Page 185 - Membranes for Industrial Wastewater Recovery and Re-Use

154 Membranes for Industrial Wastewater Recovery and Re-use

retain the dissolved chemicals whilst removing colour down to very low levels.
Ultrafiltration membrane processes allow selective recovery of sparingly soluble
dyes, such as indigo, but downstream reverse osmosis is then usually required to
remove soluble dyes (Cheryan, 1998; Woerner et al., 1996; Marcucci et a].,
2001; Sojka-Ledakowicza et al., 1998). Having said this, some interesting results
have been reported by Porter and his various co-workers whereby low molecular
weight anionic dyes were found to be substantially rejected by a titanium dioxide
microfiltration membrane (Zhang, 1996; Porter and Gomes, 2000). This was
attributed to charge rejection, and charged UF membranes have been
successfully employed in South Africa for selectively recovering water and
dissolved salts (Erswell et al., 1988).
Most operating membrane-based dyewaste recovery and reuse systems
ultimately make use of reverse osmosis or nanofiltration to decolourise. Payback
periods are invariably long even where freshwater costs are high, although two
bench-scale studies have reported calculated operating costs of between $0.5
and $1 per m3 product (Voigt et a]., 2001; Ciardelli et al., 2001). For two South
Africa installations the payback ranges from just less than three years for an NF
process (Buckley, 1992), to over seven years for a system combining ceramic UF
with RO (Short, 1993). A payback period of 1.37 years was quoted for a UF/NF
plant in North Carolina (NCDENR, 1995), although no further detail was given.
Given the continuing downward pressure on membrane costs, the increasing
stringency of environmental regulations worldwide and the history of the
employment of membranes for textile wastewater treatment and recycling in
certain regions of the world, and South Africa in particular, it seems inevitable
that the use of membranes for this duty will become more widespread in years to
come.

References
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Akbari. A,, Desclaux, S., Remigy, J.C. and Aptel, P. (2002). Treatment of
textile dye effluents using a new photografted nanofiltration membrane.
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