Abstract: Plastic particles of a selected one of various shapes are usage in a floating media bed of a biological aerated flooded filter. They may be manufactured by injection moulding or by an extrusion-and-chopping method comprising advancing molten plastics through a channel-form extrusion die slot 30, cooling the extruded plastics in water, and chopping-off transversely by a die head face cutter 34 pieces of the cooled channel-form plastics, the distribution of the volumetric rate of flow of the plastics over the outlet mouth of the die slot 30 being such that the external surface of the base of the cooled channel-form plastics is longitudinally convex.