Influencing effect of intra-granule mass transfer in expanded granular sludge-bed reactors treating an inhibitory substrate

Two expanded granular sludge-bed (EGSB) reactors (superficial velocity us = 6.0 and 9.0 m/h) were used to treat an inhibitory substrate phenol. The granule diameter (dp) increased with increasing organic loading rate (OLR) and us. At the OLRs of 1.67–4.44 kg phenol/m3 d, the accumulation of volatile fatty acids (VFAs) was insignificant; whereas at the OLR of 5.11 kg phenol/m3 d, both the accumulation of VFAs and the washout of large hollow granules (average dp = 2.90–3.12 mm) occurred. The comparative experimental and simulated results showed that the proposed kinetic model is suitable for design and predicting purposes. The calculation results of mass transfer parameters (Thiele modulus, Biot number, diffusion layer thickness, and overall effectiveness factor) and parametric sensitivity analysis results (half-saturation constant Ks and dp) showed that the intra-granule mass transfer would lead to a more influencing effect than the external mass transfer on the overall substrate removal rate in EGSB reactors.