Imagine a city bus cruising down the street, quiet and clean. Now, imagine its fuel came from, well, us. That’s exactly what happened in Barcelona, Spain, where a five-year experiment turned an ordinary bus line into a beacon of green transport. Researchers successfully ran the V3 bus line using pure renewable natural gas made from human waste. And no, there was no unpleasant smell.
This bold step shows how cities can turn overlooked resources into valuable fuel. For half a decade, experts from the water company Veolia teamed up with Barcelona’s public transport company TMB and the Autonomous University of Barcelona. Their shared goal was simple: make public transport cleaner, even in surprising ways.
The idea came from a specific challenge. While electric buses are great, they sometimes fall short for city outskirts. These routes often need buses that can carry more people and travel further. So, the team looked for a different, green solution.
Fueling Buses with What We Flush
The key to this unusual fuel was the Baix Llobregat treatment plant. This facility is one of Europe’s biggest industrial water purification sites. Here, researchers found a way to create a renewable fuel. This new fuel emits 80% less carbon dioxide than regular natural gas.
The Baix Llobregat plant is a marvel itself. It handles about 400,000 cubic meters of wastewater every single day. A huge 95% of this water gets cleaned and reused. It helps water farm fields, keeps city parks green, and even refills underground water sources. During very dry times, this super-clean water could even be made safe for drinking.
Beyond the water, the plant also produces a lot of dry material from human waste. We’re talking 250 metric tons daily. This material usually goes to farms. But a group of Spanish researchers saw a different future for it.
The project, called Nimbus, took this waste and transformed it. They converted four cubic meters of waste per hour into many kilograms of biomethane. Biomethane is a clean fuel. It’s made by refining biogas until it’s over 90% methane. This process removes carbon dioxide and other things that make biogas dirty. What’s left is pure enough to go into existing gas pipelines. It can also power vehicles that run on natural gas.
Biogas from human sludge naturally contains about 65% methane and 35% carbon dioxide. Veolia’s process didn’t just separate these two gases. Instead, they cleverly combined the carbon dioxide with hydrogen. This hydrogen came from the treatment plant’s water and other clean energy sources. By doing this, they managed to turn almost all the biogas into biomethane. This fuel doesn’t put more carbon dioxide into the air than it already holds. It does create very tiny amounts of nitrogen oxides and fine particles, but overall, it’s a huge step towards cleaner air.
The Nimbus team produced enough fuel daily for Barcelona’s V3 bus line to travel 100 kilometers. That’s quite a distance for a fuel made from waste!
After five successful years, the Nimbus project is ready for its next act. It will transform into a new initiative called SEMPRE-BIO. The Spanish researchers plan to build on what they learned. They aim to boost biomethane production to 10 to 12 cubic meters per hour. This increased output will be enough to power an additional bus line. Looking further ahead, the team hopes these small pilot projects will grow. They want to see them become large industrial efforts. The ultimate goal is to power public transport using truly renewable sources like the very waste we create every day.