Nanomembranes - Nanotechnologies for Water Environment Applications - page 394

detergents, acetic acid and complexing agents). In addition, it has high temperature (50-
60 o C) and high pH (typically about 11) (Tang and Chen, 2005). Both water-soluble and
non water-soluble dyes are found in the textile effluent due to the varying classes of dyes
used in the process. Nanomembranes were found to be suitable for removing even the
water-soluble dyes with molecular weight ranging from 200 to 1000 Da and divalent
salts for the softening effects (Fersi et al.; 2005, Tang and Chen, 2005).
Table 12.12 Classes of dyes and the associated pollutants (extracted from US EPA,
1997).
Dye Class Description Applied to Fixation (%)

Water-Soluble
Acid anionic compounds wool and nylon 80-95

Basic applied in weakly acidic

dyebaths, very bright colors.

acrylic and some
polyesters.

97-98

Direct anionic compounds, can be

applied directly to cellulosic
without mordants (or metals like
chromium and copper).

cotton, rayon and other
cellulosics

70-95

Reactive anionic compounds, largest dye

class.

cottons, other cellulosics
and wool

60-90

Non Water-Soluble
Disperse

- polyester, acetate and other

synthetics

80-92

Vat oldest dyes, more chemically

complex

cotton and other cellulosics. 80-95

Fersi et al. (2005) compared the performance characteristics of three different
categories of membranes, namely MF (tubular; alumina m pore size), UF (tubular;

zirconium oxides; 50 nm and 20 nm pore size) and NF (flat sheet; polyamide; 2 nm
pore size) for treating textile effluent that had been pre-treated with activated sludge.
Direct MF with a paper filter as a pre-treatment was able to remove 40% of the turbidity
and color. Direct UF (both 50 and 20 nm) was able to achieve more than 80% turbidity
removal, but the color removal remained relatively poor (up to ~55% only). This finding
shows that the UF membrane even down to 20 nm is not effective in retaining low
molecular weight dyes in the effluent. Pretreatment using MF could be a realistic option
prior to the UF process, while direct NF is able to achieve retention of more than 90%
bivalent cations and 60% of monovalent cations. Retention of TDS, turbidity and color
exceeded 90% while conductivity was reduced by 70%. Similar high dye removal
efficiency (99.2%) was achieved using NF membranes, namely, Desal-5 (Osmonics) and
NF-70 (Saechan) when operated at an average pressure of 0.61 MPa. However, both NF

α ; 5 μ