The firm pleaded guilty yesterday at Llandudno Magistrates Court to causing the pollution incident in 2018 in which around 9km of the river Clywedog was affected.
The court found that Welsh Water illegally discharged raw sewage from the Five Fords wastewater treatment plant, leading to the death of more than 3,000 fish, including brown trout, bullhead, lamprey, stone loach, salmon, parr, chub, eel, stickleback, perch and minnow.
Natural Resources Wales (NRW) operations manager David Powell said: “Pollution incidents like this can devastate ecosystems and we hope that this outcome will send out a positive message that NRW will not tolerate those who pollute Wales’ rivers, damaging the environment and risking harm to the local wildlife.”
He added: “NRW has worked closely with Dŵr Cymru Welsh Water to restore the river following this incident. I hope this fine will send out a clear message that environmental legislation is to be taken seriously and that damaging the environment, whether deliberately, or out of negligence, comes with consequences.”
Welsh Water said in a statement that it takes its responsibility for protecting the environment in its care seriously "but on this occasion we recognise we fell short and for this we are very sorry".
It said the incident took place while emergency maintenance work was being carried out, "which accidentally led to partially treated wastewater overflowing from one of our storage tanks for about one hour into the river Clywedog".
"Unfortunately, we were unaware this had occurred while undertaking the work, however as soon as we realised we took immediate steps to try and minimise its impact. We also liaised with NRW on the matter and on any additional steps we could take to minimise the impact on the stretch of river affected," it said.
It pointed out that it entered a guilty plea at the earliest opportunity.
"We welcome that it was acknowledged that we are a responsible company and were fully open and transparent during the course of investigations and would like to assure our customers that we have introduced robust processes at the works to minimise the risk of the same thing happening in future," it added.