As the City of Montreal started releasing eight billion litres of raw sewage into the St. Lawrence River on Wednesday, to allow for repairs of a massive main sewer line that runs along the southeast side of the city, officials from lakeside municipalities in the GTA say a scenario where they would have to release raw sewage into Lake Ontario is unlikely.
Montreal’s wastewater system is built with one large wastewater treatment taking in all of the city’s wastewater while Toronto, Peel and Durham have several plants each to process wastewater. Outside of the core of Toronto, much of the suburban GTA was built with separate sewer systems, one that handles rainwater runoff, and other that deals with waste from buildings. Much of Montreal does not have separate sewers to handle rainwater runoff. This is why officials believe that Lake Ontario will be safe from the emergency measures taken in Montreal.
John Presta, director of environmental services for Durham Region, said he toured Montreal’s main wastewater treatment plant last week.
“All of their flows, including rainwater and snowmelt, going into large deep trunk sewers and all of that flow goes into their one main plant, which discharges into the St. Lawrence,” Presta said.
In Durham for instance, Presta said there are six separate plants. But in Montreal, there is only one large plant to handle the city’s wastewater, and one of the main pipes that supplies it needed to be repaired this week.
With all the sewage flowing to a single point, Frank Quarisa, the City of Toronto’s director of wastewater treatment, said he believes Montreal decided to take a drastic course of action.
“That is a trade-off against an event that could last years in the running if you had a major failure.”
Along Lake Ontario, Quarisa said wastewater treatment plants are allowed to release wastewater during extreme rain events that has not undergone the full treatment process.
But it cannot be considered “raw.”
“It’s still removal of grits, there’s still screening, there’s still primary treatments which is removal of solids,” Quarisa said.
The wastewater is also chlorinated before it’s released.
“That’s not raw sewage,” Quarisa said.
If the wastewater plants didn’t release partially-treated excess rain runoff during periods of heavy rain, there is a risk the overflow could damage the plant’s sensitive biological water treatment processes, which can take weeks of work to restore.
In Peel Region, only one wastewater treatment plant, located in Mississauga south of Lakeshore Road East and Dixie Road, discharges partially-treated wastewater and it only does so once or twice per year, officials said Thursday.
In Durham, Presta said wastewater treatment plants release partially treated wastewater into Lake Ontario less than 0.1 per cent of the time they operate.
In 2014, the City of Toronto’s Ashbridges Bay Wastewater Treatment Plant released partially-treated water on 20 separate occasions, releasing a total of 2.175 billion litres into Lake Ontario, according to an annual report.
But Quarisa said there is no correlation between days where high levels of bacteria were located on Toronto’s beaches, and days where its wastewater treatment plants released partially-treated wastewater.
The City of Montreal has discharged raw sewage on three other occasions in the past twelve years, sending 10 billion litres into the St. Lawrence in the spring of 2003, 7.6 billion litres in the fall of 2003 and 800 million litres in the fall of 2007.
