Irish Water has said it cannot justify extra treatment of discharges from Dublin’s Ringsend sewage plant outside the traditional bathing season, which has been sought by year-round sea swimmers.
Tests on more than 3,000 bacterial samples following ultraviolet (UV) light disinfection between September of last year and January did not show sufficient improvement to support the additional costs of operating the system during the winter, which ran to more than €400,000, it said.
The presence of bacteria such as E. coli, associated with complaints of vomiting and skin infections following off-season swimming, could be attributable to “near-shore pressures” such as dog faeces, run-off from lands and misconnected houses and businesses, Irish Water said.
Minister for Housing Darragh O’Brien and Minister of State at the Department of Housing Malcolm Noonan last year asked Irish Water to extend use of the UV system, which disinfects the water, beyond the traditional bathing season to determine if it could improve bathing water quality. The request was in response to a growing number of year-round swimmers reporting illness, possibly associated with dips in Dublin Bay.
Irish Water will on Thursday publish the results of the four-month trial of the UV system, which included the assessment of 3,100 bacterial samples from 15 locations, including Sandymount, Dollymount, Bull Island and Merrion Strand.
“Detailed analysis of the data collected did not demonstrate any material improvement in bathing water quality at bathing locations in Dublin Bay as a result of the operation of the UV system at the Ringsend [plant] during the winter period,” it said.
“The results were independently peer reviewed and endorsed by the UCD Acclimatize project, which found the data set that was developed was both comprehensive and appropriate for the assessment.” The Acclimatize project is researching pollution sources at urban and rural bathing waters.
The findings were, Irish Water said, consistent with previous analyses carried out by UCD and Dublin City Council which identified “near-shore pressures on bathing waters” as the primary reason for “occasional failures” in bathing water quality.
“Near-shore pressures on bathing water quality can include run-off from agricultural land and roads, urban wastewater, dog and bird fouling, and misconnected houses and businesses,” Irish Water said.
“In the absence of a demonstrable improvement in bathing water quality, the significant financial and carbon cost of operating the UV system outside of the bathing water season cannot be justified.”
Irish Water said it will continue to operate the UV system at the Ringsend plant during the bathing water season from June to September in accordance with the requirements of its wastewater discharge licence.