Environmental Engineering Reference
In-Depth Information
3.2 Effects of Temperature
Temperature is another important environmental factor affecting various processes
associated with microbial activity. There exists certain temperature range for the
growth and reproduction of microbe cells and the effective function of enzymes.
The decolorization performance of bacteria was generally observed to increase up
to the optimum temperature, and then decreased. A decline performance after the
optimum temperature was usually attributed to the loss of cell viability or the
denaturation of enzyme responsible for decolorization (Saratale et al. 2011 ; Sol
í
s
et al. 2012 ).
3.3 Effects of Dissolved Oxygen/Agitation
Although there are few studies which suggested that agitation could improve the
decolorization process by increasing mass transfer and nutrient distribution, we
generally believe that the presence of oxygen will compete with azo dye for
electrons and inhibit most bacterial decolorization performance. It should be noted
that in some studies of azo decolorization under agitation conditions, the dissolved
oxygen could be rapidly depleted by high amounts of cellular biomass in the
nutrient media. Thus, the decolorization reaction was actually performed under
anoxic conditions. Nowadays many studies suggested to incubate speci
c strains
under aerobic conditions
rst to obtain the needed biomass rapidly, and then to use
the harvested cells to decolorize azo dyes under static or anaerobic conditions. Of
course, the aerobic conditions are required for further mineralization after color
removal. Anaerobic decolorization process combined with subsequent aerobic
treatment was typically applied for the complete treatment of azo dye wastewater.
3.4 Effects of Salinity
In textile dyeing, various salts were utilized to separate organic contaminants (brine
rinse), to help precipitation of dyestuff (salting out), and to mix with concentrated
dyes to standardize them (Dennis 1996 ). Sodium level could also be elevated by
addition of sodium hydroxide into dye bath to increase the pH (Khalid et al. 2008 ).
Besides azo dye residues, textile ef
uents also contain substantial amounts of
various salts. High salt concentrations may cause plasmolysis and reduce biological
activity (Kargi 2002 ). Azo dye removal ef
ciencies under saline conditions were
usually decreased due to inhibitory effects of salt on microbial
ora (Khalid et al.
2008 ). Dilution of ef
uents before entering into traditional activated sludge systems
might alleviate salt stresses but this may produce a large volume of wastewaters
which could enhance the treatment burden (Khalid et al. 2008 ). In recent studies,
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