Canadian Wastewater Survey (CWS)

Detailed information for 2024 and 2025

To detect and measure trends in the viral loads of infectious diseases including SARS-CoV-2, influenza A and B, and respiratory syncytial virus (RSV) through wastewater epidemiological surveillance.

To measure, on a bi-monthly basis, loads and consumption of various illicit and licit substances based on the analysis of municipal wastewater.

Data release - April 3, 2025

• Description
• Data sources and methodology
• Data accuracy

Description

The Canadian Wastewater Survey (CWS) was launched in March 2019. Wastewater-based epidemiology applies analytical techniques to wastewater in order to estimate consumption of substances, and levels of infectious diseases and antimicrobial resistance (AMR) at the community level. It has the potential for generating near real-time data on geographical and temporal trends without the need for self-reported surveys for use as an early warning system and monitoring the effectiveness of public health measures.

There are currently two components to the CWS:

- Monitoring and estimating levels of various licit and illicit drugs in wastewater conducted in collaboration with Health Canada.

- Detecting and monitoring levels of infectious diseases in wastewater in collaboration with the Public Health Agency of Canada.

Reference period: Daily

Collection period: Variable - currently seven consecutive days starting on the second Wednesday of each odd month for the drug component, and twice a week for the infectious disease component.

Subjects

• Health
• Lifestyle and social conditions

Data sources and methodology

Target population

Wastewater treatment plants in Canadian municipalities and regions.

- Controlled Substances: Metro Vancouver, Edmonton, Prince Albert, Saskatoon, Toronto, Montreal, and Halifax.
- Infectious Diseases: Victoria, Metro Vancouver, Calgary, Edmonton, Sudbury, London, Toronto, Region of Peel, Kingston, Montreal, Moncton, and Halifax.

Instrument design

A one-time electronic questionnaire is comprised of variables compiled by relevant personnel at wastewater treatment plants and municipal/regional departments based on the treatment plant design and coverage area.

The Excel questionnaire is a monthly or weekly questionnaire, comprised of variables compiled by engineers and operators at wastewater treatment plants such as water flow data, water quality testing results, and weather or other events that could have an impact on results.

The questionnaires were developed in consultation with subject matter experts, potential respondents, data users and questionnaire design specialists. They are submitted via email, or through an electronic file transfer (EFT) service.

Sampling

This survey is a census with a longitudinal design.

Sampling unit:
Wastewater treatment plants.

Data sources

Responding to this survey is voluntary.

Data are collected directly from survey respondents.

For the drug component, personnel in wastewater treatment plants (WWTPs) collect 24h composite wastewater samples and record wastewater flows for a one-week period during the reference month. The samples are delivered to a Health Canada laboratory for analysis following the end of the sample collection period. WWTP-specific metadata including influent wastewater quality parameter information at the time of sampling are sent to Statistics Canada by the WWTPs. The results of the lab analysis by Health Canada are sent to Statistics Canada following completion of sample analysis.

For the infectious disease component, personnel in wastewater treatment plants collect 24h composite wastewater samples twice weekly during the collection period. The samples are delivered to the National Microbiology Laboratory (NML) for analysis. The results of the lab analysis by NML are sent to Statistics Canada. WWTP-specific metadata including influent wastewater quality parameter information at the time of sampling are sent to Statistics Canada by the WWTPs.

Error detection

For the drug component, the chemical analysis procedure was validated for accuracy and consistency. The results from the wastewater samples are compared to results from historical samples to identify and remove extreme outliers.

For SARS-CoV-2, until April 2022, wastewater samples were analyzed in technical duplicates against two targets each (N1/N2) to validate the measurement of SAR-CoV-2. Since the detection of omicron in wastewater in December 2021, the N1 test began to exhibit decreased sensitivity compared to N2. As such, the N200 assay was used instead to conduct wastewater surveillance along with N2 since both of these tests correlated well with each other. Over time, the N200 assay also began to show decreased sensitivity compared to N2. For this reason, use of the N200 assay was stopped on December 2024 and only the N2 assay is now used for SARS-CoV-2 surveillance.

