Page 120 - Industrial Wastewater Treatment, Recycling and Reuse

94 Industrial Wastewater Treatment, Recycling, and Reuse

coagulants significantly reduces treatment costs with significant enhance-
ment in coagulation efficiency. Thus, formulation of inorganic and organic
coagulants can provide a better techno-economically feasible operation
in wastewater treatment. This is especially true for reactive dyes, which
pose the greatest color removal problems (even a concentration as low as
0.005 ppm is visibly detectable). Further, reactive dyes are not easily biode-
gradable; even after extensive wastewater treatment, color is likely to remain
in the effluent. In the absence of any effective conventional treatment meth-
odology for reactive dyes, it is imperative to develop an effective formulation
in the form of combined coagulants. The objective in developing such for-
mulation would be to have an efficient coagulation process operating over
a wide pH range with better settling properties and minimum sludge volume.
The efficacy of development of coagulant formulations in altering the
coagulation process performance is shown in Figures 2.4, 2.5, and 2.6 using
data from our own studies (Ashtekar, 2007; Ashtekar et al., 2010). The coag-
ulant formulations here were made by physical blending at ambient temper-
ature, one of the most popular techniques that does not involve any new
chemical bond formation. Figure 2.4 shows coagulation efficiency using
an inorganic coagulant, PAC, in the removal of different dyes as a function
of coagulant concentration. As stated earlier, in an electrical double-layer
suppression-type mechanism common to inorganic coagulants, there is little
or no dependence on coagulant dosage after minimum/optimum dosage of
PAC, which is 100 ppm for most dyes reported. This is, however, not always
the case, and for all the dyes and contributions of other mechanism types can
be seen at substantially high concentrations in the form of reduced efficiency
for some dyes. In comparison, it is to be noted that ferrous sulfate was largely

% Color removal 100 80 40 500

60

20
300
0 200
150
Vat brown 100 Coagulant concentration (mg.L –1 )
50
Ferrous yellow
30
Sulfur black
20
Reactive blue
Figure 2.4 Dye removal behavior using inorganic coagulant poly-aluminum chloride.