70   AZO DYES' METABOLISM BY PSEUDOMONAS PUTIDA
Ajit K. Ghorpade, Senior Engineer,
Merck & Co., Inc.
Elkton, Virginia 22827
Hugh T. Spencer, Professor
Department of Chemical Engineering
University of Louisville
Louisville, Kentucky 40292
INTRODUCTION
Azo dyes are used extensively in textile and leather dyeing operations and contribute to problems
associated with disposal of the wastewater generated from these operations. Generally, these waters
contain high concentrations of sodium chloride and are brightly colored. Regulations controlling
release of these effluents to Publicly Owned Treatment Works (POTWs) vary from state to state;
however, regulators will decrease discharge limits on total residual chlorine and color of treated
effluent. At present, sodium chloride and color concentrations in typical dye waste streams are 2,000
to 3,000 mg/L and 1,000 to 1,500 American Dye Manufacturer's Index (ADMI) units, respectively.
Textile and leather dyes are, by design, built to resist degradation in the open environment. This
keeps a dyed product usable for a longer period, but it also makes the treatment of dye waste more
difficult; the same properties which help the dye to hold up to repeated washings also make it resistant
to degradation in the wastewater treatment plant. At present, the industries facing this problem have
found it possible to meet the color regulations by hyper-chlorination (chlorination beyond that needed
for simple disinfection). Because of the anticipated change in residual chlorine regulation, textile
industries will be looking for alternative ways to treat their dye waste. One such alternative will be to
use designer organisms created to degrade azo dyes.
Bacteria metabolizing the food materials in a wastewater stream tend to degrade first those substrates which have a biological origin and are in high concentration. Azo dyes are xenobiotics,
compounds which do not have a biological origin. Therefore, a low azo dye concentration contained
in textile wastewaters alone cannot be a source for bacterial growth. This study evaluates the extent of
bacterial degradation of azo dye when using another substrate as a primary food and energy source.
The bacterium, Pseudomonas putida, PRS 2015 with pAC 27, 3 Cba + , a genetically altered strain,
was used to degrade three azo dyes in a chemostatic setting using 3-chlorobenzoic acid as a primary
substrate.
LITERATURE REVIEW
Sulfonated azo dyes are produced and used in textile dyeing. They are xenobiotics since aromatic
azo groups are not synthesized in the biosphere. As a consequence, they are recalcitrant in aerobic
wastewater treatment. Hence, there is a major scientific interest in isolating microorganisms from
nature capable of utilizing azo dyes as a sole carbon source. Most of the prior studies used oxygen as
an electron acceptor and were aerobic. The anaerobic studies1"3 that used an electron acceptor other
than oxygen will not be discussed in this paper. A summary of aerobic investigations is presented
below, divided based on the nature of bacterial systems used during the experiments (i.e., mixed or
pure culture).
Azo Dyes Biodegradation by Mixed Cultures
The studies in this class used sludge from POTWs or textile mill outfalls. Urushigawa and Yone-
zawa developed an understanding of the relationship between the biodegradability of azo compounds
and their molecular structures.4 The experiments were performed in Erlenmeyer flasks; the sludge and
dye concentration used in the study were 1000 mg/L and 50 mg/L, respectively. The azo dye concentrations were determined by measuring UV absorbance of the centrifuged broth. The azo dyes studied
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Primed in
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