The treatment processThe inhabitants of Borås sort their food waste into black sacks and incinerable waste into white sacks. At Sobacken, they are sorted optically; photocells in a mechanical sorter sense what color a sack is and ensures that each one goes to the right place. White sacks are crushed and used as fuel at Ryaverket, a combined power and heating plant (CHP) in Borås. When the waste is combusted in the CHP, it heats water that is then pumped into the district heating network.
Black sacks are opened automatically. The food waste is transferred to a3,000 cubic meter tank where it is digested. The temperature in the tank is a constant 55 degrees — the optimum conditions for the bacteria — and gas production is thus very efficient. It takes about 30 days for the bacteria in the digester to break down the organic waste and produce raw gas.
Black sacks are tumbled again to ensure that no waste, or rather energy, is lost. The empty sacks are burnt at Ryaverket together with the white sacks.
But it is not only households´ food waste that is digested to make biogas. About half the waste that is digested comes from industry. In Borås, waste comes from a local meat packing plant, a local fruit juice producer, and the city´s restaurants, shops, and hypermarkets.
The raw biogas is transported in pipelines to Gässlösa where it is combined with gas from the municipality´s wastewater treatment plant, which also has a digestion process for its sewage slurry. Three quarters of the gas produced in Borås today comes from waste and the remaining quarter from sewage slurry.
The raw gas is transported by pipeline to an upgrading plant, where the carbon dioxide is removed. The raw gas consists of 65% methane and 35% carbon dioxide, but after cleaning it consists of almost 99% methane.
The gas is then transported, once again by pipeline, to the filling stations operated by Borås Energi och Miljö. A filling station that can accommodate 43 buses and a filling station for private motorists in the city center will open early next year.
In Borås today 29 buses, 9 garbage trucks and some hundred private cars run on biogas. From 2010, Borås municipal transport is planning to run all its city buses on biogas.
The residue that remains after digestion is called bio-fertilizer. This is mixed with garden refuse from the recycling plants and manure, among other places from Borås Zoo. The resulting compost is then sold to the municipality´s inhabitants.
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A few facts:
About 100,000 people live in Borås municipality. The city has become a model for other municipalities since it was the first one in the country to invest in a biogas plant. There is also a well-established cooperation with the University of Borås and trade and industry.
The first plant in Borås opened as long ago as 1995. Ten years later, biogas production really took off in a new, more modern plant. The city´s first biogas-powered garbage truck entered service in 2002.
