CFD simulations of membrane filtration zone in a submerged hollow fibre membrane bioreactor using a porous media approach

The current membrane bioreactor (MBR) design methods and the popular bio-kinetic models rely on the assumption that membrane bioreactor is completely mixed, neglecting the real hydrodynamic condition within the reactor. MBRs differ from conventional reactors in so far as the spatial distribution of reactor discharge points is very broad for an MBR compared with a conventional bioreactor. Computational Fluid Dynamics (CFD) provides a possibility to investigate the hydrodynamic behaviour of large scale MBRs. The CFD modelling of whole MBR plant requires a macro-scale approximation which can keep the mesh size and computation effort within the reasonable limit. However, the simulation of the flow behaviour surrounding the membranes requires high mesh resolutions and hence large element numbers. Therefore, it is impossible to model each individual hollow fibre using the Navier–Stokes equations as it is too computationally costly. In this paper, the effects of Siemens Memcor Memjet® hollow fibre membrane bundle on flow field were transferred to a porous media model. This porous media model was coupled with a three-dimensional multiphase model to account for the hydrodynamic behaviour of a full-scale submerged MBR. An experimental approach was developed to calibrate the inertial loss caused by the hollow fibre bundle against various liquid velocities at different flow direction and fluid viscosity. The experimentally determined inertial losses were compared against those estimated from the empirical correlations for tube banks. These experimental calibrations were then applied to the porous media model. Significant improvement on the hydrodynamic descriptions was observed by coupling the porous media model compared with the previous developed MBR CFD model.