All sewerage systems in Dublin city and most systems in the greater Dublin region have reached or exceeded their capacity and require "immediate" upgrading, the Greater Dublin Strategic Drainage Study has found.
The sewerage network, parts of which are more than 100 years old, has become "completely overwhelmed" as a result of the population growth and expansion of Dublin city.
The system is unable to deal with the volumes of sewage now produced, but its capacity is also diminished by leaks in older pipes that allowed ground-water to infiltrate the system, the study found, resulting in frequent flooding and sewage spills at times of heavy rainfall.
The upgrade of the eight sewage systems in the greater Dublin region is predicted to cost about €727 million of the €2.6 billion estimate for the entire drainage and sewerage needs of the region.
Most of the money, just over €500 million, will be spent in the first six years to relieve the most serious problems.
The systems presenting the most urgent upgrading needs are the city centre and Docklands system, the Grand Canal system, which runs to the west of the city, and the Rathmines and Pembroke system, all of which have exceeded their capacity.
The Dodder Valley sewer, serving south Dublin, is at capacity, with the risk of local flooding. The north Dublin sewers have capacity "for the foreseeable future" in the north fringe area; however, the older sewers nearer the city are at risk of flooding.
The Dún Laoghaire system is limited to the capacity of the pipeline to Ringsend and will require new sewage storage capacity, the report said.
The Shanganagh and Bray systems require upgrading to cope with future predicted development of the area, as do the Oberstown and Leixlip systems in Kildare.
The main city centre sewage system, the city centre and Docklands system, has exceeded its capacity, leading to "excessive spills" causing pollution of the river Liffey during rainy periods. Serious on-street flooding is also caused by heavy rain.
This system includes major pipelines from Parkgate Street to Eden Quay and Heuston Station to Burgh Quay and takes sewage flows from south of the River Tolka, with nearly 100 combined sewers.
The Grand Canal system serves around one-fifth of the population of the greater Dublin area. Described as the backbone of the city's drainage network it has two major branches, Lucan/Clondalkin and Blanchardstown.
Both have exceeded their capacity, with the Lucan branch needing upgrading as "a matter of urgency", while the Blanchardstown catchment is "completely overwhelmed".
The Rathmines and Pembroke system extends from Harold's Cross in the west to Ballsbridge in the east and includes Rathmines, Ranelagh, Rathgar, Miltown and Donnybrook. Some sewers in this area, which are the subject of ongoing maintenance work, are in "very poor condition", the report said. Flooding is worst in Ranelagh, Rathmines and Donnybrook.
The drainage study, commissioned by the four Dublin local authorities and including adjacent catchments in Meath, Kildare and Wicklow, was conducted over five years by a consortium led by Hyder Consulting (formerly Welsh Water) and Dublin drainage and engineering firms, McCarthy Consulting and MCOS.
It aims to address the region's short-term needs to 2011 and in the long term up to 2031, with projected costs of €2.6 billion.
