Odorless, quiet, sustainable. On the last day of July, passengers boarded Barcelona's V3 bus line with no idea where its fuel came from. Written in large letters on the bus façade, just below its name "Nimbus," a sign clearly stated: "This bus runs on biomethane produced from eco-factory sludge." Still, the explanation was likely too vague for most to grasp its full meaning: The moist matter from wastewater treated at the Baix Llobregat treatment plant was used to produce the biomethane. In other words: the human waste of more than 1.5 million residents of the Catalan city.

"I think it's a fantastic idea. We already put animal waste to good use, so why not ours?" said Rosa Maria Gay, a 68-year-old retiree, when the bus's fuel system was explained to her. "I had never heard of this before – it's a great idea. If our excrement is useful for something, all the better!" added Leire Muños, a 32-year-old receptionist, laughing. "I thought it was an electric bus; as long as it's renewable energy and doesn't smell bad, I think it's great," said Alessandra Spano, an 18-year-old computer science student, eyes wide and smiling.

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