Page 43 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technology 2 3

which is no more than 14% porous. Unfortunately, the membrane thickness of
60 pm makes the hydraulic resistance of the Anopore membranes relatively high
and, more crucially, they can currently only be produced in small batches as disc
filters for use in the laboratory.

2.7.4 Membrane configurations
A membrane is only useful if it takes a form which allows water or pollutants to
pass through it. The configuration of the membrane, i.e. its geometry and the
way it is mounted and oriented in relation to the flow of water, is crucial in
determining the overall process performance. Other practical considerations
concern the way in which the membrane elements, i.e. the individual discrete
membrane units themselves, are housed to produce modules, the complete
vessels through which the water flows. The optimum membrane configuration is
one that has the following characteristics:

(a) a high membrane area to module bulk volume ratio
(b) a high degree of turbulence for mass transfer promotion on the feed side
(c) a low energy expenditure per unit product water volume
(d) a low cost per unit membrane area
(e) a design that facilitates cleaning
(f) a design that permits modularisation.

All membrane module designs, by definition, permit modularisation (f), and
this presents one of the attractive features of membrane processes per SE.
However, some of the remaining listed characteristics are mutually exclusive.
For example, promoting turbulence (b) results in an increase in the energy
expenditure (c). Direct mechanical cleaning of the membrane (e) is only possible
on comparatively low area:volume units (a) where the membrane is accessible.
Such module designs inevitably increase the total cost per unit membrane area
(d). Finally, it is not possible to produce a high membrane area to module bulk
volume ratio without producing a unit having narrow feed channels, which will
then adversely affect turbulence promotion.
There are five principal configurations currently employed in membrane
processes, which all have various practical benefits and limitations (Table 2.5).
The configurations are based on either a planar or cylindrical geometry and
comprise:

0 Pleated filter cartridge
0 Plate-and-frame
0 Spiral wound
0 Tubular
0 Hollowfibre

Of the flat plate geometries the lowest cost is the pleated filter cartridge (Fig.
2.4), used exclusively in microfiltration and designed as a relatively inexpensive