Future housing development in Dublin will be limited unless there are more sewage facilities according to Irish Water.
The company is making its case at an oral hearing into a proposed new wastewater treatment plant in Clonshaugh near Dublin Airport.
Head of Asset Management at Irish Water Sean Laffy told An Bord Pleanála that the plant is needed to cope with an estimated population growth of 450,000 in the Greater Dublin Area by 2040.
Irish Water is applying for planning permission and a compulsory purchase order to build a 13km underground orbital sewer from Blanchardstown to the planned wastewater treatment plant at Clonshaugh that will intercept existing flows to Ringsend.
It wants to also build a 12km outfall pipeline to bring the treated wastewater from there to Baldoyle and out to sea for discharge around 1km north east of Ireland's Eye.
The water authority said sea conditions will allow for optimal dispersal and it will not have a perceptible impact on coastal waters or interfere with bathing areas.
Mr Laffy said that the wastewater load had exceeded treatment capacity in the region back in 2002 and even with a planned upgrade the existing Ringsend plant will reach capacity around 2025.
A previous study completed in 2008 warned of future development constraints unless a new wastewater treatment plant was built.
He said the proposed treatment plant at Clonshaugh which will cover nearly 30 hectares would cope with projected population growth until 2050.
Mr Laffy said there constraints preventing further development at existing plants in Ringsend and Leixlip.
And he said a new single plant is preferred to multiple smaller sites because it has more benefits in terms of odour control, sludge treatment and energy recovery.
However, campaigners against the plan who represent local residents, farmers, water sports enthusiasts protested outside the hearing accompanied by local politicians.
Spokesperson Bette Brown pointed out that the proposed plant will not give the same level of treatment as Ringsend.
Catherine McMahon, representing local sea swimmers, said there are fears the planned outflow near Ireland’s Eye would affect Fingal’s only Blue Flag beach at Portmarnock.
Irish Water’s project manager Ciarán O’Keefe told the hearing that it had been decided to apply Ultra Violet disinfectant treatment to the effluent before discharge at sea.
He said this followed submissions from fishermen and advice from a marine ecologist that this would provide an "abundance of caution" to ensure protection of shellfish.
The hearing before Inspector Mairead Kenny is expected to last over two weeks.