WATER difficulties at the Ballymore Eustace treatment plant first bubbled to the surface more than three weeks ago.
Engineers managed to get the problem under control twice.
However, last weekend the problem escalated to a point where it could no longer be contained.
Padraig Doyle, Divisional Engineer for Water Services at Dublin City Council, said initially the problem occurred "at quite a low level and was resolved."
"It happened again at a low level and was resolved again but it got really bad last weekend," he told the Herald.
He explained that when water comes down off the mountains and from the rivers into the reservoir the first part of the treatment process is to remove big items like leaves and branches.
Then water is moved to tanks where two chemicals are added.
"These combine with the particles like sand, grease and smaller vegetation matter to form a kind of sludge -- a bit like wallpaper paste," he explained.
The tanks are up to 15ft deep and the 'sludge blanket' floats about halfway up the tank where there are pipes to suck it out.
The cleaned water is then run through sand filters before being sent out to consumers.
Mr Doyle said that unfortunately at the moment the 'sludge blanket' is floating to the top of the tank and some of it is ending up in the sand filters and clogging them.
This has reduced capacity at the plant from about 310 million litres of treated water a day to about 210 million litres.
The engineer pointed out that no-one knows why the 'sludge blanket' is floating so high.
"We don't know what's causing this but our emphasis is on what we are going to do to solve the problem," he said.
The solution lies in finding the proper chemical mixture to add to the water to make the sludge drop.
He said that multiple tests are being carried out to find the right solution.