This work describes the experimental activities developed to study the biomass transport
phenomena occurring in upflow anaerobic reactor influencing the biomass washout. The
experimental investigations have been carried outon pilot UASB fed with the aim to determine the
height to which washout is affected by: daily flow distribution; upflow velocity; concentration and
sedimentation properties of the biomass. The experiments had shown a considerable influence on
the biomass behavior of the time interval between two successive feeds of the reactor. It was found
that, if this period is more than 1 hr larger losses of the biomass into the effluent were obtained,
independent of the upflow velocity. Shorter time periods give rise to a regular sludge expansion of
the interface even with very high upflow velocities (up to 4 m/hr), and accordingly exhibit limited
sludge washout.