Science·CBC Explains

Why testing sewage for COVID-19 could be more useful than ever

The COVID-19 testing system has been overwhelmed by the Omicron variant, making it impossible to know how many people are infected. Testing for the virus in wastewater could help plug the gap. But how does it work? Where is it being used? How can it inform public health policies? Here's a closer look.

What wastewater surveillance can — and can't — tell us, and why only some provinces are using the data

Tracking COVID-19 through wastewater as clinical testing systems overwhelmed

1 year ago
Duration 2:04
As testing systems become maxed out across the country, some experts are turning to wastewater testing as a way to determine how much COVID-19 is really in our communities.

The COVID-19 testing system has been overwhelmed by the Omicron variant, making it impossible to know how many people are infected.

Monitoring for the novel coronavirus in wastewater could help fill the gap, says Rob Delatolla, a professor of civil engineering at the University of Ottawa and one of dozens of researchers across Canada that have been developing and using the technique.

"I think it just became that much more valuable," he said.

But how does it work? What can and can't it do? And how much can it really help inform public health policy?

Why is wastewater used to test for COVID-19?

People who are infected shed the SARS-CoV-2 virus that causes the disease in their feces. This happens even before the onset of symptoms. People can keep shedding genetic material from the virus in the form of RNA for several weeks. That RNA can be detected and monitored in wastewater and used to detect trends in COVID-19 infection.

Bernadette Conant is CEO of the Canadian Water Network, which co-ordinates the COVID-19 Wastewater Coalition. She said this technique was first used in the Netherlands early in the pandemic, adapted from those used in the surveillance of other diseases, such as polio.

The Canadian coalition started up in April 2020 to help share knowledge about the technique, its reliability and how it could be used to inform public health.

Where across Canada is wastewater being tested for the virus?

It's being used in "pretty much every province across Canada," said Conant, but is more developed in some areas than others. The technique is most heavily used in Ontario and Alberta, where wastewater surveillance covers 75 per cent of the population. In Quebec, the program ended in December, as it was funded by a research grant that ran out.

It's been primarily used in urban areas because they tend to have wastewater treatment plants that regularly sample water anyway to monitor for other things and tend to have universities with expertise needed to develop and run this kind of surveillance.

However, Conant said researchers are looking into how to apply the technique in areas without sewers.

How does the technique work?

There's no standard method, said Conant, because it is often developed quickly, using whatever resources are locally available. 

But most sites follow a similar procedure:

  • Wastewater is sampled on a regular basis at a particular location. That may involve collecting a container of water at a wastewater treatment centre or it may simply pass through a filtering device at a site like a sewer.

  • The sample is concentrated; this may involve removing the water and keeping only the solids, which the RNA tends to stick to.

  • RNA is extracted from that sample.

  • An analysis is done to look for a COVID-19 RNA signal and to measure how big it is.

WATCH | A look inside Ottawa's wastewater facility where COVID-19 samples are collected:

A look inside Ottawa's wastewater facility where COVID-19 samples are collected

2 years ago
Duration 0:37
U of O researcher Robert Delatolla explains how people with COVID-19 shed the virus into the city's wastewater.

What can the technique be used to do?

Early detection. Since even asymptomatic and pre-symptomatic people shed the virus, outbreaks can be detected through wastewater before it shows up through clinical testing.

Detecting outbreaks at specific locations. It's been used to monitor for outbreaks at places such as university residences, schools, prisons and long-term care homes across Canada.

Monitoring trends in larger populations. It's been used to monitor whether cases are rising or falling in populations of up to a million people by sampling at water treatment plants. Up until recently, it could also be compared to case counts from clinical testing. Eric Arts, a professor of microbiology at Western University in London, Ont., who has been involved in wastewater monitoring, said because it has tracked closely with that other data, researchers can now project from the data "pretty accurately" the infected population in some areas.

Measuring the effect of public health interventions. Researchers can see whether (or when) the wastewater COVID-19 signal increases or decreases following interventions, such as vaccinations or school closures — even if testing criteria have changed at the same time.

WATCH | How does wastewater help with surveillance of COVID-19:

How does wastewater help with surveillance of COVID-19?

2 years ago
Duration 4:19
Ottawa has been testing its wastewater to get a better understanding of how much of the coronavirus is in the city's sewage. The project's co-lead investigator explains how it helps with surveilling COVID-19.

Tracking the rise of new variants. Because it relies on genetic sequencing, and the sequences of different variants are different, researchers can use wastewater testing to detect new variants and potentially what fraction of the population is infected. In some areas, it was able to document the emergence of the Alpha and Delta variants, said Mark Servos, Canada Research Chair in Water Quality Protection at the University of Waterloo, who has been working on wastewater monitoring in Ontario for almost two years. "And now the decline of Delta, as Omicron has skyrocketed so rapidly."

