Winter swimmers would not benefit from extra Ringsend sewage plant cleaning

Dog faeces, land runoff and badly connected houses to blame for Dublin Bay pollution

Irish Water’s decision not to extend its disinfection of effluent from Ringsend sewage plant using UV light beyond the summer season will likely come as a disappointment to year-round sea swimmers.

These hardy souls take the plunge in all weathers and all they want is to have a dip in January without ending up with a nasty rash or vomiting bug.

Irish Water has concluded, however, that extending the use of the UV system, which involves employing ultraviolet lamps to irradiate the effluent and kill bacteria, will not help them in the winter.

It would be hard to argue with the science. Irish Water collected more than 3,000 bacterial samples over a four-month period from 15 sites, including designated bathing waters and other places popular with swimmers, in what it says was one of the largest microbial sampling programmes ever completed in Dublin Bay.


In addition to its own research, Irish Water examined historic and current bathing water quality sampling data from Dublin City Council and Dún Laoghaire Rathdown County Council. Information was sourced from Met Éireann, the Environmental Projection Agency and the Office of Public Works. Further sampling research was provided by the UCD Acclimatize project, which is researching the sources of pollution in bathing waters.

The UCD team also reviewed and endorsed the findings of the Irish Water report which concluded there was “no discernible benefit to winter bathing water quality” through the use of UV treatment and the “failure to achieve good bathing water quality is not driven by discharges from Ringsend”.

The report does, however, state that Irish Water would continue to use the UV system during the summer season. Which begs the question of why the system would work at one time of year, but not another?

Ronan Kane, the environmental modelling specialist who oversaw the trial, said there were “very different conditions meteorologically during the winter months” which would influence UV use.

“We have increased rainfall which makes other sources of bacteria pollution more significant in winter months and we have shorter daylight hours, which means there is less natural decay of bacteria that occurs due to UV radiation. So it is a very different environment between summer and winter.”

It is also the case that Irish Water is required under its operations licence to use UV treatment in the summer, so it hasn’t had to opportunity to switch it off to determine if it is making a material difference during the bathing season.

The cost of running the UV for the trial was €400,000 in January 2021 electricity prices, but while this will now be significantly dearer, would it not be worth doing, just in case it might make some improvement? No, Kane says.

“The data tells us that it wouldn’t make any difference, so I don’t think it would represent a belt and braces approach.” Financial considerations were not part of study, he says.

What the trial did achieve, he says, was to add to the store of information on the “other sources” of bacterial pollution, including storm water overflows, pipes from houses misconnected into the drainage rather than the sewage system, and animal faeces.

Earlier this year, Dublin City Council and UCD reported dog faeces was the most frequently detected faeces in Dublin’s bathing water, with one animal having the ability to contaminate a water body the size of a tennis court.

Those who pooh-poohed this finding might have been hoping for a different outcome from the Irish Water research, as there appears to be little appetite to tackle irresponsible dog owners.