Canadian Wastewater Survey (CWS)

Detailed information for March 2018 to February 2019





Record number:


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

Data release - August 26, 2019


The purpose of this experimental program is to track over time drug consumption by measuring its concentration in wastewater. In the first instance, cannabis is being reported; other drugs will follow.

The frequent (monthly) recording of data allows for more reliable estimates of drug consumption trends, particularly that of cannabis following legalization.

The collection of data from wastewater offers the benefit of reporting all consumption (avoiding under-reporting), whether it be from legal or illegal sources. An estimate of the share of the black market may be made by subtracting drug loads associated with legal sales from total drug loads found in wastewater.

Reference period: Month

Collection period: The second week of the reference month


  • Health
  • Lifestyle and social conditions

Data sources and methodology

Target population

Wastewater treatment plants in Vancouver, Edmonton, 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.


This survey is a census with a cross-sectional design.

Wastewater sampling, as well as the calculations of target drug loads and consumption, are performed on a monthly basis for the five metropolitan areas.

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 laboratory for analysis. The results of the analysis, as well as the recorded wastewater flows at the time of sampling, are sent to STC.

The chemical analysis of target drug metabolites in wastewater, together with the flows recorded at the time of sampling at the wastewater treatment plant, lead to a calculation of drug load (in units of weight of metabolite / day). To arrive at calculations of population consumption of the respective drugs (in units of weight of raw drug / 1 000 population-day), the drug loads have to be multiplied by excretion rates (units of weight of raw drug / units of weight of metabolite), and then normalized using estimates, supplied by the Population Census of the populations served by the respective wastewater treatment plants.

Error detection

Wastewater samples are analyzed in triplicate to validate the measurements.


Isolated instances of missing data required imputation.


The chemical analysis of target drug metabolites 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 weight of metabolite / day). To arrive at population consumption estimates of the respective drugs (in units of weight of raw drug / 1,000 people-day), the drug loads are divided by estimates of the excretion rate (weight of metabolite excreted/weight of raw drug consumed), losses, and drug potency, then normalized using estimates of the population contributing to the wastewater.

The methodology for deriving uncertainty estimates has been released.
Estimating cannabis consumption using markers in wastewater: methodological paper, May 21, 2019.
Wastewater-based estimates of cannabis and drug use in Canada: Analytical methods and supplementary information, August 26, 2019.

Quality evaluation

Data are verified for reasonableness and coherence. Where concentrations of target drug metabolites are unreliable, estimates of loads and consumption 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 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 will be made available at a later date explaining how the uncertainty arising from each of flow measurement, chemical analysis, excretion rates and population estimates lead to uncertainty in drug consumption estimates.

Non-sampling error:
A technical paper will be made available at a letter date that will explain sources of non-sampling error.

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