A Manitoba Hydro power failure this week resulted in the largest known outflow of untreated sewage in Winnipeg since early last year.

​At 3:46 p.m. on Wednesday, Winnipeg's waste-water control centre on McPhillips Street received a power-failure alarm emanating from a pumping station on Mager Drive in the Elm Park neighbourhood of St. Vital.

The city operates pumping stations to send the contents of combined sewers — both stormwater runoff from the surface and untreated sewage — uphill to sewage-treatment plants.

While combined sewers can overflow into the city's rivers on their own during rainstorms, a power failure at a pumping station will increase the volume of the diluted sewage that winds up in the waterways.

The Mager Drive combined sewer was already overflowing when the power failed at the pumping station, the city states in an online incident report.

Crews arrived at the station with a mobile generator at 5:25 p.m. and restored power at 6:02 p.m., according to the report. They didn't close an outfall gate because that could have resulted in sewer backups into basements at nearby homes, the city said.

Manitoba Hydro restored the power at 6:13 p.m., by which time 2.69 megalitres of diluted, untreated sewage wound up in the Red River, the city said.

That volume is slightly more than the volume of water it takes to fill a full-size Olympic swimming pool. It's also the largest confirmed combined-sewer outflow in Winnipeg since Jan. 10-19, 2016, when five megalitres of diluted sewage flowed into the Red River at St. John's Park in the North End due to a pipe blockage.

The city reported the outflow to provincial environmental authorities the day of the accident, city spokesperson Michelle Finley said.