The city of Oslo is converting 80 municipal buses to run on biomethane captured from human waste, a novel, if somewhat disgusting, approach to cutting CO2 emissions and meeting Norway's ambitious plan to be carbon-neutral by 2050.

Beginning in September, the two sewage treatment plants in Norway's capital will collect methane, a byproduct of the microbial process that breaks down sewage, and pump it into city buses. City officials say the switch will cut fuel costs and reduce the amount of carbon dioxide released into the atmosphere by both the treatment plants and the buses.

"Oslo aims to be one of the most environmentally sustainable capitals in the world," project manager Ole Jakob Johansen told the Guardian. "Using biomethane makes sense. Not only would the biomethane otherwise be wasted, but the reduction in emissions per bus will go a long way to achieving our carbon-neutral target."

Biomethane advocates say using the wonder gas — keep those jokes to yourself, please — for fuel not only improves air quality, it can turn rural communities into alt-fuel producers. California's pro-biomethane camp claims collecting methane from the state's 1.7 million dairy cows would produce eight billion cubic feet of methane a year, the equivalent to more than 150 million gallons of gasoline.

Of course, that's less than half the 390 million gallons of gasoline the United States sucks down each day, but it's still nothing to sneeze at.

Poo-to-pump fuel may sound gross, but it isn't like there are guys in hazmat suits shoveling raw sewage into gas tanks. Biomethane is produced during a four-phase process called anaerobic digestion that uses microorganisms to break down everything from human waste and leftover food to lawn clippings and stuff swept from the slaughterhouse floor.

Until now, much of the methane produced by Oslo's sewage treatment plants was flared off into the atmosphere, releasing about 17,000 tons of CO2. But with the city concerned by a big jump in transportation-related air pollution, which has climbed 10 percent since 2000 and contributes to more than half of the country's annual CO2 emissions, officials decided to capture the stuff and burn it in buses.

It's a net-zero carbon equation because the carbon originally came from the atmosphere, city officials say. Even when you account for the electricity used to make the gas, the switch will save 44 tons of CO2 per bus per year.

Setting up the initial infrastructure required for biomethane production ain't cheap, but the gas that gets produced is. Bean counters in Oslo say that on average biomethane gas will be €0.40

($0.50) cheaper per liter than diesel, while conversion will require only minor modification to the buses.

And it turns out Oslo has more than enough sludge to work with. The city's two sewage plants can crank out enough biomethane to power all

80 of the buses involved in the trial. Should the program succeed, it will be expanded to all 400 city buses by producing methane from household and restaurant food waste.

Photo: Stor-Oslo Lokaltrafikk.