The MBR Book: Principles and Applications of Membrane

(Cui et al., 2003) by inducing liquid flow fluctuations and local tangential shear
transients, the shear rate � (/s) being given by:
k
g �
d
U L
Bubble flow Slug flow Churn flow Annular flow Mist flow
Increasing air flow
Figure 2.31 Air–liquid flow regimes in a cylindrical channel (Judd et al., 2001)
Fundamentals 85
(2.31)
where U L is the liquid CFV (m/s), � is the separation (m) and � is a constant depending
on the membrane geometry. The effect is to increase back transport and promote
mass transfer of liquid through the membrane (Section 2.1.4.4). Tangential shear
at the membrane surface prevents large particle deposition on the membrane surface.
However, since it is proportional to the cube of particle diameter, lateral migration
velocity for smaller particles is much less, leading to more severe membrane
fouling by fine materials (Choo and Lee, 1998).
It has long been recognised through studies of model systems (Cabassud et al.,
1997; Ghosh and Cui, 1999; Cui and Wright, 1994; Mercier et al., 1997) as well as
MBRs themselves(Le-Clech et al., 2003a, b) that gas bubbles (or “slugs”) passing up
through a tubular membrane are able to enhance the flux over that attainable from
liquid crossflow at the same velocity. This type of two-phase air–liquid flow is termed
“slug flow” (Fig. 2.31) and represents the most effective type of air–liquid flow for
promoting flux. Much work has been conducted, principally by Cui and his various
co-workers, to model membrane aeration in channel flow. Thus far, models have
been produced which describe the spatial variation of shear with time for rising
bubbles as a function of bubble (or slug) size, channel dimension and geometry for
Newtonian fluids. It is also possible, within certain boundary conditions, to relate �
to the flux, J, from first principles, provided assumptions can be made about the particle
size and concentration, the system hydrodynamics and the fluid and membrane
homogeneity. Such assumptions, however, are not pertinent to an iMBR where
three-phase flow prevails in a highly heterogeneous non-Newtonian fluid containing
solutes, colloids and particulates. Moreover, the system becomes yet more complicated
when the geometry deviates from well-defined channels, as provided by FS
or tubular configurations, to HF modules.
Aeration also affects HF iMBR performance by causing fibre lateral movement (or
sway) (Côté et al., 1998; Wicaksana et al., 2006), which imparts shear at the membrane
surface through the relative motion of the membrane and the surrounding liquid.