COLUMBUS - Columbus Water Works is investigating what caused a major spill on the Chattahoochee Riverwalk a week after heavy rainfall caused the same 72-inch sewer line to dump 1.1 million gallons of untreated sewage and rainwater into the river.

The latest spill, totaling 64,000 gallons, was reported at 10:53 a.m. Thursday. An employee taking water samples along the river noticed sewage flowing from a manhole cover.

"While he was standing there, it was gurgling up over the manhole," said Jim Patterson, vice president of communications at the Water Works.

Construction was under way at the waste water treatment plant about a quarter-mile away when crews had to shut off the electric power and switch to backup power generators to keep the system operating, a process that normally takes five minutes, said Cliff Arnett, senior vice president of water resources operations and maintenance. A glitch delayed the move for 20 minutes, causing sewage to back up in the huge line.

But Arnett said that length of power outage has not caused an overflow in the past.

"All the lines are about as full as they can get. That is probably the answer, but I don't know that for sure," he said.

The Water Works typically treats about 32 million gallons of sewage daily, but that has increased recently to 40 million a day.

During heavy rainfall, Arnett said, sewer lines get increased flow from illegal storm drains tied into the sewer system and house connections in poor repair.

This year, Columbus has received 71.7 inches of rainfall, 26.2 inches over the 45.57 inches of normal rainfall for the year.

"The sewer system is not designed to take that," Arnett said. "This was very unusual."

The Water Works could face a fine and will be required to monitor the water in the area for a year.

But Arnett said the spill had no affect on the river.

"In the real scheme of things," he said, "it is like a spit in the ocean."