The River Liffey underwent a €20 million spring-clean yesterday when new sewage treatment works were formally commissioned.
The new facilities form an extension to the existing waste water treatment plant at Oberstown, Co Kildare, which discharges effluent from much of the county into the upper Liffey valley.
The project, which began in 1998, doubles the capacity of the plant which is to be further extended under a second phase of investment by 2005.
Oberstown had long been considered inadequate to cope with the county's fast-growing population.
Environmentalists and anglers had blamed the overloaded plant for pollution and fish kills in the Liffey in Kildare and Dublin in recent years.
The stretch of the Liffey below Oberstown had also become a regular target for criticism in the Environmental Protection Agency's annual water quality reports.
Kildare County Council said yesterday the new facilities would result in effluent of a quality that was "remarkably close to that of the river water in its natural state".
Testing laboratories have been built on site to allow more efficient monitoring and water quality control.
The chairman of Kildare County Council, Mr John O'Neill, said the new facilities would protect public health, allow the enjoyment of the Liffey for amenity purposes and provide for further growth in the county's residential and industrial population.
"The Liffey is a precious resource and preservation of its quality forms a key part of the County Council's environmental policy," he said.
The original Oberstown plant was built in 1981 to serve a catchment area of 40,000., including Naas, Newbridge, Clane, Sallins, Kill and Johnstown.
Following yesterday's switch- ing-on ceremony, performed by the Minister for Finance, Mr McCreevy, the plant has the capacity to take in Kilcullen and Prosperous and to handle waste from a total population of 80,000. The next phase of development will increase its capacity further to 130,000.