Sewage treatment. Storage tanks (at upper left) & pools (from bottom left to upper right) at a sewage works used for treatment of industrial and domestic effluent. The sewage is first treated with organic chemicals which make it soluble and stored in the tanks which exclude air. The effluent is broken down by bacteria into various gases (mainly methane) and a humus-like material. The solids are passed on to ponds (brown) where, in the presence of oxygen, different bacteria break down the solid material into carbon dioxide. The bacteria and inorganic solids are filtered out of the water (clear ponds) before being discharged into the sea or a river. Photographed in Germany.
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