Consecutive reaction kinetics involving a layered structure of the granule in UASB reactors

A consecutive-reaction kinetic model for the sucrose-fed upflow anaerobic sludge bed (UASB) reactor that accounts for a layered structure of the granule and the mass fraction of methanogens (f) is proposed. When the UASB reactor was maintained at the volumetric loading rates (VLR) of 7.9-13.8 kg chemical oxygen demand (COD)/m(3) d, the accumulated volatile fatty acids (VFAs) increased with increasing VLR, whereas the experimental f decreased with increasing VLR. This was primarily because methanogenesis was the rate-limiting step and the sucrose-fed granule was a layered structure. The calculated residual concentrations of sucrose and the intermediates VFAs using the layered-structure model are less deviated from the experimental measurements than those using the homogeneous-structure model. The calculated effectiveness factors for sucrose uptake and intermediates VFAs uptake (eta(1); eta(2)) ranged from 0.18 to 0.35 and 0.65 to 0.96, respectively, indicating that the overall substrate (sucrose or intermediates VFAs) removal in the UASB reactor was diffusion-controlled, especially at the VLRs of 7.9-10.6. kg COD/m(3) d. This finding was also confirmed by the simulated concentration profiles of sucrose and VFAs in the UASB-granule. From the simulation results, the effect of internal mass transfer resistance on overall substrate (sucrose) removal should not be neglected, especially for a granule size of greater than 2.0 mm.