Relationship between size and mass transfer resistance in aerobic granules

Aims: To investigate the size effect of aerobic granules on mass transfer efficiency by introducing the effective factor and the modified Thiele modulus. Methods and Results: Batch experiments of aerobic granules with different sizes were conducted to study the size effect of granules on mass transfer resistance. Results showed that both specific substrate removal and biomass growth rates were size dependent, i.e. reduced rates were observed at big sizes. It was found that the diffusion resistance described by the effective factor and the Thiele modulus increased with the increase of the size of aerobic granules. Conclusions: The effective factor should be controlled at values higher than 0.44 and the Thiele modulus lower than 1.05 for efficient mass transfer in aerobic granules. Significance and Impact of the Study: Based on the coupled effective factor and Thiele modulus, an operation guidance including granule radius, kinetics of biomass and environmental conditions could be proposed for stable aerobic granulation.