Upwards of 100 objectors are expected to address an oral hearing into a planned €500 million sewage plant and orbital sewer pipe in Fingal.

The An Bord Pleanala hearing on the Irish Water project got under way at the Gresham Hotel in Dublin on Wednesday and will last until at least next Tuesday.

It is feared that the Greater Dublin Drainage Project, which involves tunnelling under UNESCO- and European-protected sites, will pose a health and safety risk and damage marine and wild life.

The monster plant at Clonshaugh close to Dublin Airport will treat sewage to secondary level, which does not kill all bacteria, as opposed to tertiary level, which is the case at Ringsend.

Objectors to what will be the State’s second-largest sewage plant include Environment Minister Richard Bruton, Fianna Fail deputies Darragh O’Brien and Sean Haughey, and Sinn Fein TD Louise O’Reilly.

Also opposed to the plans are Minister of State Finian McGrath, TDs Clare Daly and Tommy Broughan, and Social Democrats Cllr Cian O’Callaghan.

Irish Water will address the oral hearing first and is expected to outline its case for the project which aims to take waste from up to 500,000 households across Dublin, Meath and Kildare.

It has said that the plant, which is due to be operational by 2026, will be a “vital development to protect public health, safeguard the environment and support the sustainable social and economic growth of the Greater Dublin Area”.

The orbital sewer, which will also take some waste from the Ringsend facility, will involve tunnelling pipes about six feet in diameter from a site close to the National Aquatics Centre in Abbotstown, Blanchardstown.

These will travel across the city to Belcamp with the outflow pipe from the plant crossing Baldoyle Estuary to the Irish Sea, close to Ireland’s Eye