Fouling is a common issue in membrane bioreactors, significantly reducing
their separation efficiency. Increasing surface shear force is an effective
strategy for mitigating or reducing fouling. This study employed
a shear-enhanced lab-scale membrane bioreactor, featuring a stationary
circular membrane and a rotating impeller with a diameter of 35 mm,
housed within a cylindrical cell with a diameter of 56 mm. Flow field
simulations inside the membrane module were performed using the commercial
Computational Fluid Dynamics (CFD) software, ANSYS Fluent.
The influence of different rotational velocities on shear stress distribution
was analysed, and the CFD predictions were compared with theoretical
models. Shear stress contours at a rotation speed of 250 rpm, and the
radial distribution of shear stress on the membrane under various rotation
speeds are shown in Fig. 1a and b, respectively. The results
indicate that increasing impeller speed leads to a corresponding increase
in shear stress at the membrane surface, driven by a higher velocity gradient
between the impeller and membrane. For instance, the maximum
shear stress at 1000 rpm was 94%, 84%, and 47.5% higher than that
observed at 100 rpm, 250 rpm, and 500 rpm, respectively. Shear stress
increases with radial distance due to the rise in tangential velocity generated
by the rotating impeller. Maximum velocity is observed near the rim of the impeller, while the minimum is at the cell center. Consequently,
shear stress is lowest at the center of the cell and highest near
the impeller rim (r = 0.0175 m). As illustrated in Fig. 1a, the shear
stress on the membrane surface decreases steadily from r = 0.0175 m
towards the wall, due to the wall’s obstruction. Furthermore, because of
the impeller’s short length, the region between the impeller rim and the
housing wall experiences less exposure to the high shear forces generated
by the impeller.