Concerns in Westport at the potential damaging environmental effect of leachate - a toxic liquid produced when water filters through landfill waste - being pumped into Clew Bay, Co Mayo, were expressed at a Bord Pleanála oral hearing in Westport yesterday.
The total cost of the dual project would be about €16.4 million.
Shellfish farmers are among those objecting to the proposals on the grounds that their livelihoods would be endangered.
In a submission to the hearing, Tony O'Flynn of the Department of Local Government said the key concern for the nature conservation service of the department is the risk of severe negative effects on the EU habitats directive species, freshwater pearl mussel.
He said the proposed changes at the Derrinumera landfill facility required a significant change in the nature and scale of the activities licensed by the EPA under the existing waste licence and would therefore be the subject of a review by the EPA as a separate process.
Evidence from Mayo County Council officials was given to the hearing, which is chaired by planning inspector Daniel O'Connor.
Brian O'Reilly, chartered engineer with the authority, said that at present dewatered sewage sludges are taken out of Mayo by licensed contractors and disposed of in a number of ways, including landspreading.
At present, landfill leachate is taken by a fleet of tankers from the landfill through the streets of Castlebar to the wastewater treatment plant where treatment capacity is limited and is needed for the development of Castlebar itself. The treated waste water was discharged at the confluence of the Manulla and Castlebar rivers into the Moy catchment.
An environmental impact statement (EIS) was commissioned by Mayo County Council due to the sensitive nature of Clew Bay which is a designated shellfish water and also designated under the water framework directive for transitional and coastal waters and is a candidate special area of conservation.
Ecological consultant Chris Emblow told the hearing that in the event of project or mitigation measure failure, the receiving environment could be polluted by toxic substances such that profound and irreversible consequences would occur.
He said that during the construction stage, pollutants and chemicals used could contaminate the area.
Chemical contamination could also occur from accidental spillages, such as oil and other chemicals, through poor operational management, the non-removal of spillages, poor storage, handling and transfer of oil and chemicals.
However, if standard precautions were taken and best practice followed, impacts would be minimal.
Vincent Roche, manager of the North Western Regional Fisheries Board, welcomed the county council's proposals. He said that for almost 30 years, the landfill site had discharged raw sewage into a river which eventually flows into Lough Beltra and on to Clew Bay.
The hearing continues today.
The Irish Times