Yes, yes it is a big system, which should have been carefully prepared and tested for at least one or two years and they did it in 2 months.
A leak was found on a joint of PVC pipes that transport the treated water.
In earlier Yomiuri article, 4 kilometer long PVC pipes feed the treated water to the reactors, and TEPCO people were worried about the leak all along.
I guess they were too preoccupied with Kurion's zeolite vessels (which require frequent change and system downtime) and didn't have time to test the 4 kilometer long network of PVC pipes.
From Yomiuri Shinbun (1:31AM JST 6/28/2011):
TEPCO started the cooling of the reactors using the water treated by the contaminated water treatment system at 4:20PM JST on June 27.
However, a water leak from the pipe joint of a water feeding pipe was discovered, and the water injection into the reactors were stopped after one and a half hour. TEPCO will investigate in the morning of June 28.
According to TEPCO, the contaminated water treatment system itself continues to operate. Cooling of the reactors is unaffected, as they are using the water from the storage tanks as before.
A leak was found on a 10-centimeter diameter PVC pipe. It is a pipe that feeds fresh water to the pump that pumps water into the reactors. A worker who went to adjust the flow rate found the leak. TEPCO says the leaked water was treated, and there is no effect on the work at the plant.
To clarify, this "fresh water" was made "fresh" by going through the 4-step contaminated water treatment system.