Imputation

Some of the scheduled samples were unable to be analyzed, whether because of situations at the wastewater treatment plants or issues related to the shipping and handling of the samples (e.g. samples arrive thawed or broken).

For the drug component, some samples were deemed extreme outliers and so the samples were treated as missing. In these cases, the missing samples were imputed to create valid monthly estimates and variances.

For the purpose of estimating viral loads, concentration measurements below the limit of detection are imputed with a value below the limit.

Estimation

The chemical analysis of target drug metabolite concentrations in the wastewater, together with the flows recorded at the time of sampling at the wastewater treatment plant, lead to a calculation of drug metabolite load (in units of milligrams of metabolite / one thousand people / day). The load is divided by an estimate of the contributing population to standardize municipalities of different sizes. The CWS also reports on the detection rate. For each wastewater sample, a positive detection occurs when the target compound is present in a concentration exceeding the limit of detection. At this concentration, there is high confidence that the compound is present in the sample and the measurement was not a false positive. The detection rate is never imputed; missing samples are simply excluded from the calculation. This was done because positive detections represent a high level of certainty about the presence of the drug that could not be matched with imputation.

The concentration of viral particles of SARS-CoV-2 in wastewater primary influent as measured in duplicate by RT-qPCR. In addition, the concentration of the pepper mild mottle virus (PMMoV) is also measured by the same means for quality assurance purposes. Metadata encompassing epidemiology indicators and wastewater plant metadata is released to assist in interpreting the data.

Quality evaluation

Data are verified for reasonableness and coherence.

For the drug component, standard errors and 95% confidence intervals are available for the load per capita estimates. For the infectious disease component, where lack of reliability is suspected, results are withheld.

Disclosure control

Statistics Canada is prohibited by law from releasing any information it collects that could identify any person, business, or organization, unless consent has been given by the respondent or as permitted by the Statistics Act. Various confidentiality rules are applied to all data that are released or published to prevent the publication or disclosure of any information deemed confidential. If necessary, data are suppressed to prevent direct or residual disclosure of identifiable data.

Revisions and seasonal adjustment

Data may be subject to revision if errors in calculation are detected, if laboratory analyses are repeated, or if additional scientific knowledge of the supporting data becomes available. As of now, no revision calendar has been established.

Data accuracy

A technical paper is now available that explains sources of non-sampling error and how the uncertainty in drug consumption estimates arises from uncertainty in flow measurements, chemical analysis, excretion rates, losses, potency, and population estimates.

https://www150.statcan.gc.ca/n1/pub/13-605-x/2019001/article/00006-eng.htm

Estimating cannabis consumption using markers in wastewater: methodological paper, May 21, 2019.

SARS-CoV-2 concentration from wastewater is determined at NML based upon previously developed methodologies for viruses similar in structure to SARS-CoV-2. RNA is extracted from wastewater solids on an automated nucleic acid platform, and subsequently quantitated by the reverse transcriptase quantitative polymerase chain reaction (RT-qPCR).

The limit of detection (LOD) was estimated by NML to be 2.02 genome copies per milliliter of raw wastewater influent. The performance of the method was compared to peer-reviewed and pre-print studies employing similar methods directed against the same SARS-CoV-2 molecular targets in longitudinal studies of real-world wastewater samples. The range of viral concentrations reported in the previous studies was greater than the LOD determined by NML for the majority of samples, suggesting that the test employed here would be sensitive to the same range of viral input material, and provides confidence that NML's wastewater test is performing adequately.

The NML has also participated in numerous rounds of an interlab study led by the Ontario Clean Water Agency with other laboratories across Canada performing similar analysis of SARS-CoV-2 in wastewater. Based on the results of this study, the NML method has better than average sensitivity.