Partially treated wastewater enough to fill 500 Olympic-sized pools dumped into Lake Ontario during heavy rainfall: city

The city dumped more than 1,300 megalitres of partially treated wastewater into Lake Ontario during Tuesday’s torrential downpour, which is enough to fill 500 Olympic-sized swimming pools.

A city spokesperson said three wastewater plants were “bypassing,” a process in which stormwater and sewage are diverted from a secondary treatment procedure.

IN PHOTOS: A look back at the massive flooding across Toronto

“The bypassed wastewater still goes through screening, grit removal, primary treatment, phosphorous removal and full disinfection to ensure the treated water always meets strict federal and provincial regulations,” the spokesperson said, adding that the process is done to protect the plants’ ability to continue treating wastewater during an extreme rainstorm.

Health officials have advised against swimming in Lake Ontario for 48 hours after rainfall due to the possible presence of high levels of bacteria. On Wednesday, all of Toronto’s beaches were under a red flag.

The 10 beaches were marked unsafe for swimming due to high levels of E.coli in the water.

“Much of the sewage that may have been deposited after the storm should be disinfected through exposure to UV through the sunlight. It can be unsafe to swim, even at designated swimming beaches, for up to 48 hours after a major rainfall,” the city said.

“Storms increase bacteria in the lake, and 48 hours is long enough for bacteria levels in the lake water to drop or dissipate to ensure that the beach water is again safe enough for swimming.”

On Tuesday, thunderstorms brought heavy rainfall within three hours, causing widespread flooding across the city. According to Environment Canada, 97.6 millimetres of rain fell, which is more than a month’s worth of precipitation. In July, Toronto usually receives around 75 millimetres of rain.