Page 50 - Membranes for Industrial Wastewater Recovery and Re-Use

30 Membranes for Industrial Wastewater Recovery and Re-use

manufacture. Hence, some companies have started producing cassettes
alongside their pressure-sealed module products.
One noteworthy recently development concerns ceramic membranes. As
already stated, the onus is on suppliers of these membranes to reduce costs to
make them economically competitive with the much less expensive polymeric
materials. It appears that this might be achievable by extruding ceramic
membrane substrates to produce channels of rectangular geometry (Fig. 2.12).
The cost of these membranes, produced from silicon carbide substrate monolith
coated with titania, alumina or zirconia, can be as low as $100 per m2 for
narrow-membrane channels (2 mm).
Another fairly recent development is the use of aeration combined with
submerged membrane systems. Submerged (or immersed) membrane systems,
where large-area membrane filtration modules are submerged in a tank and the
permeate removed at very low transmembrane pressures, were originally
developed for membrane bioreactors (MBRs) by companies like Kubota and
Zenon in the late 1980s as an alternative to the more conventional sidestream
(or external) filtration systems. MBRs are an example of a hybrid process, in this
case combining the activated sludge aerobic biotreatment process with
membrane filtration (Stephenson et al., 2000). The high fouling propensity of the
sludge liquor being filtered, which can have a suspended concentration of up to
20 g lP1, demands a high degree of turbulence promotion to enhance the flux. In
the submerged system turbulence promotion is provided by coarse bubble
aeration. It has been shown by a number of researchers (Ghosh and Cui, 1999;
Mercier et al., 1997; Cabassud et al., 1997) that air-water two-phase flow can
substantially enhance the membrane flux over that attained by single-phase

Figure 2.12 Cerameni silicon carbide monolith substrate (reproduced withpermission)