Method for treating an aqueous fluid comprising a biodegradable organic substance in an installation comprising an upflow bioreactor containing a granular sludge bed, said granular sludge bed comprising granular biomass, and an external separator, wherein the method comprises - the feeding of the aqueous fluid into a lower part of the bioreactor, contacting the fed fluid with the granular biomass, which granular biomass comprises anaerobic microorganisms and wherein the biodegradable organic substance is converted by the anaerobic micro-organisms, thereby forming biogas from the biodegradable organic substance; - the withdrawing of the fluid that has been contacted with the granular biomass from an upper part of the bioreactor, which withdrawn fluid comprises granular biomass; and - the feeding of the aqueous fluid comprising the granular biomass into the external separator comprising a separation chamber provided with tilted internals, inside which external separator granular biomass settles, wherein in the separation chamber the aqueous fluid comprising the granular biomass is separated into a liquid phase, which has a reduced granular biomass content or is essentially free of granular biomass, and a fluid phase enriched in granular biomass, which fluid phase enriched in granular biomass comprises settled granular biomass and is returned to the bioreactor via a conduit and an inlet for the fluid enriched in granular biomass into the bioreactor, wherein the external separator is positioned at the same height or below the bottom of the bioreactor, whilst the inlet for the fluid enriched in granular biomass into the bioreactor via the conduit is positioned at a higher level than at least an outlet of fluid enriched in granular biomass of the external separator, and wherein biogas from the bioreactor is collected by a biogas collector connected to a biogas injector for injecting said biogas into the conduit for returning said fluid phase enriched in granular biomass from the external separator to the bioreactor, which injected biogas promotes a flow of the fluid enriched in granular biomass from the external separator towards the bioreactor. - the feeding of the aqueous fluid into a lower part of the bioreactor, contacting the fed fluid with the granular biomass, which granular biomass comprises anaerobic microorganisms and wherein the biodegradable organic substance is converted by the anaerobic micro-organisms, thereby forming biogas from the biodegradable organic substance; - the withdrawing of the fluid that has been contacted with the granular biomass from an upper part of the bioreactor, which withdrawn fluid comprises granular biomass; and - the feeding of the aqueous fluid comprising the granular biomass into the external separator comprising a separation chamber provided with tilted internals, inside which external separator granular biomass settles, wherein in the separation chamber the aqueous fluid comprising the granular biomass is separated into a liquid phase, which has a reduced granular biomass content or is essentially free of granular biomass, and a fluid phase enriched in granular biomass, which fluid phase enriched in granular biomass comprises settled granular biomass and is returned to the bioreactor via a conduit and an inlet for the fluid enriched in granular biomass into the bioreactor, wherein the external separator is positioned at the same height or below the bottom of the bioreactor, whilst the inlet for the fluid enriched in granular biomass into the bioreactor via the conduit is positioned at a higher level than at least an outlet of fluid enriched in granular biomass of the external separator, and wherein biogas from the bioreactor is collected by a biogas collector connected to a biogas injector for injecting said biogas into the conduit for returning said fluid phase enriched in granular biomass from the external separator to the bioreactor, which injected biogas promotes a flow of the fluid enriched in granular biomass from the external separator towards the bioreactor.