Over a billion gallons of sewage pass through New York City’s wastewater treatment plants every day. Since the spring, the city has been one of the many places around the country to use that stream of excrement to track the spread of COVID-19.
When people are infected with the coronavirus, some of the virus exits their bodies via their feces. It starts to show up reliably in their waste even before they start to display symptoms. Those virus-laden feces end up in wastewater, along with everything else we flush down the drain. Research shows that upticks in the levels of virus in sewage for a particular area come about a week before the number of people testing positive spikes.
There are 14 wastewater treatment plants in New York City, and at each one, facility workers regularly siphon off vials of incoming sewage and send them to a lab in Brooklyn to be tested for traces of genetic material from the coronavirus. There’s usually at least some bits of virus in each sample — a sign that people across the city are sick. What they’re really looking at, though, is the amount of virus. If concentrations from a particular plant go up, it’s a signal that the area that sends waste to the plant might be facing an outbreak.