Dŵr Cymru Welsh Water is accelerating development of the £667.2 million Cwm Taf Water Supply Strategy, the largest infrastructure scheme in its history and one of the most substantial drinking-water investments currently underway in the UK. The programme will replace and upgrade ageing treatment assets across the Merthyr Tydfil region — some dating back nearly a century — to ensure long-term water security for almost half of Welsh Water’s three million customers.
The strategy delivers a split-site solution designed to modernise the network while reducing environmental and community impacts. At its core is the Dan-y-Castell Water Treatment Works, a new facility valued at £362 million (excl. VAT) and located on a screened brownfield site near the A465. Complementing it is the £194 million (excl. VAT) upgrade of the Llwyn-onn Water Treatment Works within Bannau Brycheiniog National Park. Together, these two components total £556 million before VAT, aligning with the scheme’s £667.2 million overall value once VAT is included.
The Dan-y-Castell works will introduce modern treatment processes — including solids removal, filtration, manganese treatment and disinfection — alongside buried treated-water storage and wash-water recovery systems. A new raw-water pumping station at Pontsticill will support the transition and eventually enable the decommissioning of older facilities.
The Llwyn-onn upgrade, the only existing site suitable for expansion, will combine repurposed buildings with new structures designed to blend into the landscape. It will assume the raw-water treatment role currently met by the Cantref works, boosting capacity and resilience under future climate scenarios.
While project development continues, Welsh Water has also launched statutory pre-application planning consultations to gather views from local residents and stakeholders ahead of planning submissions.
Extensive environmental surveys — including ecology, hydrology, heritage and landscape studies — are shaping the design, with sustainability features such as solar generation and low-energy process layouts under review. Construction is expected to begin in 2027, subject to planning approval, with regulatory completion targeted for 2032 followed by a year of testing before full operation.
