Insight into mass transfer by convective diffusion in anaerobic granules to enhance biogas production

Mass transfer in anaerobic granule is crucial for efficient biogas production, but so far the fundamental understanding remains poor. This study aims at gaining an insight into convective diffusion to enhance mass transfer within small (0.5-1 mm), medium (1.5-2 mm) and large (3-3.5 mm) granules. The convective diffusion rates (F-CD) of granules at a superficial liquid velocity of 3 mill were 2-3 magnitudes greater than molecular diffusion rates (F-MD), and significantly enhanced the biogas production rate of small, medium and large granules by 4, 1.5 and 1.3 folds. The granules were permeable in nature as fluid collection efficiency (eta) of small, medium and large granules of 0.97, 0.89 and 0.58 indicated, and were of cluster-cluster formation as fractal dimension (Df revealed. The large granules possess best mass transfer condition and highest biogas production rate because they had highest F-CD due to the biggest permeable area despite lowest eta. This work for the first time elaborates the important roles of internal structures in terms of permeability and fractal dimension on the convective mass transfer in anaerobic granules, and it also highlights the importance of molecular diffusion. The results could facilitate upgrading the understanding of mass transfer process in anaerobic granules to enhance biogas production. (C) 2017 Elsevier B.V. All rights reserved.