MANKATO, Minn. — It’s a chemist’s version of finding a needle in a haystack.
Mankato’s wastewater treatment plant receives about 11.3 million gallons of sewage each day, enough to fill 17 Olympic-size swimming pools. Researchers at the University of Minnesota Medical School in Duluth are looking for the SARS-CoV-2 virus within that waste, and the virus is so small it would take more than 8,700 of the particles placed end to end to cross the eye of a needle.
But the UMD study is not only spotting the virus, it appears to be measuring it precisely enough to capture the rise and fall of COVID-19 cases within the Mankato area.
“It’s incredible what they can do,” said Jim Archer, an industrial chemist for the city of Mankato.
Archer and his colleagues have been pulling samples of the wastewater twice a week since May and sending them to the researchers at UMD. The wastewater surveillance study is potentially a valuable source of data in a pandemic.
Right now, public health officials need to rely on results of COVID-19 tests given to a small percentage of the population to get a sense of how prevalent the virus is in a particular population. By contrast, every person older than diaper age provides several samples a day to Mankato’s wastewater treatment plant, which also handles waste for North Mankato, Eagle Lake, Madison Lake, Skyline, Lake Washington and parts of South Bend Township.
“Obviously, there may be a difference in the number of people being tested and the general public,” said Assistant City Engineer Michael McCarty of the much larger sample size in the wastewater study.
Mankato was one of the initial cities to sign on to the UMD study, which has now grown to 44 communities around the state. McCarty said it involves minimal time to participate and may lead to a promising technique for better tracking and controlling the next epidemic.
“Maybe it becomes a screening tool in the future to help prevent these things from happening again,” he said.
The data collected so far isn’t being used regularly by local public health leaders to respond to the ups and downs of COVID transmission in the area, partly because the UMD study hasn’t yet been reviewed by other scientists. Although the city gets weekly reports, they’re filled with caveats.
“They always caution us, ‘This is research, this is research, this is research, so be careful,’ because they’re working on methodology,’” Archer said.
Still, the researchers are encouraged about the potential of wastewater surveillance in epidemiology.
“They tell us in the long term, it’s going to be something that’s done frequently ... that it’s going to have a lot of merit,” Archer said.
The samples being supplied to UMD every Monday and Wednesday represent the combined waste of all of the communities sending their sewage to Mankato for processing. But the collection system includes pumping stations where samples could be collected for each of the communities if public health officials wanted to track disease in each town, Archer said.
Wastewater samples could be collected in an even more targeted way — perhaps from university dorms or other concentrated residential facilities to provide an early warning when a virus is present — although collection of the samples would be more labor-intensive.
A big potential advantage of detecting the amount of virus in feces is that it shows up there well in advance of when people typically start to experience symptoms — the point when someone is more likely to get tested or to end up seeking treatment in a hospital.
“It seems to have a seven-day advance in uptick,” Archer said of the wastewater testing.
Looking at the data coming back to the city, there appears to be a clear correlation between the amount of virus in Mankato-area waste and the number of cases, according to Archer, who noted that October sewage results presaged November’s steep spike in cases.
“I think it’s something that’s going to hold up and it’s probably going to be a tool for health officials going forward, not just for this virus but for a lot of things,” he said.
One of the improvements the UMD researchers are working on is speed — studying ways to streamline the testing to get results out in the most timely manner possible, Archer said. And he noticed they’re asking for larger samples in recent weeks, possibly because they’re working to track not just the presence of SARS-CoV-2 but also the presence of new variants that appear to be more contagious than the initial coronavirus.