Ottawa is keeping more sewage than ever out of the Ottawa River, and the city hopes to stop even more, an official says.

Over the spring and summer, 180 million litres of local sewage mixed with storm water ended up in the river — a lot but less than half the 400 million litres discharged into the river last year.

Dixon Weir, the city’s general manager of environmental services, says the sewage decrease is the result of a series of system improvements involving regulators inside the pipes. The regulators won’t release storm water mixed with sewage into the river unless the system is full, he said.

"The system is designed to direct as much towards treatment as it can."

To keep neighbourhoods safe from flooding during heavy rains, it’s still necessary to release some of the discharge, Weir said.

Temporary storage planned

But the city is capturing 99.5 per cent of the combined sewage before it reaches the river and it’s considering how to capture even more.

The plan would involve building an underground storage tunnel six kilometres long and almost three metres in diameter, he said.

"During a rainfall event, that wee bit of combined sewage that currently is being released would be directed to a temporary storage," Weir said. "And it would be collected in that storage facility, for later release to the environmental centre, where it's treated."

An environmental assessment of the plan is in the works and a few different designs are being considered. If the plan goes ahead, the tunnel could be built within the next five years.