Dŵr Cymru Welsh Water has submitted planning applications for a £600 million overhaul of drinking-water infrastructure serving more than 400,000 properties across South Wales.
Featured image: AI-generated illustration © Fish 4 News. It is not an official rendering of the proposed Cwm Taf facilities.
The Cwm Taf Water Supply Strategy, described by the company as the largest water infrastructure project in its history, would replace two ageing treatment works and upgrade a third. The plans were submitted on 27 October 2026 to Merthyr Tydfil County Borough Council and Bannau Brycheiniog National Park Authority.
The proposed infrastructure would support water supplies to Merthyr Tydfil, surrounding communities and parts of Cardiff, including hospitals and schools. The applications have been lodged, but planning permission has not yet been granted.
What is being proposed?
- A new water treatment works at Dan-y-Castell Farm near Merthyr Tydfil, together with treated-water storage and new pipelines.
- A new raw-water pumping station at the existing Pontsticill site.
- Upgrades at Llwyn-onn Water Treatment Works, including additional storage and pipeline connections.
- The eventual disconnection and decommissioning of the older treatment works at Pontsticill and Cantref, once replacement arrangements are operational.
The company says the designs include green roofs and habitat improvements intended to support biodiversity. Details of the effects on local roads, construction activity and the environment will be considered through the planning process.
Why is Welsh Water replacing the facilities?
The existing works have become increasingly difficult and costly to maintain. Welsh Water says major investment is necessary to meet drinking-water standards, improve resilience and reduce the risk of future supply interruptions.
This is also a regulatory requirement, not simply a voluntary expansion. A Drinking Water Inspectorate legal notice, updated in January 2025, requires the company to construct and commission the new Dan-y-Castell works, upgrade Llwyn-onn and physically disconnect Cantref and Pontsticill from the network by 30 September 2032.
The notice identifies historic water-quality risks requiring mitigation. It does not mean the water currently supplied to every customer is unsafe: the regulatory document also sets out ongoing treatment, monitoring and risk-control measures.
How the plans changed after consultation
Welsh Water originally consulted on a larger single treatment works at Gurnos Farm. Following local feedback and further assessment, it adopted the current split-site approach: a smaller new plant at Dan-y-Castell alongside improvements to the existing Llwyn-onn facility.
The company says the revised scheme would reuse more existing infrastructure and reduce the scale of new pipework and pumping facilities compared with its earlier proposal. Local communities will have further opportunities to comment as the applications are considered.
What happens next?
The proposals must pass through the relevant planning processes before construction can proceed. Welsh Water’s indicative programme places construction between 2028 and 2032, subject to permissions, procurement and delivery.
The company is engaging with investors, lenders and engineering firms before planned procurement through Ofwat’s Direct Procurement for Customers model. It has also announced a supplier event at ICC Wales, Newport, on 10 November 2026.
The headline £600 million figure is the company’s stated programme estimate. The final delivery arrangements and any precise effect on household bills have not been established by the announcement.
