The MBR Book: Principles and Applications of Membrane

28 The MBR Book
(a) (b) (c)
Figure 2.6 Schematics showing flow through membrane configured as: (a) FS, (b) CT or MT and (c) HF
permit turbulence promotion, cleaning or, preferably, both. Turbulence promotion
can arise through passing either the feedwater or an air/water mixture along the
surface of the membrane to aid the passage of permeate through it. This crossflow
operation (Section 2.1.4.2) is widely used in many membrane technologies, and its
efficacy increases with increasing membrane interstitial distance (i.e. the membrane
separation).
Because the MT module operates with flow passing from inside to outside the tube
(“lumen-side” to “shell-side”), whereas the HF operates outside-to-in, the interstitial
distance is defined by (Fig. 2.6):
● the tube diameter for a MT,
● the distance between the filaments for an HF,
● the channel width for an FS.
The membrane packing density of the HF thus becomes crucial, since too high a
packing density will reduce the interstitial gap to the point where there is a danger of
clogging. CT modules, which are, to all intents and purposes, HF modules with
reversed flow (i.e. lumen-side to shell-side), are too narrow in diameter to be used for
MBR duties as they would be at high risk of clogging.
Physical cleaning is most simply affected by reversing the flow (i.e. backflushing),
at a rate 2–3 times higher than the forward flow, back through the membrane to
remove some of the fouling layer on the retentate side. For this to be feasible,
the membrane must have sufficient inherent integrity to withstand the hydraulic
stress imparted. In other words, the membrane must be strong enough not to
break or buckle when the flow is reversed. This generally limits backflushing of polymeric
membranes to those configured as capillary tubes or HFs. At low filament
diameters the membranes have a high enough wall thickness: filament diameter
ratio to have the inherent strength to withstand stresses imposed by flow
reversal.