This animation attempts to show the dynamic nature of a UASB reactor. The yellow circles represents evolving biogas bubbles and the brown circles represent sludge granules. The upward liquid flow and uplifting action of gas bubbles causes  the partial fluidization of the sludge bed and hydraulic mixing. The background is a scanning electron micrograph inside a sludge granule cultivated on sucrose-containing wastewater, show a diverse population of microorganisms.