The present invention comprises a white comprising a bed (1) capable of directing the water stream (12) between the top inlet and the bottom outlet of the bed and a water-supply means for directing the water stream (12) at the top inlet of the bed. It''s about the water river. The lower bed 1 consists of a series of adjoining unit phase modules 6a which are assembled along each transverse contact surface. The unitary module 6a is supported by the grid-shaped module bearing structure 16 along the lower surface 61.
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