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 ward through the granular sludge bed. Here the organic compounds are biologically degraded and biogas is produced. In the top of and just above the granular sludge bed, a fluidized zone develops due to production of the biogas. In this zone further biological degradation can take place. The biogas is separated from the liquid in the gas-liquid separator. Granules with good settling abilities settle back through the fluidized zone to the granular sludge bed, while flocculated and dispersed bacteria wash out of the reactor with the effluent.