Campaigners are gearing up for a legal battle to prevent a giant sewage plant from being built in north Dublin.

An Bord Pleanala is due to rule on the project next week – with objectors vowing to take their fight all the way to the European Courts if they have to in order to stop it.

The Greater Dublin Drainage Project, which Irish Water says is a vital piece of infrastructure to take waste from 500,000 households in Dublin, Kildare, Meath and Wicklow, was the subject of an oral hearing.

The controversial scheme includes a €500 million sewage plant at Clonshaugh, an orbital sewer from Blanchardstown and an outfall pipe through the protected Baldoyle Estuary into the Irish Sea.

If it proceeds, it will have the capacity to dump 300 million litres of wastewater every 24 hours just off Ireland’s Eye – something experts warn will have a catastrophic effect on the Dublin Bay Biosphere.

Campaigners from across Dublin have united in their opposition with a panel of experts including marine and microbiologists ready to argue their cause.

Sabrina Joyce, of the Environmental Conservation and Habitats Organisation, said Irish Water needs to go back to the drawing board.

She told Dublin Gazette: “Our experts have come up with a very workable solution and we feel they need to look at that.

“We’re ready for a judicial review in the High Court and, if needed, we will push it on to the European Court which will say ‘you need to protect your coastline’.

“They [the project’s developers] got it wrong. The whole project was rushed through. They chose this route because it costs the least amount of money, with the shortest length of pipeline.

“But it will destroy habitats and marine life. It’s not just our back garden – this is the Irish Sea we are talking about.”

During a four-day oral hearing, planners were told that dumping wastewater – which will include heavy metals from industrial sources – will devastate a vital and fragile marine eco-system.

The Environmental Protection Agency has identified water treatment plants as being among the main sources of micro-plastics in our waters.

Objectors argued that between Ringsend and the Clonshaugh proposal, sewage from more than two million people will exit off our coastline – a disproportion which is “hugely unjust to the people of Dublin”.

The Mayor of Fingal, Cllr Eoghan O’Brien (FF), who made a joint submission with his TD brother Darragh O’Brien, said he is hopeful that common sense will prevail.

He told Dublin Gazette: “There are very strong planning grounds as to why it shouldn’t go ahead. I hope An Bord Pleanala will take those on board and adjudicate accordingly.”