A few minutes before midnight struck to mark the Year of the Monkey, Ma Shanting’s trash wagon stood at the curb on Hefei Road in Shanghai as he and his fellow garbage haulers chatted about their unexpectedly quiet night. A policeman ambled by and joked they’d all soon be out of jobs.

With a shovel and broom in the back of his aluminum pedal-powered wagon, Mr. Ma had been prepared to sweep up piles of red paper, charred cardboard boxes and other fireworks garbage. For as long as anyone can remember, fireworks detritus litters...