Colleges across the nation — from New Mexico to Tennessee, Michigan to New York — are turning tests of waste into a public health tool. The work comes as institutions hunt for ways to keep campuses open despite vulnerabilities like students’ close living arrangements and drive to socialize. ... The tests work by detecting genetic material from the virus, which can be recovered from the stools of about half of people with COVID-19, studies indicate. ... Sewage testing is especially valuable because it can evaluate people even if they aren’t feeling sick and can detect a few cases out of thousands of people, experts say.