Bacterial Degradation of Textile Dyes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 242

was a matter of concern, research on bacterial azo dye reduction has focused on the
activity of facultative anaerobic bacteria from mammalian intestines (Walker 1970 ;
Chung et al. 1992 ). Later, when the removal of dyes from wastewater became a
problem, bacteria from other origins were also used to investigate anaerobic azo dye
reduction by pure cultures (Wuhrmann et al. 1980 ), mixed cultures (Haug et al.
1991 ), anaerobic sediments, digester sludge (Brown and Laboureur 1983 ), anaer-
obic granular sludge and activated sludge under anaerobic conditions. A large
number of azo dyes that can be reduced by different bacteria indicate that azo dye
reduction is a non-speci

c reaction and that the capability of reducing azo dye can
be considered as a universal property of anaerobic bacteria.

2.1.5 Aerobic Oxidation of Aromatic Amines

Various substituted amino-benzene, amino-naphthalene and amino-benzidine
compounds have been found aerobically biodegradable (Ekici et al. 2001 ). The
conversion of these compounds generally requires enrichment of specialized aer-
obes. In some cases, biodegradation was only achieved in nitrogen-free medium
(Konopka 1993 ). Especially sulfonated aromatic amines are very dif

cult to be
degraded. Their low biodegradability is due to the hydrophilic nature of the sul-
fonate group which obstructs membrane transport. The biodegradation of sulfo-
nated aromatic amines has been demonstrated for relatively simple sulfonated
aminobenzene and aminonaphthalene compounds. Another transformation, that
aromatic amines may undergo, is auto oxidation when exposed to oxygen. Espe-
cially aromatic amines with ortho-substituted hydroxyl groups are susceptible to
auto-oxidation (Kudlich et al. 1999 ). Many aromatic amines, e.g. substituted ani-
lines, amino-benzidines and naphthylamines, have been found to oxidize initially
oligomers and eventually dark-colored polymers with low solubility which are
easily removed from the water phase (Field et al. 1995 ).

2.1.6 Combined Anaerobic-Aerobic Treatment

The prerequisite for oxidative degradation of azo dye is reductive ssion of the azo

linkage under anaerobic condition, followed by aerobic degradation for complete
removal of azo dyes (Field et al. 1995 ). Two different approaches can be discerned:
sequential treatment in separate reactors and integrated treatment in a single reactor.
The integrated approach is based on temporal separation of the anaerobic and the
aerobic phase, as in sequencing batch reactors or on the principle that diffusion of
oxygen in microbial bio m, so that anaerobic and

aerobic conditions co-exist in a single environment (Lens et al. 1995 ). The removal
of color in the anaerobic stage was generally high, mostly higher than 70 % and in
several cases, even achieved almost 100 %. Color removal ef

lms is usually limited to 10 - 100 μ

ciencies differed
widely in dyes, when the removal of different azo dyes was tested under similar
conditions.