Page 38 - Membranes for Industrial Wastewater Recovery and Re-Use

18 Membranes for Industrial Wastewater Recovery and Re-use

Backing Cloth
80 - 100 pm

Figure 2.3 Composite ROmembrane structure (FT30, Film-Tec)

raw material but also on the ease with which pores of the desired size or size
distribution can be introduced. This can vary considerably from one material to
the next according to the method used and the corresponding precision of the
pore size distribution (or degree of isoporosity).
The range of available membrane materials employed in water and
wastewater treatment is very broad, and they vary rather more widely in
chemical composition than in bulk morphology. Membranes can be produced by
stretching, sub-atomic particle bombardment combined with etching and, in the
case of ceramic materials, sintering (Table 2.2). These membranes are formed by
the pressing and sintering of fine powders onto a pre-prepared porous support,
which can produce pore diameters down to around 2 pm, followed by sol-gel
processes to produce successive deposits of progressively finer porous layers
down to a minimum pore size of around 3 nm. Ceramic nanofiltration
membranes, however, have only recently arrived in the marketplace. The
production process tends to be very expensive if a highly selective membrane
layer of even thickness and narrow pore size distribution is to be produced. The
cost of microfiltration or ultrafiltration membranes derived from titanium and/or
zirconium usually exceeds $1000 per m2, although there appears to be some
progress in producing low-cost ceramic membranes. At the opposite end of the
spectrum are simple, homogenous polymeric membranes produced by extrusion
(stretching) of partially crystalline sheets perpendicular to the orientation of the
crystallites, possibly with the assistance of a fibrulating agent. Such materials
cost less than $1 per m2 to produce, since the process can be made continuous,
and the cost of the membrane module is then determined almost entirely by the
module fabrication cost. However, such membranes are limited in their
permeability, isoporosity and mechanical strength.
Supported liquid membranes (SLMs) have not yet been produced on a
commercial scale, and currently appear to be more appropriate to gas
separations where the improved mass transport of gases through liquids over
that attainable in solids becomes important. SLMs currently being developed for