A simplified analysis of reaction and mass transfer in UASB and EGSB reactors

The paper is focused on the study of the hydrodynamics, reaction kinetics and mass transfer in lab-scale Upflow Anaerobic Sludge Blanket (UASB) and Expanded Granular Sludge Bed (EGSB) reactors treating wastewaters with low volatile fatty acids concentrations. The fluid pattern in the UASB approached a plug-flow reactor because of the low turbulence provided by the upflow velocity and gas production. The EGSB behaved as a continuous-stirred tank reactor due to the effects of the high recirculation rate. Liquid film mass transfer resistance seemed negligible in the EGSB but important in the UASB reader. This should be the consequence of the different hydrodynamic conditions in the two reactors. The rate of acetate degradation was best fitted by an apparent half-order kinetics in both reactors. Internal mass transfer limitations, associated to an intrinsic zero-order kinetics, was detected in the degradation of low acetate concentrations by the granular biomass.