Abstract: A biological contactor with a granular bed including a layer based on activated carbon, the material of which satisfies the following conditions:density between 0.18 and 0.32 g/cm.sup.3,carbon tetrachloride adsorbing power between 60% and 120% by weight,carbon tetrachloride retention less than 25% by weight of the carbon tetrachloride adsorbed mass,the carbon tetrachloride adsorbed mass, methylene blue: between 5 and 30 ml,particle size: between 0.5 and 1.5 mm effective size with a coefficient of uniformity less than 2.A washing cycle (air only; air plus water; water only) of the contactor is triggered when a parameter representative of the population of algae in said layer reaches a predetermined set point threshold.

Abstract: A biological contactor (8) comprises a granular bed including a layer (17) based on activated carbon the material of which satisfies the following conditions:density between 0.18 and 0.32 g/cm.sup.3,carbon tetrachloride adsorbing power between 60% and 120% by weight,carbon tetrachloride retention less than 25% by weight of the carbon tetrachloride adsorbed mass,methylene blue: between 5 and 30 ml,particle size: between 0.5 and 1.5 mm effective size with a coefficient of uniformity less than 2.A washing cycle (21, 24) (air only; air plus water; water only) of the contactor is triggered (38) when a parameter (36, 37) representative of the population of algae in said layer reaches a predetermined set point threshold.

Abstract: Method of treating liquid by sedimentation in which reagents are injected into a flow of untreated liquid and there is created in the flow a colloid mixing and stabilization area; the flow is passed into an intermediate aggregation area and then into a sedimentation area equipped with separator plates from which clarified liquid is taken; characterized in that insoluble granular material which is more dense than the liquid is injected into the liquid in predetermined proportions in the mixing area where the flow is made turbulent, turbulence is induced in the intermediate aggregation area to maintain the granular material in suspension, virtually all of the granular material is brought into the sedimentation area, the sludge recovered in the sedimentation area is removed, the granular material is removed therefrom (D) and is recycled after cleaning.