Page 44 - Membranes for Industrial Wastewater Recovery and Re-Use

24 Membranesfor lndustrial Wastewater Recovery and Re-use

Table 2.5 Membrane configurations
Configuration Area/vol. cost Turbulence Backflushable? Applicationa
ratio (m2/m3) promotion

Pleated 500-1 500 Very low Very poor No DEMF. low TSS
cartridge waters
Plate-and-frame 100-300b High Fair No ED, UF. RO
Spiral wound 800-1200 LOW Poor No RO, NF. UF
Tubular 150-300 Very high Very good No CFMF. high TSS
waters
Capillary tubec 1500-5000 Low Good Yes UF
Hollow fibred 10 000-20 000 Very low Very poor Yes MF, RO

a Most important application in bold type: DE = dead-end, CF = cross-flow.
Can be 2/m3 for a cassette.
Capillary tube used inUF water flows from inside to outside the tubes.
Hollow fibre used in MFand RO water flows from outside to inside the tubes.

disposable unit to be employed for polishing relatively pure waters. Pleated
media provide a high area to volume ratio and also ensure the minimum number
of sealing operations, i.e. a side seal and two end seals. The construction is such
that the loss of filter area at the folds of the pleat is avoided by using a suitably
designed membrane separator. The configuration is suitable for both organic
polymeric media and porous metal media, the latter having a minimum pore size
of around 2 pm. For these filters, and for other membrane modules applied to
pure waters, key design parameters are integrity testability, sterilisability and,
most importantly, propensity to leach contaminants from the module or unload
retained particles from the filter medium.
Plate-and-frame modules comprise rectangular or (pseudo) circular flat sheet
membranes with separators and/or support plates. The modules can be pressure
sealed, such as with electrodialysis stacks, or be designed as cassettes. Cassettes
allow membrane elements to be inserted and removed individually without the
necessity of partially dismantling the module. They are, however, limited to low-
pressure operation, and hence to microfiltration or ultrafiltration processes. An
example is the Kubota membrane, employed exclusively in a membrane
bioreactor (Fig. 2.5). This technology operates at transmembrane pressures as
low as 0.05 bar, and therefore does not demand as robust a construction as a
pressure-sealed unit would provide. The membrane element comprises two
rectangular sheets, 0.4 m x 0.8 m, heat sealed at the perimeters to a plastic
frame. The elements are inserted into a cassette holding up to 150 elements, with
the permeate collected from inside each one individually.
An example of a circular pressure-sealed module is the Rochem reverse
osmosis module (Fig. 2.6). This module employs support plates of 200-400 mm
diameter and 5 mm thick at the outside edge. The plate surfaces are covered with
tiny protuberances that promote turbulence as the water flows over them.
Sandwiched between adjacent plates is a membrane element comprising two
membrane sheets, heat sealed at the edges and separated by a thin plastic mesh.
The feed is introduced at the top of the stack and flows radially outward at the top