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Sewage treatment. Sewage treatment plant buildings which are used for thickening sewage sludge (in foreground) and bacterial anaerobic treatment of sewage (in background). The sewage is treated in closed chambers or digesters by bacteria which digest it in oxygen-free (anaero- bic) conditions. This digestion process lasts for 10-30 days and reduces the sewage to carbon dioxide and methane and a humus-like material. The methane can be collected and used as fuel, whilst the solid residue can be dried and used as a fertiliser or burnt. Photographed in Bavaria, Germany.
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