A few months ago, in a clanging, hissing plant on the outskirts of Newark, a tanker truck backed up to a deep reservoir and delivered thousands of dollars’ worth of raw material—what people in the rendering industry sometimes refer to as “liquid gold.” The plant’s owner is a company called Dar Pro, and the C.E.O., Randall Stuewe, looked on while a hose from the truck gushed a brown fluid, filled with fine sediment and the occasional mysterious solid. Slowly, the pit became a pool, whose surface frothed and eddied and gave off a potent odor of old French fries, onion rings, and batter-fried shrimp. “Used cooking oil,” Stuewe told me. “We process two billion pounds a year.”