Sewage spills such as the one last Saturday comprise very little of the sewage that winds up in the Ottawa River, says the City of Ottawa wastewater manager.

"The spill itself is frankly a rather minor issue compared to the fact that we have combined sewer overflows on a regular basis when it rains," said Dave McCartney. "That's the real issue."

That is because sewage spills happen relatively rarely, he said, while sewage is dumped by design into the river whenever there is a rainstorm.

Ottawa's sanitary and storm sewers share the same pipes, and those pipes tend to overflow during heavy rainfalls. The system is designed to send the overflow into the Ottawa River.

But accidental spills, caused by sewage gates being jammed open, have received much attention since it was revealed such a spill was responsible for fouling Petrie Island beach in 2006.

Earlier this month, city council commissioned a $400,000 study to evaluate, among other things, the impact of spills on the health of the river. Results are expected in December, and McCartney said they will help council come up with a list of priorities.

In the meantime, Ottawa's sewer system is getting an upgrade, but McCartney said even that won't put an end to spills.

Coun. Bob Monette, who represents the ward that includes Petrie Island Beach, said whatever the findings of the study, the city must prevent future spills.

"I would not be satisfied if the report came back and said that spills is not going to be a major issue," he said.

Petrie Island beach was closed for 45 days in the summer of 2006 due to bacterial contamination. Earlier this year, it was revealed that a sewage gate upstream from the beach had remained jammed open for 15 days, spilling more than a billion litres of raw sewage into the river.