Method for treating a fluid aqueous waste stream comprising a biodegradable organic substance, comprising—feeding said 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;—taking a first flow from the bioreactor and using the first flow as a feed to a membrane filtration unit, in which said feed is subjected to filtration, thereby forming a permeate stream and a retentate stream;—returning the retentate stream to the bioreactor;—taking a second flow from the bioreactor as feed to a sludge treatment unit, in which sludge treatment unit the second flow is separated into a third flow, having an increased organic substance content compared to the second flow, and a fourth flow, having a decreased organic substance content compared to the second flow.