UASB REACTOR
The advantage of the UASB reactor compared to traditional anaerobic treatment, e.g., the contact process, is the ability to retain high biomass concentrations despite the upflow velocity of the wastewater and the production of biogas. Consequently, the reactor can operate at short hydraulic retention times since the sludge retention time is almost independent of the hydraulic retention time. Successful operation under these conditions requires a highly active biomass with good settling abilities. In UASB reactors, the biomass is retained as aggregates, called granules, formed by the natural self-immobilization of the bacteria, i.e., this kind of immobilization does not employ any supporting material such as Rasching rings or clay. The formation and stability of the granules are essential for successful operation. The UASB reactor is typically divided into four compartments: (1) the granular sludge bed, (2) the fluidized zone, (3) the gas-solids separator, and (4) the settling compartment (Fig. 1). The granular sludge bed is located in the bottom of the reactor. The wastewater is pumped into the bottom of the reactor and passes up