Removal of Organic Dyes from Industrial Effluents: An Overview of Physical and Biotechnological Applications - Green Chemistry for Dyes Removal from Wastewater - page 12

1.3.1 ChemicalMethods
Chemical methods consist of many techniques, such as coagulation
or flocculation combined with flotation and filtration, precipitation-
flocculation with Fe, Al and Ca hydroxides, electroflotation, electroki-
netic coagulation, conventional oxidation methods by oxidizing agents,
irradiation or electrochemical processes. These chemical techniques are
often high cost, and the accumulation of concentrated sludge, along with
the decolorization leads to a disposal problem. These techniques may
also cause a secondary pollution problem based on excessive chemi-
cal use. Recently, there have been other emerging techniques known as
advanced oxidation process and ozonation. The advanced oxidation pro-
cess (AOP), which is based on the generation of very powerful oxidiz-
ing agents such as hydroxyl radicals, has been applied with success for
pollutant degradation [49,50]. Oxidation by ozone (ozonation) is capable
of degrading chlorinated hydrocarbons, phenols, pesticides and aromatic
hydrocarbons [51,52]. The dosage applied for the dye-containing efflu-
ent is dependent on the total color level and residual COD. Ozonation
shows a preference for double-bonded dye molecules, which leaves the
colorless and low-COD effluent suitable for discharge into environment
[50, 52-55]. A major advantage of ozonation is that ozone is applied in its
gaseous state and therefore does not increase the volume of wastewater
and sludge. Lin and Liu [56] used a combination of ozonation and coagu-
lation for treatment of textile wastewater.
Although these methods are efficient for the treatment of waters con-
taminated with pollutants, they are very costly and commercially unattract-
ive. The high electrical energy demand and the consumption of chemical
reagents are common problems.
1.3.2 PhysicalMethods
Physical methods, which are widely used in industry because of their high
dye removal potentials and low operating costs, such as adsorption, ion-
exchange and irradiation, filtration and membrane-filtration processes
(nanofiltration, reverse osmosis, electrodialysis), are the most applicable
methods for treatment of textile wastewater in plants [57,58]. Some adsor-
bents such as activated carbon [59] and coal [60], fly ash [61,62], silica,
wood, clay material [63,64], agriculture wastes and cotton waste are used
in dye effluent treatment processes. The irradiation process is more suitable
for decolorization at low volumes within a wide range, but degradation of
dye in textile effluents requires very high dissolved oxygen. Ion exchange