Diffusion of substrate and oxygen in aerobic granule

In this study, one-dimensional models were developed to simultaneously describe diffusion of substrate and dissolved oxygen in aerobic granule under various conditions in sequencing batch reactor (SBR), meanwhile the models were further applied to predict changes of the bulk substrate concentration in the course of SBR operation. The proposed models were solved numerically by the Numerical Finite Difference Methods without making any additional assumptions. Results showed that diffusions of substrate and dissolved oxygen in aerobic granule would be a dynamic process, and they were closely interrelated. For aerobic granules with a size smaller than 0.4 mm, no diffusion limitation of substrate and dissolved oxygen occurred. However, when aerobic granules grew to a size larger than 0.5 mm, dissolved oxygen became a major limiting factor of metabolic activity of aerobic granule over substrate. These imply that the optimal size of aerobic granule would be less than 0.5 mm in the sense of mass transfer. It was revealed that the substrate removal rate was inversely related to the size of aerobic granules, the substrate removal rate by aerobic granules with a size of 0.5 mm was almost three times of that by aerobic granules with a size of 1 mm.