Page 42 - Membranes for Industrial Wastewater Recovery and Re-Use

22 Membranes jor Zndustrial Wastewater Recoverg and Re-use

neutral solutions, such that a highly negatively charged membrane would be
expected to repel these contaminants allowing operation at higher fluxes than
would normally be attainable without fouling. This has focused attention upon
highly negatively charged membranes and, in particular, blends of sulphonated
poly(ether ether ketone) (SPEEK) or sulphonated poly(ether ether sulphone)
(SPEES) with polysulphone or polyethersulphone, respectively (Bowen et al.,
2001; Knoell et al., 1999). These membranes are likely to be commercialised
within the next five years.
Another important constraint on the application of membranes to industrial
process waters is their resistance both to extreme pH conditions and, more
especially, key organic solvents. A few porous membrane materials, notably
PTFE and PVDF, are intrinsically resistant to attack from both chlorinated and
hydroxylated or carboxylated organic solvents, whereas PSU, PES and PEI will
withstand some organic solvents but not chlorinated reagents like chloroform.
Recently, a range of organic nanofiltration membranes (SelRO'KI,
commericalised by Koch) have become available that can apparently be
employed in pure, non-chlorinated organic solvents. These membranes are
probably a modified cross-linked PAN material. Details of other solvent-resistant
UF and NP membranes either recently available or under development are
provided by Nunes and Peinemann (2001), and this information is summarised
in Table 2.4.
So far as ceramic filters are concerned, there is a general drive to increase their
competitiveness by reducing costs to allow them to be sold at less than $1000 per
m2, although it is still the case that such a price is only attainable for large bulk
orders. However, very recently the opportunities offered by extrusion of certain
ceramic materials have started to be explored, for example by Ceramem,
allowing the production of membrane materials of much lower cost (Section
2.1.4). Perhaps the most intriguing of the ceramic filters, however, is the
Anopore filter. This is produced by electro-oxidation of aluminium to produce an
almost perfect honeycomb structure with an extremely narrow pore size
distribution and a surface porosity of up to 50%. This porosity value compares
well with the similarly homoporous Nuclepore membranes, produced by
neutron bombardment followed by track-etching of polyethylene terephthalate,

Table 2.4 Solvent-resistant UF and NF membranes, new or under development (taken from
Nunes and Peinemann, 2001)
Material Application Supplier(s) or Stability
developer(s)
Hydrophilic NF Koch SelRO" DMF, NMP, DMSO resistant:
PAN 0-14 pH, 70°C
NF Desal D-series, Osrnonics 35%H*S04,25%H3P04
- NF Somicon/Nitto 15% NaOH, 60°C. pH < 1
PVDF/Cellul. UF/NF Dow Danmark Etna pH 1-12
PI UF Nitto. Bend Research -
PEEK UF Dow, Ionics -
PAEK/PEI UF Bend Research -