Until recently raw sewage has been seen as a waste disposal problem rather than a power source, but 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 carbon dioxide 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. They decided on biomethane, which 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 the gas was flared off, emitting 17 tonnes of carbon dioxide. From September 2009, this gas will be trapped and converted into biomethane to run 200 of the city's public buses.
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. 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.
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 carbon dioxide per bus per year. The city's two sewage plants have enough biomethane to provide fuel for 80 buses, but if the trial is successful there are plans to convert all 400 public buses to run on biogas.
