Method for treating an aqueous fluid comprising a biodegradable organic substance in an installation comprising an upflow bioreactor containing a sludge bed, said sludge bed comprising biomass, an external separator, which external separator is positioned at the same height or below the bottom of the bioreactor, and a conditioning tank, wherein the method comprises - treating the aqueous fluid in the conditioning tank; thereafter - feeding the aqueous fluid from the conditioning tank into a lower part of the bioreactor, contacting the fed fluid with the biomass, thereby forming biogas from the biodegradable organic substance; - withdrawing the fluid that has been contacted with the biomass from an upper part of the bioreactor, which withdrawn fluid comprises biomass; - feeding the aqueous fluid comprising the biomass withdrawn from the upper part of the bioreactor into the external separator comprising a separation chamber provided with tilted internals wherein the aqueous fluid comprising the biomass is separated into a liquid phase, which has a reduced biomass content or is essentially free of biomass, and a fluid phase enriched in biomass; - returning said fluid phase enriched in biomass from the external separator to the bioreactor, wherein biogas from the bioreactor is collected by a biogas collector connected to a biogas injector injecting said biogas into a conduit for returning said fluid phase enriched in biomass from the external separator to the bioreactor, which conduit for returning the fluid enriched in solids is provided with an inlet to discharge the returned fluid into the bioreactor, which inlet to discharge the returned fluid into the bioreactor is at a higher height than the biogas injector; and - returning a part of said liquid phase having a reduced biomass content or being essentially free of biomass, from the external separator to the conditioning tank. - treating the aqueous fluid in the conditioning tank; thereafter - feeding the aqueous fluid from the conditioning tank into a lower part of the bioreactor, contacting the fed fluid with the biomass, thereby forming biogas from the biodegradable organic substance; - withdrawing the fluid that has been contacted with the biomass from an upper part of the bioreactor, which withdrawn fluid comprises biomass; - feeding the aqueous fluid comprising the biomass withdrawn from the upper part of the bioreactor into the external separator comprising a separation chamber provided with tilted internals wherein the aqueous fluid comprising the biomass is separated into a liquid phase, which has a reduced biomass content or is essentially free of biomass, and a fluid phase enriched in biomass; - returning said fluid phase enriched in biomass from the external separator to the bioreactor, wherein biogas from the bioreactor is collected by a biogas collector connected to a biogas injector injecting said biogas into a conduit for returning said fluid phase enriched in biomass from the external separator to the bioreactor, which conduit for returning the fluid enriched in solids is provided with an inlet to discharge the returned fluid into the bioreactor, which inlet to discharge the returned fluid into the bioreactor is at a higher height than the biogas injector; and - returning a part of said liquid phase having a reduced biomass content or being essentially free of biomass, from the external separator to the conditioning tank.