Objectors have warned that a €500 million sewage plant at Clonshaugh will result in Fingal’s “coastal goldmine” being transformed into an “environmental nightmare”.

An oral hearing on the controversial Irish Water proposal which started last Wednesday has heard from more than 100 people vehemently opposed to it.

In its submission on day one of the planning hearing, Irish Water revealed that it plans to use ultraviolet light to disinfect waste to tertiary treatment standards.

Consultant Ciaran O’Keeffe said the decision was taken in light of concerns raised by Howth fishermen and others over the outflow into the Irish Sea near Ireland’s Eye.

In an impassioned plea, Independent TD Tommy Broughan said running the outfall pipe to sea through the Portmarnock peninsula into Baldoyle Bay was “unconscionable”.

He argued that it was “not now remotely feasible” to tunnel a sewer pipe through a unique marine biosphere given UNESCO conservation status in 2015.

Deputy Broughan said Irish Water must go back to the drawing board and change its 2012 proposal in light of the UNESCO designation.

To applause, he insisted that the plan for Clonshaugh and Baldoyle Bay “should be abandoned forever”.

A Malahide resident who addressed the planning board warned: “If this project gets the go-ahead it could very easily transform our coastal goldmine into an environmental nightmare.”

The €500 million Clonshaugh site, on a 75-acre site, would take four years to build and represents the second largest wastewater scheme in the country after Ringsend.

The outfall pipe would discharge 1km from Ireland’s Eye and less than 4km from the Blue Flag Velvet Strand beach in Portmarnock.

Irish Water says it is needed to take waste from more than 500,000 households in Dublin, Meath and Kildare as the population expands.

Plans include a pumping station at Abbotstown in Blanchardstown and a 25km orbital sewer pipe.

Deputy Darragh O’Brien (FF) presented a submission lodged jointly with his brother, Cllr Eoghan O’Brien, predicting that the project could bring “disaster”.

The hearing is due to conclude this week with An Bord Pleanala expected to issue its ruling in July.