Can sewers overflow in dry weather?
The conventional wisdom is that the combined sewers in the City of Toronto only overflow during storms. You may have heard this said so many times with so much conviction that you believe it’s true. But is it? Are storms the only reason for combined sewage spills? Looking at the results of the water samples from the inner harbour of Toronto, this “fact” might not be the whole truth.
What is a “Combined Sewer”?
Firstly, it is important to understand what a combined sewer is. In Toronto, three types of underground pipes exist, sanitary pipes, stormwater pipes, and combined sewers. Sanitary pipes are the ones connected to the toilets, showers, dishwashers, and sinks in homes and businesses. This wastewater and sewage go directly to the local water treatment plants. These pipes are typically found in newer Toronto suburbs.
Stormwater pipes collect rain and snowmelt through the grates seen on the sides of roads, and in parking lots. These flow directly into rivers, streams, and eventually, it all ends up in Lake Ontario, without treatment.
Combined sewers are exactly that, a pipe that collects both wastewaters from homes, and stormwater from roads. They are also connected to water treatment plants, but also have pipes that extend into the lake. When working perfectly, these pipes can serve a great purpose as they can help treat the polluted stormwater before it goes back into the environment.
Illustration of combined sewer overflows during dry weather, and also wet weather. The adapted image is provided by Daniel Overbey. For more information read Indianapolis Digs Deep to Fix its Enormous Sewer Overflow Problem.
The problem is that these pipes rarely work perfectly. The volume of stormwater and wastewater is more than the combined sewers can carry. When there is too much mixed storm and wastewater in the pipe, it flows over the barrier in the pipe directly into the river, stream or lake. It is said in Toronto that these combined sewer overflows (CSOs) only occur during heavy rain. However, at three sites in the inner harbour of Toronto, the levels of E. coli measured paint a very different picture.
We found evidence of sewage spills in dry weather
Our community monitoring program tests Toronto Harbour for E. coli bacteria because it’s a good indicator of sewage pollution. E. coli thrives in humans’ warm (37°C) and dark digestive systems; after 48 hrs outside of the human body, E. coli dies. (This is illustrated in a research paper by Fasil Ejigu Eregno and his colleagues in 2018.) E. coli also dies when exposed to the UV light, including sunlight.
As a general rule, E. coli levels should drop 48 hours after a sewage spill. If sewage spills are only caused by storms, then there should be very limited E. coli levels in the water. That’s not what we found at the Marina Four, Bathurst Quay and Yonge Street sites we sampled in the inner harbour of Toronto. The results for 1 water sample at Yonge Street, 3 water samples at the Marina Four site and 15 water samples from the Bathurst Quay site revealed high levels of E. coli with no prior rainfall within 48 hrs.
The graphs below illustrate the relationship between E. coli levels and precipitation at Marina Four and Bathurst Quay. Using the Precipitation Toggle at the bottom of both graphs you can filter for sample results during dry periods (0 mm rain over 48 hrs).
The reasons CSOs flow during dry weather
This leads to many other questions and concerns. Question one is, is the City of Toronto’s combined sewer system overcapacity even without rainfall? If this is the case, it means that we’re generating more wastewater than sewer system can hold. This could be one reason raw sewage is constantly flowing into the harbour waters.
The next question is, why would this happen? Could it have anything to do with the many new developments going up in Old Toronto? The Old Toronto area is still connected to the combined sewer system (a separated sanitary system does not exist in the downtown core). If this is the case, it’s worrisome; the city’s population is growing and bigger storms with more rain are rolling through.
Other reasons for dry weather overflows could include cross-connections, illegal hookups, buried rivers, groundwater pumping, or infiltration.
Why does this matter?
Toronto is spending $3-billion to upgrade the sewer system. If the plan focuses entirely on controlling flows during wet weather, those dollars may not translate into a swimmable, drinkable, fishable harbour. To truly protect the harbour Toronto needs to eliminate sewage spills entirely.
You can help
Read and share the Toronto Harbour Monitoring Report 3.0;
Read the one month update for the report: An Update on the Toronto Harbour Monitoring Report 3.0 and a Call to Action;
Write a letter to your local City Councillor endorsing the recommendations - download a draft letter here;
Volunteer to help take water samples for the 2019 sampling season - Monitoring Team signup; and
Donate to support our monitoring program in 2019.