The Environmental Protection Agency has begun an investigation into discharge into Dublin Bay at the Ringsend Wastewater Treatment Plant.
The overflow, which was blamed on the breakdown of a processing tank, led to discharge that began at 9am on Saturday and was still occurring when photographed by an amateur drone operator at 5.45pm that evening.
The Ringsend plant is operating at 20% over capacity treating sewage for a population of 1.9 million people, instead of the 1.6 million it was designed to take.
As a result of the overload the plant has failed to meet EU urban waste water standards.
It has experienced overflows before such as in October 2017 during Storm Brian.
In the years after it opened in 2005, residents in neighbouring areas like Sandymount experienced foul smells emanating from the plant.
Irish Water last year announced a €80 million investment as part of a €400m total upgrade to cater for the extra demand.
It will use nutrient-reduction treatment technology known as Aerobic Granular Sludge (AGS). This will require a biosolids storage facility to be built in Dublin 11 subject to planning permission.
An Bord Pleanála recently announced that it will hold an oral hearing on 20 March into plans for another large sewage treatment plant in Clonshaugh, north Dublin to cater for future population growth there.
The population of the Greater Dublin Area is expected to grow from its present level of 1.9 million to 2.2 million by 2031.
In a statement, Irish Water said a failure at one of the tanks at Ringsend caused a "discharge of sludge" into the Lower Liffey estuary through an outfall located around 1km from the plant.
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