What are its strengths compared to other measures, such as clinical testing and hospitalizations?

Wastewater surveillance is independent of clinical COVID-19 testing, so it has the potential to not be influenced by changes "like who gets tested or whether there's enough testing," said Conant.

And "everybody in the community" contributes to sewage, notes Sarah Dorner, a professor at Polytechnique Montreal who has been involved in wastewater monitoring for COVID-19.

Because it can detect pre-symptomatic infections, it can provide earlier warning of trends compared to other measures, such as clinical testing or hospital admissions.

"It's a predictor of what's going to happen with hospitalizations, which is a lagging indicator," said Arts.

What data gaps can't it fill?

The coverage across Canada isn't as widespread as clinical testing and hospitalization data, and the data wastewater testing provides is local and variable. That's because each site covers a different population, from a few hundred people to a million, and uses different sampling techniques in different conditions.

It also can't pinpoint individual cases, so can't be used to inform treatment, contact tracing and isolation of infected individuals. Nor can it provide an exact count of cases.

"We have an idea of when high is high and low is low, but coming up with a very specific estimate is very difficult," said Servos, adding that researchers also don't yet know whether Omicron has changed the relationship between wastewater measurements and actual case counts.

Researchers say the technique's strengths and weaknesses mean it's complementary to other sources of information, such as hospitalizations and test positivity rates.

So how can it be used by public health officials?

In Ontario, which has one of the most developed monitoring networks, Lindsay Davidson, a spokesperson for the provincial Ministry of the Environment, Conservation and Parks, said in an email that the government considers it an "important tool to help identify trends in COVID-19 spread at relatively low cost."

Researchers involved in wastewater testing say that's particularly useful now that accurate case counts aren't available, helping officials make informed decisions about things like public health restrictions.

"I think the wastewater could provide a reasonable estimate of where we are with cases in the population," Arts said. "If they keep going up, we can anticipate that hospitalizations are going up. But if we're on the way down, we may be over the peak and then we can start planning, you know, eventually removing certain restrictions."

WATCH | Why wastewater data might soon be more important than COVID-19 case counts:

Why wastewater data might soon be more important than COVID-19 case counts

1 year ago
Duration 0:53
Tyson Graber, co-lead investigator on the COVID-19 wastewater project in Ottawa, says wastewater data may fill the gap in official case counts as the Omicron variant outpaces the ability of public health officials to test and report cases.

Servos and Delatolla said their teams have been working closely with municipal public health officials in Ontario from the beginning, discussing the data regularly.

"It's certainly being used by public health units on a pretty regular basis," Servos said.

In some areas, such as the Greater Toronto Area, local measurements allow officials to identify infection hot spots, he added. The University of Waterloo has used it to inform students of outbreaks at certain residences

Similarly, Calgary Mayor Jyoti Gondek recently told CBC News that wastewater data from the University of Calgary made it "clear that many Calgarians are sick with COVID, probably with the Omicron variant." And Susan Henry, the city's Emergency Management Agency chief, said it was a warning for what could be coming.

Why are some governments reluctant to use it?

It's a relatively new technique, said Conant, and some governments felt there was too much uncertainty over its utility amid limited pandemic resources. But she thinks that's changing. 

"Over the last six to eight months, you're seeing a much larger uptake and an acceptance by public health," she said.

Still, not everyone is convinced. Dr. Robert Strang, Chief Medical Officer of Health for Nova Scotia, said health officials in that province continue to have conversations with local researchers involved in wastewater testing.

"But there's a lot of questions from a public health perspective," he said. "We don't feel that it is yet at a point where we can accurately use it as a surveillance tool."

Can it be used by the public?

While wastewater data is shared mostly with public health officials, some regions such as London, Ont., and Ottawa, do make the data publicly available. In Ottawa, Delatolla said, "we've been really surprised to see how much uptake there has been by the public in terms of looking at the data."

Dorner said she used wastewater data herself to gauge the risk of COVID-19 in the community when planning her family's activities in December.

"We don't have PCR tests. So how does the individual access the information they need to assess what the probability is of an infection if they partake in certain activities that are perhaps higher risk?," she said. "I definitely think I'll be looking at the wastewater more regularly."


Emily Chung

Science, climate, environment reporter

Emily Chung covers science, the environment and climate for CBC News. She has previously worked as a digital journalist for CBC Ottawa and as an occasional producer at CBC's Quirks & Quarks. She has a PhD in chemistry from the University of British Columbia. In 2019, she was part of the team that won a Digital Publishing Award for best newsletter for "What on Earth." You can email story ideas to

With files from Kayla Hounsell, Adam Miller and Michelle Song

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