Abstract: The invention concerns a filter whereof the reactor comprises: in its bottom part, an inlet for the effluents to be filtered and an outlet of oxygenated gas; in its upper part, an outlet for the filtered effluents; filtering means including media layers and an attached biomass, of density lower than that of the effluents to be filtered and interposed between the inlet and the outlet. The reactor is subdivided by means of racks retaining the filtering means into at least two filtering stages and into an upper outlet stage, wherein emerges the outlet for the filtered effluents. Each filtering stage is provided with a layer and its own filtering means and comprises, in its bottom part, a withdrawing outlet for excess biomass.

Abstract: A method is provided for introducing a fresh sludge flux whose concentration in terms of dry material is less than 40 g/l into the upstream part of digester, maintaining a thin sludge layer in the digester, circulating substantially horizontally effluents above and across the sludge layer and removing the effluents from the digester simultaneously with soluble products of sludge digestion. The cuvette of the digester is divided into sections by transverse walls, which are provided with communication openings in the lower part thereof.