Regulator Ofwat has approved a £44.7million enforcement package against Dŵr Cymru Welsh Water following "serious failures" in the company's sewage and wastewater operations.
The settlement, which was initially proposed in March, was formally confirmed on Friday following a public consultation period.
The package includes £40.6million allocated to reducing sewage spills at specific overflow sites and addressing environmental damage linked to the company's wastewater operations.
A further £4.1million will be invested in improving water quality and biodiversity within sensitive river catchment areas.
Welsh Water has also been ordered to carry out sealing works on private sections of the sewer network to tackle groundwater infiltration, which regulators identified as a major cause of frequently spilling overflows.
Ofwat's investigation found the company had failed to properly operate, maintain and upgrade its wastewater infrastructure to cope with the volume of sewage moving through the system.
The regulator also concluded the utility lacked adequate operational processes and sufficient oversight from senior management to ensure the network could manage demand effectively.
According to Ofwat, these failures resulted in excessive sewage discharges into the environment.
Welsh Water previously acknowledged the findings of the investigation and apologised for falling below standards expected by customers and regulators.
The company said it invested £617million into its water and wastewater network during 2025-26 as part of a longer-term programme to improve environmental performance and strengthen ageing infrastructure.
Lynn Parker, Ofwat senior director for enforcement, said: "Our investigation found serious and unacceptable breaches in how Dŵr Cymru Welsh Water has operated its wastewater assets which has resulted in excessive spills to the environment."
Ms Parker added: "With this investigation now concluded, we expect the company to focus on putting things right so that customers can regain trust in their water company."
She said Ofwat would continue holding water companies accountable for environmental and operational failings across the sector.
The £44.7million enforcement package exceeds the £40million fine Ofwat said it would otherwise have imposed on the utility.
The regulator said directing the money towards environmental improvements rather than returning it to HM Treasury would deliver greater benefits for customers and affected communities.
Ofwat confirmed the measures are legally enforceable and that delivery of the commitments will be monitored by the regulator.
The case represents the seventh conclusion reached as part of Ofwat's wider industry investigation into wastewater practices across the water sector.
Welsh Water will be required to complete the redress measures between 2025 and 2030 as part of investment commitments beyond those agreed during the 2024 Price Review.
Ofwat said the costs of the package must be absorbed entirely by the company and cannot be passed on to customers through increased bills.
The regulator also confirmed Welsh Water must address all identified breaches and demonstrate future compliance with its legal obligations.
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