Canadian Wastewater Survey (CWS)

Detailed information for March 2019 to July 2019

To measure, on a monthly basis, loads and consumption of various drugs based on the analysis of municipal wastewater.

Data release - July 26, 2021

• Description
• Data sources and methodology
• Data accuracy

Description

The Canadian Wastewater Survey (CWS) has been testing wastewater samples from various wastewater treatment plants in five cities across Canada (Halifax, Montréal, Toronto, Edmonton, and Vancouver) since March 2019 to track drug consumption over time by measuring the concentration of drug metabolites in wastewater. This information will help inform decision makers around public health strategies, and allow public health authorities, law enforcement agencies, and other organizations to target their approach. The latest CWS data provide monthly estimates of drug metabolites in wastewater for 14 drugs of concern based on samples collected from March to December 2019 and from January to December 2020. The estimates are based on the amount of drug measured in wastewater (i.e. grams) and presented on a load per capita basis, per day, or levels.

Reference period: Month

Collection period: Seven consecutive days starting on the 2nd Monday of each month.

Subjects

• Health
• Lifestyle and social conditions

Data sources and methodology

Target population

Wastewater treatment plants in Metro Vancouver, Edmonton, Prince Albert, Saskatoon, Toronto, Montreal and Halifax.

Instrument design

This Word and Excel-based questionnaire is comprised of variables compiled by engineers at wastewater treatment plants based on treatment plant design and coverage area, water flow data, water quality testing results, and weather or other events that could have an impact on results. The items on the questionnaire changed, starting in January 2020, and now include more detailed questions that have been added to provide greater information on monthly differences that could impact the results of the drug analysis. The questionnaire was developed in consultation with subject matter experts, potential respondents, data users and questionnaire design specialists. It is submitted monthly, 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.

Engineers in wastewater treatment plants collect 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. The results of the analysis, as well as the recorded wastewater flows at the time of sampling, are sent to Statistics Canada.

Error detection

The chemical analysis procedure was validated for accuracy and consistency.
The wastewater samples are compared to other samples from the same week and from the prior three months to identify and remove extreme outliers.

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). Additionally, 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.

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 grams of metabolite / 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.

Quality evaluation

Data are verified for reasonableness and coherence.

Standard errors and 95% confidence intervals are available for the load per capita estimates.

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.

In order to prevent any data disclosure, confidentiality analysis is done using the Statistics Canada Generalized Disclosure Control System (G-Confid). G-Confid is used for primary suppression (direct disclosure) as well as for secondary suppression (residual disclosure). Direct disclosure occurs when the value in a tabulation cell is composed of or dominated by few enterprises while residual disclosure occurs when confidential information can be derived indirectly by piecing together information from different sources or data series.

Revisions and seasonal adjustment

Data may be subject to revision if errors in calculation are detected, if chemical analyses are repeated, or if additional scientific knowledge of the supporting parameters becomes available. As of now, no revision calendar has been established. No seasonal adjustment is being made as the number of months over which the data have been collected so far is too small to reveal a seasonal pattern.

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.