Model Development of a Sponge Carrier Process Using CFD-DEM with Permeable Particles

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Sponges, as moving bed biofilm carriers, have shown promising nitrification performance but there is limited information on the mathematical models. With the aim of calculating mass transfer and reaction within sponge carriers, pore diffusion and advection inside the sponges were modelled. Computational Fluid Dynamics (CFD) was used to model the hydrodynamics and was combined with Discrete Element Method (DEM) to model sponges and air bubbles as particles in a tank with three different configurations of aeration. The slip velocity of sponges was used as a model interface to link external hydrodynamics to sponges' internal velocity. The effect of pore convection resistance on biofilm reaction could be characterized with a single non-dimensional variable: Damköhler number, <I>Da</I>, as the ratio of reaction rate to the pore convection rate. For <I>Da</I> › 1, pore diffusion inhibited reaction and an empirical correlation of the reduction in the effectiveness factor could be obtained. Under normal operating conditions, sponge diameters and aeration intensity had significant impact on internal mass transfer. In the case of a fixed bulk oxygen concentration, there was no significant difference in the internal mass transfer for different aerator types.