Management Facility. Find out more on the Wellington City Council website here 

Wellington Water is replacing the ultraviolet (UV) disinfection system, which is the final stage of the wastewater treatment process. The current system is nearing the end of its serviceable life and needs to be upgraded to ensure the plant continues to operate safely and effectively.

This necessary upgrade is to avoid the risk of a system failure. The function of the UV system is to disinfect treated wastewater before it is discharged into the ocean via the Karori long outfall pipe.

Construction is scheduled to begin in January 2026 and is expected to continue through to April 2026.

The long term benefits of this UV system upgrade are that it will improve the plant’s performance and resilience and provide more efficiencies with the UV disinfection of treated wastewater. These upgrades will reduce the risk of equipment failure and support better water quality outcomes for the Karori Stream and wider environment.

The treatment plant's capacity to treat wastewater will not be affected nor will the current UV system which will continue to operate as normal. 

There have been some issues with the UV system which was a fault with a transformer that powers the UV system. The transformer was rebuilt and UV system was brought back online. It took longer than expected to fix as once the transformer was rebuilt there was another electrical issue with fuses in the main network line supplying the plant. This issue has been resolved and the UV system is operating again at full capacity. 

The plant is currently compliant and meets all guidelines for wastewater quality analysis and effluent monitoring. But during wet weather the plant can reach capacity for treating inflows. To prevent flooding inside the plant, intermittent discharges of treated wastewater to the Karori Stream can happen during peak flows.

Find out more about the Western Wastewater Treatment Plant.