Anaerobic digestors are commonly used in the treatment of municipal and industrial wastewaters. Fairly recently, anaerobic upflow systems have been applied to the treatment of wastewaters. The upflow anaerobic sludge blanket reactor was developed by Lettinga and co-workers (20). A variant of this design was subsequently produced by Guiot and van den Berg (14) and is referred to as an upflow anaerobic sludge bed and filter system. A major advantage of these upflow systems is that their design permits the retention of a greater amount of active biomass in comparison with other anaerobic reactors. It is known that the loading rate of an anaerobic wastewater treatment system is dependent on the amount of active biomass present in the reactor (19). Therefore, these upflow systems can accommodate organic loading rates several times higher than those of other anaerobic digestors.The ability of the upflow reactors to accumulate large amounts of biomass is due to the adhesion of bacterial cells to each other. The adhering bacteria form granules of biomass which can be several millimeters in diameter.