Drug-testing sewage? Yep! That’s a real thing that’s seriously being done – except the industry term is “wastewater-based surveillance,” which sounds delightfully more ominous.

Why would anyone give the sewers a drug test though? Well, the thinking is, if a piss test can tell you whether one person is smoking weed, a piss test conducted on a larger scale can tell you whether an entire city is smoking weed.

And you get bonuses when testing sewage instead of one person’s urine because sewage contains blood, hair, feces, phlegm and anything else the human body tosses down a drain. With so much to be collected from graywater rafting, the data is practically begging for scientific analysis.

OK, so maybe the real question is, what would anyone actually do with the drug test results from sewage? A lot, surprisingly. But this article examines only one application of drug-testing sewage: using the test results to create or update laws and social outreach pertaining to cannabis.

Now, what could go wrong with letting the toilets, showers and sinks throughout dozens of square miles of houses and commercial and industrial sites tell you what’s legal and whom to target with “Just Say No” campaigns? Plenty. But here are just three mitigating factors to consider.

Image Credit: Encho Enevski/123RF.com

If you grew up in the era of DARE, SANE and other school programs with slogans like “pass on grass” and “say ‘nope’ to dope,” you’ll love our trip down Memory Lane in “Growing Up in the ’80s.” 



Far from being a theoretical approach, wastewater-based surveillance (WBS) has been in use since at least 2007 – worldwide, no less. Raw influent wastewater samples are collected routinely at the inlet of most sewage treatment plants already, so these sources are readily available to anyone interested in them. (That in itself is a repulsive notion.)

Analysts assigned to sift through the variety of solids and liquids they’re given then quantify the concentrations of illicit drugs and their metabolites to estimate the total amount of drugs consumed by a community. From there, politicians and social justice warriors do their work.

Granted, WBS is a sustainable approach to lawmaking and childrearing that’s taking recycling to another level. But is this method something to use – even marginally if not primarily – to draw accurate, reasonable, proven conclusions about an entire population? Maybe not. And here are three reasons why.

Photo of Underground Sewer by Fabian Scjo;der/CC BY-NC-ND 2.0 via Creative Commons

Photo of Underground Sewer by Fabian Scjo;der/CC BY-NC-ND 2.0 via Creative Commons

1. The science lacks standardization.

WBS estimates concerning cannabis consumption have proven to be far less reliable than estimates concerning drugs, such as cocaine and meth. (Yes, that’s a real thing too – scientists are having a field day with everyone’s excrement.) This is attributed in part to how cannabis appears in feces. A “Water Research” study published in May 2024 identified three additional complicating factors:

  • Barriers related to in-sewer and in-sample behavior (e.g., adsorption/desorption mechanisms).
  • Analytical procedures used (e.g., sample preparation).
  • Pharmacokinetic aspects (e.g., administration route).

Accurate determinations of population-scale cannabis consumption (and excretion) are also hindered by discrepancies among WBS analysts’ approaches for reporting findings. And yet, proponents of WBS insist that this method provides “crucial” insight applicable to legislation and law enforcement in “problem areas,” as well as “high-resolution” trends and changes in community-level drug consumption. (Here’s the obligatory reminder that cannabis is a plant, not a drug.)

And the same cann-opponents claiming that weed “can strain healthcare systems and legal frameworks” observe that weed treats depressive disorders (i.e., reduces healthcare needs), and that its legalization has increased tax revenues and decreased arrest rates (i.e., unburdened legal frameworks). So, the motives are almost as convoluted as the method itself. Not to mention, some of these studies go after legal cannabis components, such as cannabidiol (CBD), failing to distinguish between hemp and marijuana in the way that laws do.

