Toronto Public Health (TPH) monitors COVID-19 activity in the community using a number of different data sources. With recent changes to the provincial testing strategy, TPH has been reviewing the use of additional sources of data to better understand the changes in COVID-19 activity. Wastewater surveillance involves testing for the virus in wastewater and looking at changes over time, helping us understand the true presence of COVID-19 in a community regardless of the testing strategy that is in place.
Ontario’s wastewater surveillance initiative is coordinated by the Ministry of the Environment, Conservation and Parks (MECP). Ryerson University and University of Toronto complete laboratory analyses on wastewater samples collected by the City of Toronto.
This report is generated by MECP. This page is updated weekly on Wednesdays at 3 p.m. Technical notes for this report prepared by MECP are available.
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See the COVID-19 Wastewater Surveillance Report
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As of December 31, 2021, the number of COVID-19 cases being reported is an underestimate of the true number of people with COVID-19 in Toronto due to provincial changes in testing eligibility.
Wastewater surveillance is a novel way to identify community trends of COVID-19. It is particularly useful when access to PCR testing is limited, and captures people both with and without symptoms. While it cannot be used to determine the number of cases of COVID-19 in the community, it identifies whether overall virus activity is increasing or decreasing. This recently developed method of surveillance is being implemented in many places across the world.
Wastewater surveillance does not replace traditional COVID-19 testing, but can provide a broader understanding of COVID-19 activity. It is important to review the data along with other COVID-19 data sources for a more accurate picture of COVID-19 activity in Toronto.
The majority of Toronto’s wastewater is treated at four wastewater treatment plants located across the city: Ashbridges Bay (covering approximately 48% of Toronto’s population), Humber (24%), Highland Creek (17%), and North Toronto (8%). Peel’s Lakeview treatment plant covers the remaining approximately 3% of Toronto’s population.
Wastewater sampling occurs in the four wastewater treatment plants in Toronto 3-5 times per week. These four treatment plants have large sewersheds that cover approximately 97% of Toronto’s population. Data for Peel’s Lakeview treatment plant are not included in our dashboard.
It is important not to compare viral signals across different wastewater treatment plants (WWTPs). Each WWTP in Toronto covers a different proportion of Toronto’s population, with variation in the size of the catchment area and mix of land uses for each plant. The information is best viewed by considering the results for all WWTPs together to determine the overall trend (increasing, stable, or decreasing) for the city of Toronto.
As with any source of data, there are some limitations to consider. There can be variation in the wastewater data due to environmental factors (for example rain or snow) which can affect the wastewater samples. There is also a large variation in the proportion of Toronto’s population each wastewater treatment plant serves. Combined with environmental factors, the population size may dilute or concentrate the amount of virus that is detected. Finally, there is variability in the time required to process the samples, which may reduce the ability of this data source to serve as an early warning system.
View the previous COVID-19 Wastewater Surveillance Reports: