A process and installation are claimed for the clarification of activated sludge. Sludge from a treatment vessel (11) passes (12A) into a mixing zone (13) where a dense insoluble granular material (14) is added. The mixture is held in suspension (19) while activated sludge forms aggregates with the granular material. The mixture then passes into a decantation vessel (21) where clarified effluent (22) and decanted muds (23) are separated. Granular material is separated (15) from the recovered muds and reused.