sewer manhole

Could the answer to stopping coronavirus in its tracks be found in sewage? In August, the University of Arizona made headlines after it announced that it had used its sewer system to stop a coronavirus outbreak – and the school isn’t alone. Across the United States, wastewater testing programs have cropped up in cities, counties, universities and states as policymakers scramble to gather the potentially valuable data found in our sewage. Here’s what you need to know.


Wastewater epidemiology testing involves collecting samples of water carrying feces from a sewer treatment system to gather health information about the population. The technique has been used by epidemiologists to survey population health for several years, mostly to detect polio levels in countries where the disease can still be widespread. Notably, wastewater testing has recently begun to be used in the United States to monitor the levels of opioid use in specific communities. In late March, a team of researchers found that wastewater testing could indicate higher levels of infection from Covid-19 than what had been reported through testing or other methods.

“When a person gets infected with [coronavirus], even before they start to show symptoms, they start shedding the virus in feces,” David Freedman, a professor at Clemson University and chair of the university’s Department of Environmental Engineering and Earth Sciences, said  in an interview. By testing wastewater from a community at certain points in the sewer system, Freedman said, “you can get information about the level of infected individuals that are contributing to that sewage.”

With coronavirus, virus load in feces shows up more quickly than in the nasal cavity, Freedman said. So  wastewater testing is what’s called a “leading indicator” for coronavirus infection. (Conversely, because a person can start to recover and be less infectious but still shed virus through their feces, it can also be a “lagging indicator.”) And because wastewater testing takes the temperature of large groups with one single test, it can provide valuable population-level information on infection rates to policymakers – at “a fraction of the cost of personal testing,” Freedman said.

“Everyone has to use a toilet, but only some people (usually the ones feeling quite sick) will choose to get tested for COVID-19,” Zuzana Bohrerova, associate director of Ohio Water Resources Center at Ohio State and leader of a wastewater testing project with the state Environmental Protection Agency, wrote in an email. 

The implications of widespread wastewater testing could be huge. Experts estimate that regularly sampling the 542 largest of the almost 15,000 wastewater treatment plants in the United States could provide information on viral load for half the population. As of late August, more than 170 wastewater plants in 37 states were being monitored for coronavirus.


At Clemson, Freedman and his team have been working with the university and surrounding city, Clemson, S.C.,  to conduct regular weekly testing to gain insight on infections. The project started by monitoring wastewater from the campus services treatment plant in August, before students arrived back on campus and a small population of graduate students, administrators, faculty and staff were using facilities.

“We thought that by sampling the wastewater in that interval, we would get a baseline for what things might look like once people started to return to campus,” Freedman said. “We went for many weeks when the virus levels in the wastewater…remained at a low enough level to indicate that maybe a couple of infected people were on the campus, but not nothing consistent with a significant outbreak.”

After the university opened in September, things quickly began to change. Clemson required students moving into dorms to test negative within 10 days of moving, and be tested again once they moved in to quickly quarantine anyone with the virus. But wastewater testing helped illustrate the cracks in this system.

“Starting on Sept. 14, well, the virus level in the campus wastewater jumped dramatically high,” Freedman said. “Somehow, enough infected individuals made their way on the campus to cause the wastewater levels to go up into a region that I would call concerning.”

Because many upperclassmen live off campus during the year, Freedman’s team also took samples of smaller treatment plants that served the city of Clemson at the beginning of the summer to get a base level before students returned to campus. The levels, he said, were in the “millions of virus copies per liter, which is really high and indicative of a lot of infected individuals using that watershed.” The team’s findings, Freedman said, helped influence the city to issue a mask ordinance in early July.

Closer monitoring of wastewater testing can yield even more specific results. In late August, officials at the University of Arizona, which was monitoring sewage from 20 dorms individually, noticed a higher level of viral load in the wastewater from a specific dorm. Rapid testing of all the dorm’s occupants found some asymptomatic positive students, who were quarantined.

“You think about if we had missed it, if we had waited until they became symptomatic and they stayed in that dorm for days, or a week, or the whole incubation period, how many other people would have been infected?” Richard Carmona, director of the University of Arizona’s reentry task force, said at a news conference.

The Clemson and Ohio projects test larger wastewater plants with sewage from multiple buildings, so don’t narrow down results to the level of Arizona’s. The bulk of projects across the country, like the one at Clemson and in Ohio, where Bohrerova is, aren’t meant to supplant or replace individual testing, but rather to provide population-level information for officials. 

“Our goal is not to find individuals that are infected,” Bohrerova wrote. “We really look at the trends of the signal on a community basis, since most local health district policies are also made on community scales and not based on individuals….the results of wastewater testing are reported usually faster for the entire community than individual testing and most public health decisions are done on community scale (how many hospital beds we need, where do we increase intensity of testing, do we open schools, dining in?).”


Like any widespread testing program, setting up wastewater testing requires collaboration between scientists, property managers and local utilities and waste management to create testing points and share information. (It’s no wonder that many universities, which are able to more easily coordinate this setup, have been leading the way in conducting testing.)

As with other testing supply chains, the new and widespread demand for materials and equipment is creating a backlog: so many states, cities, counties and universities are starting their own wastewater testing program, Quartz reported that producers are experiencing “shortages and shipping delays” of machines designed to collect water samples.

The lack of standardized methods or ways of reading test results, Freedman said, makes it “difficult to compare results from one community to another.”

Bohrerova agrees, adding that the lack of a standard concentration method is a roadblock in her work as well. Amid calls for more standardization of wastewater testing, the Centers for Disease Control announced in August that it was developing a central portal and national database to provide wastewater data from projects across the country. 

Regardless of the challenges, wastewater testing “is spreading quickly as a tool,” Freedman said.

Molly Taft

Molly Taft is a  staff writer for Earther, Gizmodo's climate change blog. Her writing has appeared not only at ClearHealthCosts,...