A reed bed 1, adapted to extract pollutants such as ammonia from water 12, comprising an outer structure containing a plurality of reed bed modules 13. The outer wall is formed of a wall 2 and/or an excavation that is lined with a waterproof membrane 3. Each reed bed module 13 is formed of an open-topped container (15; fig 4) with apertures or holes (18, 20; fig 4) in the walls and/or floor which allow the passage of water 12, and contains a layer of support medium 16 on which a plurality of reeds 14 have been grown. Handle holes (21; fig 4) are provided for lifting and carrying the container (15; fig 4). The reed bed modules 13, containing reeds 14 already grown to a desired maturity, are installed in the reed bed 1 so that polluted water may be passed through the modules 13 and the root structure of the reeds 14 therein. The reed bed 1 may be divided into bays 10, each containing a plurality of reed bed modules 13, by baffles 8, 9 disposed to ensure that the water 12 passes through the root structure of the reeds 14 of each bay 10.
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