Page 176 - Membranes for Industrial Wastewater Recovery and Re-Use

148 Membranes for Industrial Wastewater Recovery and Re-use

COD or TOC of dyeing wastewaters by biological treatment (Horning, 1978;
Pagga and Brown, 1986; Willmot et al., 1998; Lorenqo et al., 2001). Biological
colour removal is low, however, unless extended retention times (2-3 days) are
employed, since most dye molecules are not biodegradable. The partial treatment
of dyes by conventional biological processes is attributed to precipitation (in the
case of sparingly soluble dyes) or adsorption on the sludge (Porter and Snider,
1976; Weeter and Hodgson, 1975). Reactive and acid dyes, on the other hand,
are not adsorbed on biological sludge to any great degree (Hitz et al., 1978;
Slokar and Le Marechal, 1998). The problem of colour from reactive dyes in
dyeing effluents is further aggravated by their high commercial popularity and
generally low fixation rates (as low as 50% for cotton and viscose fibres) relative
to other dye classes. Thus, even though they form only around 2% of the COD in
dyewaste, they present the most intractable of problems.
Several physicochemical colour removal methods have been employed at full-
scale over the years (Table 3.31). Many studies have focused on ozonation (Lin
and Lin, 1993; Perkowski et al., 1996; Matsui, 1996) and advanced oxidation
(Solozhenkho et al., 1995; Lin and Peng, 1996) which reliably decolourise a
wide range of dyewastes, although there has also been some notable success
using the proprietary zeolite-based adsorbent Macrosorb. Having said this, all
treatments present particular disadvantages. Advanced oxidation is high in
capital and/or operating costs and may create mutagenic products. Coagulation
creates significant quantities of sludge, as does powdered activated carbon.
Adsorption and ion exchange are generally low in capacity, necessitating large
quantities of materials. Also, and in common with membrane processes, they
produce a concentrated regenerant product or else, in the case of granular
activated carbon, demand an expensive regeneration process. Finally, no single
process is universally effective against all dye chemicals - although reverse
osmosis is certainly the most effective decolouring and desalting process against
the most diverse range of dyewastes, and has been successfully employed for
recycling (Buckley, 1992). Crucially, only dense membrane processes offer the
opportunity to remove dissolved solids (Section 3.1,7), which would otherwise
accumulate for any chemical treatment process).

3.3.6 Demand management
Given the exigencies of textile wastewater treatment, and of dyewaste in
particular, it is unsurprising that much effort in cost and environmental impact
reduction within the industry has been directed at reducing water and chemicals
consumption. There are a large number of documented examples of cost savings
in textile fabrication facilities through basic low-cost process modification and
effective, less profligate water use. Simple reuse of process water from dyeing,
rinsing and bleaching operations without further treatment, as proscribed
through water auditing techniques such as pinch analysis (Section 4.2), can
provide substantial cost benefits (Table 3.32). There are many other examples of
cost savings made through substitution of process chemicals for ones less
environmentally onerous (such as starch with PVA/acylates and soaps with