Granular sludge reactor system comprising an external separator

A 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 and an external separator, wherein the method comprises:

• feeding the aqueous fluid 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• withdrawing the fluid that has been contacted with the biomass from an upper part of the bioreactor, which withdrawn fluid comprises biomass;
• feeding the fluid comprising the biomass withdrawn from the upper part of the bioreactor into the external separator comprising a separation chamber, wherein the fluid comprising the biomass is separated into a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 and having a density;
• subjecting the fluid phase enriched in biomass to a density reduction downstream of the external separator to provide a fluid phase enriched in biomass having a reduced density, wherein the density reduction is accomplished by introducing an external gas in said fluid phase enriched in biomass;
• and
• returning the fluid enriched in biomass which has been subjected to said density reduction to the bioreactor.