Environmental Engineering Reference
In-Depth Information
severe limitations mainly related to mass
transfer resistance or the appearance of con-
centration gradients inside the systems, slow
primary startup requiring several weeks, and
difficulty in controlling granulation process
which depends upon a large number of param-
eters.
b.
Aerobic reactors
Anaerobically treated distillery spent wash
still contains high concentrations of organic
pollutants and as such cannot be discharged
directly. Aerobic treatment of anaerobically
treated distillery spent wash has been attempt-
ed for the decolorization of the major colo-
rant, melanoidin and for further reduction of
the COD and BOD. A large number of micro-
organisms such as bacteria (pure and mixed
culture), cyanobacteria, yeast, fungi, etc. have
been isolated in recent years that are capable
of degrading melanoidin and ultimately decol-
orizing the wastewater.
Fig. 2.2
Mechanism of action for lignin peroxidase
(LiP).
ox
oxidized state of enzyme. (Breen and Singleton
1999
)
free radical. These enzymes lack substrate speci-
ficity and are thus capable of degrading a wide
range of xenobiotics including industrial colored
wastewaters. The mechanism of action of these
enzymes is as follows:
a.
Lignin Peroxidase (LiP)
LiP is a heme-containing glycoprotein, which
requires hydrogen peroxide as an oxidant. LiP
from different sources was shown to miner-
alize a variety of recalcitrant aromatic com-
pounds and to oxidize a number of polycyclic
aromatic and phenolic compounds (Karam
and Nicell
1997
).
Fungi secrete several isoenzymes into their cul-
tivation medium, although the enzymes may
also be cell wall-bound (Lackner et al.
1991
).
LiP oxidizes nonphenolic lignin substructures
by abstracting one electron and generating cat-
ion radicals, which are then decomposed chemi-
cally (Fig.
2.2
). LiP is secreted during secondary
metabolism as a response to nitrogen limitation.
They are strong oxidizers capable of catalyzing
the oxidation of phenols, aromatic amines, aro-
matic ethers, and polycyclic aromatic hydrocar-
bons (Breen and Singleton
1999
).
b.
Manganese Peroxidase (MnP)
2.6
Enzymatic Processes
for Decolorization
A large number of enzymes (e.g., peroxidases,
oxidoreductases, cellulolytic enzymes, proteases
amylases, etc.) from a variety of different sources
have been reported to play an important role in
an array of waste treatment applications (Ferrer
et al.
1991
; Dec and Bollag
1994
). Paper and
pulp mills, textiles and dye-making industries, al-
cohol distilleries, and leather industries are some
of the industries that discharge highly colored ef-
fluents. The ligninolytic system consists of two
main groups of enzymes: peroxidases (lignin per-
oxidases and manganese peroxidases) and lac-
cases (Leonowicz et al.
2001
; Arana et al.
2004
;
Baldrian
2006
). Although the enzymatic system
associated with decolorization of melanoidin
containing wastewater appears to be related to
the presence and activity of fungal ligninolytic
mechanisms, this relation is as yet not completely
understood. Laccase is a multicopper blue oxi-
dase capable of oxidizing
ortho
- and
para
diphe-
nols and aromatic amines by removing an elec-
tron and proton from a hydroxyl group to form a
MnP is also a heme-containing glycoprotein
which requires hydrogen peroxide as an oxi-
dant. MnP oxidizes Mn(II) to Mn(IIl) which
then oxidizes phenol rings to phenoxy radi-
Search WWH ::
Custom Search