In 2020, raw sewage was released into the country's rivers more than 100,000 times, according to official figures from Dŵr Cymru.
The discharges occurred across more than 2,000 water treatment works and sewer overflows.
The not-for-profit company, Dŵr Cymru, which is responsible for the network has insisted that it is conforming to environmental regulations.
An investigation by BBC Panorama has claimed to have found evidence that the operator has been illegally dumping untreated sewage into Afon Gwy, Sir Fynwy.
However, Dŵr Cymru has refuted the claim that its sites were operating illegally.
(More to follow...)