NOVELTY - Method (M1) for treating fluid aqueous waste stream involves feeding waste stream into bioreactor (5); reacting biodegradable organic substance with biomass under anaerobic conditions, and forming methane; feeding first flow of fluid from bioreactor to membrane filtration unit (7) and forming permeate/retentate streams; returning retentate stream to bioreactor; separating second flow of fluid using sludge treatment unit (6), into third flow having higher organic substance and fourth flow having lower organic substance; and feeding fourth flow to membrane filtration unit with first flow.    USE - For treating fluid aqueous waste stream using bioreactor; to degrade biodegradable organic substance such as biodegradable pollutants.    ADVANTAGE - The method provides installation for treating fluid aqueous waste stream having reduced overall-energy consumption of the membrane bioreactor, reduced fouling of the membrane, prolonging life time of the membrane, increased availability of the membrane bioreactor (time between to stop, to clean or replace parts), increased flux over the membrane, improved bioreactor performance, increased treatment capacity, increased treatment efficiency, reduced overall Capex (capital expenditures) i.e. the cost of long-term improvements, reduced overall Opex (operational expenditure) with respect to energy and chemicals. By using present method it is possible to operate the membrane bioreactor at a satisfactory organic loading rate, while controlling the membrane fouling. The biogas is injected in a tubular filter membrane as a gas lift technique, thus controlling membrane fouling.    DETAILED DESCRIPTION - Method (M1) for treating fluid aqueous waste stream comprising biodegradable organic substance, involves feeding aqueous waste stream into bioreactor (5) containing fluid which contains biomass; reacting biodegradable organic substance with biomass in the bioreactor under anaerobic conditions, and forming methane; taking first flow of fluid from bioreactor and using the first flow for feed to membrane filtration unit (7) (filter feed), in which feed is subjected to filtration, and forming permeate stream and retentate stream; returning the retentate stream completely or partly to the bioreactor; taking second flow of fluid from bioreactor as feed to sludge treatment unit (6) positioned outside the bioreactor or integrated in bioreactor, where the second flow of fluid is separated into third flow having a higher organic substance content than the second flow, and fourth flow having a lower organic substance content than second flow; using fourth flow completely or partly for the feed of membrane filtration unit, typically after mixing fourth flow with first flow that is to be fed to membrane filtration unit. An INDEPENDENT CLAIM is included for installation suitable for carrying out method (M1), comprising bioreactor (5); sludge treatment unit (6) for separating fluid stream from bioreactor (second flow) into a stream (third flow) having higher organic substance content than fluid stream from the bioreactor and a stream (fourth flow) having lower organic substance content than fluid stream from the bioreactor, and membrane filtration unit (7), the bioreactor having inlet (8, 8a) for fluid waste stream, outlet (12, 12a, 12b) for fluid flow and outlet (11) for a gas; the sludge treatment unit comprising inlet (13) for fluid originating from the bioreactor, having inlet (13) connected with outlet for fluid (12, 12a, 12b) of bioreactor via conduit (2) for transporting fluid from bioreactor to sludge treatment unit comprising outlet (14) for fourth flow, and outlet (15) for third flow; the membrane filter unit comprising an inlet (16) for fluid which is connected with outlet (12, 12a) for fluid from bioreactor via conduit (1) for transporting fluid from bioreactor to filter unit (7); the membrane filtration unit comprising outlet (18) for permeate and outlet (17) for retentate, where outlet (17) is connected to recycle conduit (9) for returning retentate from filtration unit to bioreactor; where outlet (14) for fourth flow is connected with conduit (4, 4a) for introducing fourth flow, preferably after mixing with fluid taken from the bioreactor via outlet for fluid (12, 12a), via inlet (16) filter feed into the filtration unit and/or where outlet (14) for fourth is connected with conduit (4, 4b) for recycling fourth flow to the bioreactor via the same inlet (8) or separate inlet (8b) into the bioreactor.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram of combination of installations.    Conduit (1)    Conduit (2)    Conduit (4, 4a, 4b)    Bioreactor (5)    Sludge treatment unit (6)    Membrane filtration unit (7)    Inlet (8, 8a, 8b)    Recycle conduit (9)    Outlet (11)    Fluid (12, 12a)    Inlet (13)    Outlet (14)    Inlet (16)