SEM(Scanning Electronic Microscope) observations were made to compare the microbial characteristics between UASB and fixed film (FF) reactors with various wastes: concentrated vs diluted, and complex vs simple volatile waste. It appeared that the sludge granulation was influenced by the wastes fed to the reactor rather than the reactor type itself. Stable granulations were not developed with the concentrated waste as well as the simple volatile waste. Microbial washout was pronounced as the volatile fraction of waste was increased. Rod and cocci shape bacteria were easily washout but filamentous bacteria seemed to entangle those bacteria not to washout easily.