Bacterial Degradation of Azo Dye Containing Wastes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 67

4.7 Salt Concentrations

Wastewater from dyestuff manufacturing and textile processing industries shows
presence of various acids, alkalis, metal ions and salts as impurities. Up to 15 20 %

salt concentration has been reported in these industrial wastewaters, because high
salts are needed for the dyeing process (Hazrat 2010 ). So, high salt-tolerant bacteria
capable of dye decolorization are the organisms of choice for actual ef

-

uent
treatment on a large scale. Generally, a sodium concentration above 3 % causes
moderate inhibition of most bacterial activities. Thus, azo dye removal ef

ciency is
inversely proportional to the salt concentration. However, there are a few examples
of halotolerant microorganisms, such as Exiguobacterium acetylicum, Staphylo-
coccus gallinarum, B. rmus and A. hydrophila which are able to decolorize azo

dyes even in the presence of high salts (Chen et al. 2011 ; Ogugbue et al. 2011 ).

4.8 Electron Donors

The azo dyes and the other organic content of textile wastewater are too low to act
as a suf cient substrate for the growth of anaerobic bacteria. So, it is necessary to

have an external substrate (electron-donor) supply to enhance the anaerobic
decolorization performance. The electron donors, sodium salts of acetate, formate,
succinate, citrate and pyruvate have been shown to enhance the azo dye decolor-
ization. The anaerobic bacterial azo reduction is a biochemical process that oxidizes
the electron donors and transfers the electrons to the acceptors through a multi-
component system related to the electron transport chain. During decolorization,
bacteria require NADH as an electron donor for NADH-DCIP reductase. In the
presence of arti

cial electron donors, such as glucose, methanol, sodium acetate,
sodium formate, sodium citrate, and sodium pyruvate, an induction in the reduction
activity was observed. Amongst these substrates, methanol is widely used as a cost-
effective electron donor for the biological treatment of wastewater. The donors not
only induce the reduction mechanism, but also stimulate the enzymatic system
responsible for the reduction process. In contrast, it has been observed that in the
presence of some electron donors, the electron transport process is inhibited, and
this might be due to the competition for electrons from the donors. Certain
chemicals, such as thiomersal and p-chloromercuribenzoate, inhibit the alcohol
dehydrogenase of the NADH-generating systems required to produce reducing
equivalents for dye reduction. It has also been noted that the products of cell lysate
residue can also function as electron donors for an anaerobic azo dye reduction
(Saratale et al. 2011 ).