Single and Hybrid Applications of Ultrasound for Decolorization and Degradation of Textile Dye Residuals in Water - Green Chemistry for Dyes Removal from Wastewater - page 262

7.1 Overview of the Textile Industry, Dyestuff

and Dyeing Mill Effluents

Global annual production of textiles requires 0.7 million tons of dyestuff
per year and a well-equipped dyeing/spinning mill consumes 3 tons of dye
and 600 tons of water per 100 tons of fiber [1,2]. An old-fashioned mill with
unmodified equipment may use more than 1000 tons of water to process
the same quantity of fiber. Hence, textile dyeing industries expend large
volumes of water, which is ultimately discharged with intense color, chem-
ical oxygen demand (COD), suspended/dissolved solids and recalcitrant
material as unfixed dye residuals and spent auxiliaries. A typical reactive
dyebath effluent contains 20-30% of the input dye mass (1500-2200 mgL -1 )
and traces of heavy metals (i.e., cobalt, chromium and copper) that arise
from the use of metal-complex azo dyes [2-4]. As such, proper treatment
of textile process effluent is of utmost significance to prevent transmission
of dye residuals and other recalcitrant matter to sewage treatment facilities
and the receiving waters, where they impose aesthetic, environmental and
health problems via hindering of oxygen and light transmission and pro-
duction of toxic end products by hydrolysis [5].
Textile dyestuff is classified either by reference to the dye properties
[6] or by chemical constitution and usage [7]. Accordingly, classification
of dyestuff based on Color Index chemical constitutions is presented in
Table 7.1(a); the associated chemical structures and classification based on
application are given in Table 7.1(b). As stated previously, spent dyebaths
of dyeing operations contain not only the residuals of dyestuff, but also the
spent auxiliaries (added to enhance the process efficiency), some of which
are wasted with the dyebaths, thus further increasing the complexity of the
effluent. A summary of the most commonly used auxiliary chemicals and
their expected benefits is given in Table 7.2 [8].
The expanded use of reactive dyes in textile dyeing operations (due to
their high color fastness and wide spectrum) has significantly impacted
the quality of the process effluent, which has lately become particularly
identified with the dyeing of cotton products, i.e., the use of reactive azo
dyes [10]. However, a common feature of these dyes is that they have poor
fixing properties (70-90%) and 10-30% of the input dye mass is wasted
with the spent dyebaths. This is why dyeing mill effluents are recognized
not only by their intense color, but also by their low biochemical oxygen
demand (BOD), variable pH, high chemical oxygen demand (COD), high
temperature and salinity, as presented in Table 7.3.
Reactive azo dyes constitute 50% of synthetic dye production globally

(7 10 5 tons) [12] and are non-biodegradable in conventional biotreatment