Page 52 - Membranes for Industrial Wastewater Recovery and Re-Use

32 Membranes for Industrial Wastewater Recovery and Re-use

velocity of the rotor is around 10-1 5 m s-l, depending on the diameter of the
membrane cell. Both the VSEP and CR ultrafiltration processes have been
successfully applied to liquors of high suspended solids content (Sections 3.2.5
and 5.6), achieving concentrate streams in excess of 5% solids in some cases.
Other turbulence promotion modifications to modules are still largely at the
developmental stage. These include intermittent jets, in which the feed is
pumped coaxially through a membrane tube at fixed intervals through a nozzle.
The abrupt change in velocity produces a toroidal vortex, increasing the flux by
up to 2.5 times for granular suspensions such as bentonite clay (Arroyo and
Fonade, 1993). Pulsed flow has also received much attention (Gupta et al., 1985,
1992; Rodgers and Sparks, 1993; Bertram et al., 1993). In this mode, pulses of
flow are generated in the feed or permeate channel, again creating large
temporal changes in the velocity gradient. A simpler alternative for turbulence
promotion is the use of simple inserts in tubes. This is also not practised
commercially, despite the many publications in this area, as reviewed by Gupta
etal. (1995), Belfort etal. (1994) andothers.
Finally, a process that has significantly extended the capability of
electrodialysis is the electrodeionisation (EDI), also called continuous
deionisation (CDI), process commercialised by Vivendi Ionpure and Ionics. In
this process the diluate cells of the electrodialysis stack are filled with ion
exchange resin beads. The resin effectively aids the transport of ions from the
diluate to the concentrate cells by providing a conducting pathway. This results
in extremely effective removal of all charged species in the diluate cell, producing
a product water of a quality comparable to that from a twin-bed deionisation
process but offering the advantage of being continuous.

2.2 The process fundamentals

2.2.1 Process performance definitions

Flux
The key elements of any membrane process are the influence of the following
parameters on the overall permeateflux:

0 the membrane resistance,
0 the operational driving force per unit membrane area,
0 the hydrodynamic conditions at the membrane-liquid interface, and
0 the fouling and subsequent cleaning of the membrane surface.

The flux is the quantity of material passing through a unit area of membrane
per unit time. This means that it takes SI units of m3 mP2 s-l, or simply m s-l,
and is occasionally referred to as the permeate velocity. Other non-SI units used
are 1 m-2 h-’ (or “LMH”) and m3 per day, which tend to give more accessible