Sewage overflows at Irish Water's Ringsend water treatment plant happen around 15 times a year, but mostly in winter, the utility has said.
The entire south Dublin coastline has been made off limits to bathers until next Wednesday following a sewage overflow at the plant due to heavy rainfall during the week.
Dun Laoghaire-Rathdown County Council and Dublin City Council have issued warnings not to swim at the following beaches; Dollymount, Sandymount and Merrion Strands, Seapoint Beach, Sandycove Beach, The Forty Foot Bathing area, Killiney Beach and White Rock Beach.
Irish Water said there was a “storm water overflow” from the Ringsend plant for two hours over the course of Tuesday and Wednesday.
A spokeswoman for the utility said overflows happen at Ringsend on average around 15 times a year “but mostly in winter, outside the bathing season, when the rainfall is heavier”. She said “there is potential” for an overflow to occur again over the summer period if there is “extreme rainfall”.
“It’s a function of all waste water treatment plants all around the world. It wasn’t a failure of the plant or a mechanical breakdown. This happens as it should happen,” the spokeswoman added.
Sean Laffey, head of asset management at Irish Water said the overflow from the storm water tank "would have been a very highly dilute settled sewage from the networks that came out of the Dublin area".
"All of our treatment plants have a particular design capacity. In the case of Ringsend, the maximum flow it can take is about 11,000 litres a second. Once that flow is reached, anything in excess of that is taken off into a storm water holding tank and normally what happens is once the rain has passed, the contents of the storm water tank are fed back into the plant for treatment and discharged as normal," he told RTÉ Radio 1.
“During exceptional rainfall events the storm water tanks get full. We don’t get an opportunity to pass it back into the plant so what we do is we settle the contents of the storm water tank, we screen it for debris and then we discharge it back out into the environment and that’s what happened.”
Mr Laffey said the Ringsend water treatment plant “operated exactly as it was designed to operate”.
“These type of overflows are best practice across the world. The alternative is that you build your plant big enough to take any rainfall event. From an investment and value for money point of view, that’s just not a runner for us,” he added.
Water quality samples have been taken at Dollymount, Merrion and Sandymount beaches on Friday and results are expected on Monday.
“The prohibition will be in place until a clear set of results is obtained for the bathing waters,” a Dublin City Council spokeswoman said.
“While we cannot predict the results of water quality samples, we would expect the prohibition on bathing to be in place for a few days.”
Dun Laoghaire-Rathdown County Council also said sampling will be taking place at the other affected bathing areas.
“It is HSE advice that aside from bathers and beach users any organisers of any coastline or water based leisure or sporting event along the Dún Laoghaire-Rathdown coastline should pay particular attention to this notification,” a statement from the council said.
Irish Water said bathing restrictions are in place for the entire 2019 bathing season at Sandymount and Merrion beaches “due to general water quality”.
