Method for treating a waste stream using a bioreactor and a membrane filter

Method for treating a fluid aqueous waste stream comprising a biodegradable organic substance, comprises feeding the aqueous waste stream into a bioreactor containing a fluid which contains biomass; reacting the biodegradable organic substance with the biomass in the bioreactor under essentially anaerobic conditions, thereby forming methane; - taking a first flow of fluid from the bioreactor and using the first flow for a feed to a membrane filtration unit (filter feed), in which said feed is subjected to filtration, thereby forming a permeate stream and a retentate stream; returning the retentate stream completely or partly to the bioreactor; taking a second flow of fluid from the bioreactor as feed to a sludge treatment unit, where a flocculation or coagulation additive is added to the second flow before feeding the second flow to the sludge treatment unit or where a flocculation or coagulation additive is added inside the sludge treatment unit, whereby organic substance in the flow flocculates or coagulates, which sludge treatment unit may be positioned outside the bioreactor or integrated in the bioreactor, in which sludge treatment unit the second flow of fluid is separated into a third flow, having a higher organic substance content than the second flow, and a fourth flow, having a lower organic substance content than the second flow. The method further comprises using the fourth flow completely or partly for the feed of the membrane filtration unit after mixing the fourth flow completely or the part thereof that is to be fed to the membrane filtration unit with the first flow that is to be fed to the membrane filtration unit; or - feeding the fourth flow completely or partly to the bioreactor. Installation suitable for carrying out a method according to any of the preceding claims, comprising a bioreactor; a sludge treatment unit suitable for separating a fluid stream from the bioreactor (second flow) into a stream (third flow) having a higher organic substance content than the fluid stream from the bioreactor and a stream (fourth flow) having a lower organic substance content than the fluid stream from the bioreactor, and a membrane filtration unit is disclosed. The bioreactor comprises an inlet for a fluid waste stream, an outlet for a fluid flow and an outlet for a gas; the sludge treatment unit comprising an inlet for fluid originating from the bioreactor, which inlet is connected with the outlet for a fluid of the bioreactor (5) via a conduit f