The Swiss capital's fleet of 100 buses will soon be switching to sewage gas - a cheaper and cleaner alternative to petrol.
Construction work is underway on a pipeline to connect the Bern sewage works to the natural gas network, which will supply the city's bus depot.
Bernmobil, which runs tram and bus services in the capital, expects the first 32 Volvo gas-powered buses to be delivered in May. As the rest of the fleet becomes obsolete, it will be replaced by the non-diesel models.
Basel, Geneva, Lucerne and Lausanne all have gas-run buses, but so far only Lucerne uses biogas from the sewage plant. Its system was the first of its kind in Switzerland and went into production in January 2005.
Bern expects to more than double the performance of the central Swiss plant, with a predicted annual production of 13 billion kilowatt hours.
Waste not, want not
About 450 sewage treatment plants in Switzerland produce biogas. Bern is one of three that are about to enrich the gas to bring it up to natural gas standard so it can be used as fuel.
Five more of the larger plants have applied for planning permission to build enrichment facilities.
Some 190,000 houses are connected to the Bern sewage works, which treats 35 billion litres of wastewater a year.
The resulting sludge from this treatment is fed into bioreactors, where it undergoes a digestion process from which biogas is produced with an average methane content of 65 per cent.
Since the plant opened in 1967 this gas has been used to heat and power the plant.
In the past few years however the plant has been producing more gas than it can use and a portion has to be burnt off.
Instead of wasting fuel, the directors decided to upgrade the gas so that it has the same methane content as natural gas – 96 per cent – and can be fed into the natural gas network.
The upgrading process removes carbon dioxide and water and harmful trace components such as hydrogen sulphide while increasing the relative methane content.
The Swiss government is keen to encourage the use of gas rather than petrol as a fuel source, partly because of increasing concerns over traffic pollution.
The limits for breathable fine particles are regularly exceeded in built-up areas, costing billions of francs in healthcare.
From 2007, natural gas will no longer be subject to government fuel taxes, while petrol and diesel taxes will increase.
"The amount of nitrogen emitted into the atmosphere from the new gas-powered fleet will be reduced from the current annual level of 75 tons to 34 tons," Beat Ammann, Bern sewage plant director, told swissinfo.
Gas-powered buses have other advantages over diesel. Jean-Marc Hensch, director of the Swiss Gas Industry Association, says they are quieter and their fuel reservoirs are more resistant to damage.
"If the gas should escape, it simply dissipates, whereas spilled diesel tends to pool on the ground and present a further fire hazard," Hensch told swissinfo.
The recent boost in gas production at the Bern sewage works is largely due to deliveries of waste from the food industry: the amount has doubled since 2003.
Seventy-three Swiss farms produce biogas from rotting compost, and they are also looking to maximise their incomes by adding food leftovers to their bioreactors.
So there is stiff competition for industrial clients.
Biogas production is clearly a growth industry, but it may take a long time before it can supply enough fuel to run ten per cent of Swiss buses and cars – the target of the Agency for Renewable Energy and the gas industry.
Agency director Arthur Wellinger predicts that, by 2010, only ten per cent of traffic will be powered by gas, and ten per cent of that will be made up of biogas.
swissinfo, Julie Hunt
Organic waste from one person can produce 30-35 litres of fermentation gas per day.
Bern sewage plant's biogas upgrading facility and connection pipeline will cost SFr2.3 million ($1.75 million).
It will be partly funded by canton Bern and the city's public energy and water supplier, EWB.
Sewage is a renewable, environmentally friendly source of energy and is about to be used to power the bus fleet in Switzerland's capital.
The sewage will be turned into biogas, a sustainable alternative to non-renewable energy sources. It is carbon dioxide neutral and generates lower emissions of nitrogen oxides and fine particles than fossil fuels.
Biogas will be delivered in pipelines with natural gas, which produces 20% less carbon dioxide emissions than petrol.
Purified biogas is around 40% cheaper to produce than fossil fuels and much cheaper to buy.