Photo by Pixabay/pexels.com

Photo by Pixabay/pexels.com

2. The underwhelming sample size is clinically insignificant.

The first estimation of community-level illicit drug use via wastewater analysis was calculated in Iceland’s capital, Reykjavik, where the population at the time was roughly 123,000 across a little under 40 square miles. For perspective, that’s roughly how many people live in 4 square miles in New York City, 9 square miles in Boston or 14 square miles in Los Angeles. The comparison is relevant insofar as the Reykjavik experiment’s celebrated success rate (based on matches with seized amounts and DUI cases) is what became a springboard for exporting the method.

If you can test 123,000 people’s sewage to make a law affecting 123,000 people, that’s one thing; testing 123,000 people’s sewage (for argument’s sake) to make a law affecting 7.9 million (New York City), 4.37 million (Boston) or 3.75 million (Los Angeles) people is another. Granted, the sample size will vary based on the organization carrying it out and the government funding the study, but the point remains the same. The sample size presents an issue, as it heavily influences research outcomes.

A 2020 report on WBS in “Comprehensive Analytical Chemistry” even ceded that “results often suggested an overestimation of consumption.” Elaborating on that, the report explains: “THC, the psychoactive ingredient in cannabis can be absorbed by diverse routes of administration such as smoking, oral, oromucosal, rectal, transcutaneous, and intravenous, while elimination from the body is equally diverse such as faeces, urine, sweat, oral fluid, and hair. THC pharmacokinetic processes are dynamic and may be affected by a person’s frequency and magnitude of use.” Thus, it would take disturbingly little for an outlier (e.g., the neighbor who parties all day, every day) to misrepresent the average. Moreover, timing – such as drawing samples on 4/20 – could skew the average.

Photo via Creative Commons/ CC BY-NC 4.0

Photo via Creative Commons/ CC BY-NC 4.0

3. Sample purity can’t be guaranteed, compromising accuracy.

The material under scrutiny can be contaminated from biological conditions in sewers, chemicals introduced from commercial and industrial activities, leaks in piping, vermin infestations, and innumerable other things. Otherwise put, blood, sweat, pee and poo aren’t the only things contained in those neatly packaged samples at the sewage treatment centers.

The right cleanroom laboratories and sophisticated scientific equipment can sift out extraneous materials, sure. But can they be trusted to reverse chemical reactions? According to a 2019 report on WBS from Statistics Canada, even the material’s travel time in the sewer is enough to limit test results’ accuracy, which is yet another factor beyond any lab’s control.

Photo by Barik5ive/pexels.com

Photo by Barik5ive/pexels.com


Using WBS harvesting methods for data collection and analysis could present ethical quandaries when the resultant findings determine legal consequences. Benign uses of WBS, such as estimating the size of the market for legal cannabis (as was done in Washington in 2014), can achieve something beneficial for – or at least not harmful to – the community.

The most important thing to recognize and acknowledge is that WBS presents arguably little objective data. This is self-evident in the fact that WBS is performed with contradictory test methods, applied in an overgeneralized way based on a statistically ludicrous sample size, and overlooking the less-than-ideal supplier of discarded bodily fluids. Such things merit greater consideration than they seem to have received over the past 15+ years. It’s worth considering.

Kathleen Hearons is a writer, editor, linguist and voice over actor from Los Angeles. She specializes in creative writing and research-intensive analysis and reporting.  




Sources and Suggested Reading

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Bijlsma, Lubertus, et al. “Chapter Fifteen – The estimation of cannabis consumption through wastewater analysis.” Comprehensive Analytical Chemistry, Vol. 90 (2020): 453-482. doi:10.1016/bs.coac.2020.04.005. Accessed June 5, 2024. https://www.euseme.eu/wp-content/uploads/Bijlsma_CAC202090453_482.pdf.

Bijlsma, Lubertus, Bradley Simpson, Cobus Gerber, Alexander L.N. van Nuijs and Dan Burgard. “Making waves: Wastewater-based surveillance of cannabis use.” Water Research, Vol. 255, Art. 121522 (May 2024). doi:10.1016/j.watres.2024.121522. Accessed June 5, 2024. https://www.sciencedirect.com/science/article/pii/S004313542400424X.

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