Wastewater Surveillance

About Wastewater Surveillance

Wastewater surveillance involves testing for diseases in wastewater. It can help us understand disease trends, such as if it is increasing, stable, or decreasing in an area, and how much disease activity is in an area.

It is particularly useful when access to clinical testing is limited and captures people both with and without symptoms. This data can help Toronto Public Health understand the presence of disease in a community regardless of clinical testing strategies, access to testing, and the sample size of symptomatic individuals who have taken tests.

Wastewater surveillance does not replace traditional clinical testing but can provide a broader understanding of viral activity and local trends. It is important to review the data along with other data sources for a more accurate picture of COVID-19 and Influenza activity in Toronto.

Wastewater Surveillance Locations in Toronto

The majority of Toronto’s wastewater is treated at four wastewater treatment plants (WWTPs) located across the city:

1. Ashbridges Bay (covering approximately 50 per cent of Toronto’s population)
2. Humber (covering approximately 23 per cent)
3. Highland Creek (covering approximately 18 per cent of Toronto’s population)
4. North Toronto (covering approximately six per cent of Toronto’s population)

Peel’s Lakeview treatment plant covers the remaining approximately three per cent of Toronto’s population.

Toronto’s wastewater surveillance does not include samples from North Toronto or Peel’s Lakeview treatment plants as they only cover a small fraction of Toronto’s population. Wastewater sampling occurs in the remaining three wastewater treatment plants in Toronto three to five times per week, which covers 91% of Toronto’s population. Please view the technical notes for further details on catchment areas.

How to Use this Dashboard

The interpretation section of this dashboard provides an assessment of current wastewater trends and signal strength. This can tell us how much COVID-19 or flu is found in the wastewater (signal strength) and the change in viral presence (trend).

For example:

• If the signal strength is high/very high and the trend is increasing, it means there is a lot of disease activity in that area and there is increased viral presence
• If the signal strength is high/very high and the trend is stable, it means there is a lot of disease activity in that area but the viral presence is constant
• If the signal strength is low and the trend is decreasing, it means there is low disease activity in that area and there is decreased viral presence

It is important to review wastewater trends along with other clinical indicators of COVID-19 and Influenza activity in Toronto to properly assess risk.

Limitations of the Data

As with any source of data, there are some limitations to consider, and it is important not to compare wastewater viral signals across different wastewater treatment plants (WWTPs) or diseases. There can be variation in the wastewater data due to environmental factors, such as rain or snow, which can affect the wastewater samples. Each WWTP in Toronto also covers a different proportion of Toronto’s population and varies in the size of the catchment area and land uses. The population size and environmental factors combined may dilute or concentrate the amount of virus that is detected. Additionally, 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.

In 2022-2023, wastewater data used to calculate flu signal strength ranges were only sampled once per week and included one flu respiratory season. The Humber WWTP was not sampled for flu in September 2022. Due to these limitations, the flu signal strength may not be as reflective as the current data, which is sampled more frequently per WWTP and for the whole respiratory season. Regardless, this remains the best available data for our flu signal strength ranges, and will be revised for future seasons.