DO diffusion profile in aerobic granule and its microbiological implications

By using a dissolved oxygen (DO) microelectrode, this study investigated the DO diffusion profiles in aerobic granules with different sizes under substrate-free and substrate-sufficient conditions. Results showed that DO only partially penetrated through 500 μm from the granule surface under substrate-sufficient condition. On the contrary, no DO limitation was found in aerobic granules with a radius less than 2.2 mm under substrate-free condition, i.e., aerobic condition could be maintained in the entire aerobic granules. A one-dimensional model was also used to describe the DO diffusion in aerobic granules, and the model prediction was in good agreement with the measured DO profiles. It was shown that metabolism of aerobic granules involved conversion of external TOC to storage materials followed by microbial growth on storage materials and endogenous respiration. Because of the layered structure of aerobic granules, simultaneous nitrification–denitrification and reduced aeration during famine period can be expected in aerobic granular sludge SBR.