Mixing is one of the most important operations in high-rate digesters because it maintains uniformity within the digester and prevents scum accumulation. The mixing of anaerobic digester contents significantly influences the efficiency of digestion bioprocesses. Hydraulic dead zones in the reactor reduce the effective hydraulic retention time and, thus, have detrimental effects on the reaction kinetics. In this study, the efficiency of mixing by gas recirculation was examined in a digester of the Haifa Wastewater Treatment Plant (HWWTP). There are various methods and models to evaluate the mixing efficiency of anaerobic digesters . The method used in this study was based upon injection of a fluoride tracer into the reactor at a certain feeding point, followed by a three-dimensional mapping of the fluoride distribution in the reactor as a function of time. Analyzing the results of uniform tracer distribution, it was concluded that there are no real 'dead zones' in the reactor's volume. However, when comparing with literature results of tracer distribution in other digesters, it was concluded that the mixing efficiency is very low and hardly sufficient for having an effective process. Some practical improvements, necessary to achieve higher mixing efficiency, were recommended. © by PSP 2006.