The Bromma Biogas Plant Stockholm
BACKGROUND
In the waste water treatment plant of Bromma, Stockholm, about 10000 tons of sewage sludge (dry matter) is produced annually. Before the 1970's this sludge used to be deposited on waste deposits. However, the sludge contains a large amount of organic matter which could be used as an energy source. Since the 1970's, sludge has been treated by anaerobic digestion producing biogas. In such a plant the sludge is biologically treated under anaerobic conditions. The product is biogas and a solid organic rest product. The biogas was earlier used for internal heating at the WWTP, and also sold for external use. In 1996-1997 a pilot biogas upgrading plant was installed, and in 2001 a full-scale upgrading plant was built. The upgrading plant separate methane from carbon dioxide, water and other contaminants in order to produce a vehicle fuel of natural gas quality. 1.5 million m3 gas will be produced annually, which corresponds to 1.5 million litres of petrol. Today (2004) there are four fuel stations in Stockholm providing gas as fuel. Biogas is one of the least environmentally harmful fuels commercially available today. Because it is based on biologically produced substances it is a part of the recycling process, and does not contribute to the CO2 content in the atmosphere. Emissions of hydrocarbons, CO and nitric oxides from production and use are small. The rest product has to a large extent been used as soil improvement and fertilizer thanks to the decreasing amounts of pollutants e.g. heavy metals. However, the use in agriculture is nowadays limited. In the future, with still better control of the quality of the sewage water, the rest product can be better utilized. This is important from a recycling point-of-view, because the rest product contains phosphorus, which is a limited natural resource.
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