Page 167 - Membranes for Industrial Wastewater Recovery and Re-Use

lndustrial waters 141

0 they are produced in large volumes (around 100-150 1 kg-l textile
product for the average dyeing and rinsing operation),
0 they are not readily biodegradable, such that conventional municipal
wastewater treatment plant will generally remove only around 20-30% of
colour associated with synthetic dyes, and
0 they require removal to very low levels prior to discharge if consents based
on colour are in place.

Dyes are generally small molecules comprising two key components: the
chromophores, responsible for the colour, and the auxochromes, which can not
only supplement the chromophore but also render the molecule soluble in water
and give enhanced affinity toward the fibres (Trotman, 1984). A large number of
dyes are reported in specialised literature (Colour Index, 1987). These can be
classified both by their chemical structure or their application to the fibre type
(Table 3.24). Dyes may also be classified on the basis of their solubility: soluble
dyes include acid, mordant, metal complex, direct, basic and reactive dyes; and
insoluble dyes include azoic, sulphur, vat and disperse dyes. An alternative dye
classification that refers to colour removal technologies (Treffry-Goatley and
Buckley, 1991) places the various classes of dyes (with respect to their
application) into three groups depending on their state in solution and on the
type of charge the dye acquires. Each group can be associated with potential
colour-removal methods (Table 3.2 5).
Complex chemical and/or physical mechanisms govern the adsorption and
retention of dyes by fibres. The adsorptive strength, levelling and retention are
controlled by several factors such as time, temperature, pH, and auxiliary
chemicals (Nunn, 1979; Trotman, 1984; Preston, 1986; Shore, 1990). A large
range of substances other than dyes, auxiliary chemicals used in the dyeing
process, can be found in a dye effluent at any one time. The effluent composition
and colour is further complicated by the fact that both dye fixation rates (Table
3.2 6) and liquor ratios (the volume of dye solution per weight of goods) vary, and
different dye classes may be used for a single dyeing operation (Shore, 1990;
Horning, 19 78). Moreover, continuous operation yields smaller volumes of more
concentrated dyewaste than batch operation, equating to typically a four-fold
factorial difference with respect to dye concentration and a 2.5-fold difference in
volume (Glover and Hill, 1993), some typical batch process effluent data being
given in Table 3.2 7.

Chemical finishing
Chemical finishing processes include processes designed to change the optical,
tactile, mechanical strength or dirt-releasing properties of the textile. Optical
finishes can either brighten or deluster the textile (NCDNER, 1995; OECD,
198 1). Softeners and abrasion-resistant finishes are added to improve the feel or
increase the ability of the textile to resist abrasion and tearing. Absorbent and
soil release finishes alter the surface tension and other properties to increase
water absorbency or improve soil release. Physical stabilisation and crease-
resistant finishes, which may include formaldehyde-based resin finishes,