An investigation has been launched into Welsh Water after the company admitted illegally spilling untreated sewage.

Natural Resources Wales said it was examining the severity and frequency of permit breaches by the water company.

Welsh Water said between 40 and 50 of its wastewater treatment plants are currently operating in breach of their permits after being shown data by an environmental campaigner.

We are aware of significant compliance issues at the Cardigan wastewater treatment works, and we have used our regulatory powers to enforce the improvements required at the site over the years

Analysis by Professor Peter Hammond, from campaign group Windrush Against Sewage Pollution, showed the worst offender is the Cardigan plant in west Wales, which spilled untreated sewage on 1,146 days from the start of 2018 to the end of May 2023.

The plant discharges into the River Teifi, which flows into Cardigan Bay – the home of Europe’s largest population of bottlenose dolphins.

Prof Hammond’s analysis of data over a five-year period showed there were 2,274 days when untreated sewage was released in breach of a permit.

Water treatment works are allowed to discharge untreated sewage in order to ensure it does not become overwhelmed during heavy rain, but this overflow level is regulated by permit.

Welsh Water said it plans to build a new £20 million water treatment works for Cardigan as part of a £40 million package in the county.

We have a longstanding record of being open and transparent in relation to our environmental performance

Ann Weedy, operations manager at Natural Resources Wales, said: “We are aware of significant compliance issues at the Cardigan wastewater treatment works, and we have used our regulatory powers to enforce the improvements required at the site over the years.

“The issue is now unresolvable without significant investment and upgrading of infrastructure by Welsh Water. But this is the kind of investment that we and the public expect.

“The enforcement action we are taking at this site requires Welsh Water to commit to this programme of work during their next investment period and we will work with them to drive the improvements at this site that we all want to see.

“We are currently investigating the severity and frequencies of the permit breaches at the site and this investigation is ongoing.”

A Welsh Water spokeswoman said: “We have a longstanding record of being open and transparent in relation to our environmental performance.

“We have applied this approach to compliance with our treatment works permits and have always raised any areas of non-compliance with our regulator, Natural Resources Wales, as soon as they are identified.

“We have extensive monitoring arrangements, amongst the most extensive in the sector, which we use to monitor our sites.

“Each site presents its own and sometimes very complex challenges, with NRW reviewing the information we provide, agreeing appropriate timescales for resolving the issues, and taking enforcement action where appropriate.

“We always aim to resolve any issues as quickly as possible, to limit any impact for both the environment and our customers.”

An Ofwat spokeswoman said: “We are currently investigating all 11 water and wastewater companies, including Welsh Water, for potential failures at sewage treatment works that may have led to sewage discharges into the environment.

“We have contacted Welsh Water to provide more information which will feed into our existing investigation. We are also liaising with Natural Resources Wales on this issue.”

For this to be described as the worst sewage works in terms of illegal discharges shows how Labour has dropped the ball on sewage in Wales

Politicians are calling for a review of Welsh Water and called for tougher action against sewage dumping.

Janet Finch-Saunders, the Welsh Conservative shadow climate change minister, said: “For this to be described as the worst sewage works in terms of illegal discharges shows how Labour has dropped the ball on sewage in Wales.

“Welsh Water has broken the law and must face the consequences.

“At a time when Welsh Water are planning to increase bills, even though their customers experience the second highest average bill in Wales and England, is completely unacceptable when they have been misleading the public for years and dumping harmful sewage into our waterways.

“The Labour Government need to follow the lead of the UK Government and start holding this disgraceful dumping to account by fining and start prosecuting Welsh Water, their inaction is harming our environment and waterways.”

This is a shocking admission and the Welsh Government can no longer dodge the issue

Jane Dodds, a Liberal Democrat MS, added: “This is a shocking admission and the Welsh Government can no longer dodge the issue,” she said.

“We have the second highest bills across the whole of England and Wales, company bosses have been awarded thousands in bonuses and at the same time have been illegally dumping sewage into our rivers.

“I have repeatedly led calls in the Senedd for tougher action on sewage dumping and for a full review of Dwr Cymru and its operation.

“The Welsh Labour Government must take those calls seriously or risk the future of our rivers.”