Chemically, biogas is the same as natural gas from fossil fuels, but its manufacture relies on a process where bacteria feed on fecal waste for about three weeks in an oxygen-free chamber. The result is two-thirds methane and one-third carbon dioxide, as well as a nutrient-rich residue that can be used as soil or construction material.
Once the methane is purified, it is pumped through Goteborg's network of gas pipelines to specialized filling stations, where it is pressurized for delivery. Any car with an engine and tank configured for compressed natural gas can use biogas.
After each fill-up, the corresponding amount of biogas is injected into the natural gas grid as an offset, said Bo Ramberg, chief executive of FordonsGas, which is based in Goteborg and operates the largest chain of biogas filling stations in Scandinavia.
Ola Fredriksson, an engineer at Gryaab, the sewage facility in Goteborg, said that what an average person flushed down the toilet each year created enough biogas to drive 120 kilometers, or 75 miles.
"If the oil price keeps on going up, and people are prepared to pay more for renewable energy, then it will make our company interested in producing more biogas," he said. "We have the capacity.'
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