Abstract: The invention relates to apparatus (1) in the form of a treatment vessel comprising a high voidage media bed (2) situated above a low voidage granular media bed (3). Fluid (4) to be treated, combined with an effluent recycle flow stream (5) and intermediate recycle flow stream (6) as appropriate, flows down through the flooded aerated high voidage media (2), through an intermediate section (7) containing a collector in the form of a trough (8) and down through the aerated packed bed of granular media (3). The trough (8) is known as a washout trough. The high voidage media is supported on a support means such as a grille or grid (9). During a cycle any surplus treated water passes out of the intermediate section (7 at 10), while backwash liquors pass out of the vessel at (11). The usual operating level of the liquid in the vessel is shown at (12), treated liquor or effluent passing out of the vessel at (13). Process air passes into the vessel (1 at 14).