As Robert Reed examines a mountain of trash piled three storeys high, a thin white plastic bag catches his eye. He fishes it out and holds it up. “That is a problem plastic,” he says gravely. “These get stuck in the machines, and there is no market for them.” He gives it a little wave and lets it float back down on to the heap.

We are standing inside the largest recycling facility in San Francisco, which takes in household waste, sorts it and produces tidy bales of material at the end. Broken glass crunches underfoot as Reed, a 20-year veteran of the industry, explains with pride that this plant, owned by local waste-management company Recology, is the most advanced of its kind on the West Coast, using lasers, magnets and air jets to process 750 tonnes each day.