It is available for free in huge quantities, is not owned by Saudi Arabia and it contributes minimally towards climate change. The latest green fuel might seem like the dream answer to climate crisis, but until recently raw sewage has been seen as a waste disposal problem rather than a power source. Now Norway's capital city is proving that its citizens can contribute to the city's green credentials without even realising it.

In Oslo, air pollution from public and private transport has increased by approximately 10% since 2000, contributing to more than 50% of total CO2 emissions in the city. With Norway's ambitious target of being carbon neutral by 2050 Oslo City Council began investigating alternatives to fossil fuel-powered public transport and decided on biomethane.

Biomethane is a by-product of treated sewage. Microbes break down the raw material and release the gas, which can then be used in slightly modified engines. Previously at one of the sewage plants in the city half of the gas was flared off, emitting 17,00 tonnes of CO2. From September 2009, this gas will be trapped and converted into biomethane to run 200 of the city's public buses.

Project leader, Ole Jakob Johansen said: "The city of Oslo has great visions for Oslo as a green capital. Oslo aims to be one of the most environmentally sustainable capitals of the world. 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. What's more, aside from the intial set-up costs, we expect to see an average saving of €0.40 per litre of fuel (based on an average diesel price of €0.67 per litre compared with biomethane at €0.27 per litre)".

The city's diesel public buses will only require minor modifications to their engines to run on methane, which is stored on tanks on top of the vehicles. The only noticeable difference will be how quietly they drive.

"Biogas is popular in Sweden, but they have very few vehicles powered by biomethane. We chose to focus on biomethane as this emits less carbon and is easier and cheaper to produce," said Johansen.

The net emissions from a biomethane operated bus are zero, because the carbon originally came from the atmosphere rather than fossil fuels, but electricity is used at the sewage plant to convert the gas from the waste into fuel for the buses. Oslo city council is taking the electricity used to generate the fuel into consideration and calculate that carbon emissions per bus are 18 tonnes per year, a saving of 44 tonnes of C02 per bus per year.

The city's two sewage plants have enough biomethane to provide fuel for the 80 buses, but if the trial is successful Oslo city council plans to convert all 400 of the public buses to run on biogas. The biogas will be created from a mixture of biomethane and biogas from the incineration of kitchen waste from the capital's restaurants and domestic kitchens. Eventually, the council hopes that cars will also be able to run on biogas sourced locally from biomethane and converted kitchen waste.

What do you think? Should this be deployed more widely?