22    BIOFILM PROCESSES FOR REMOVAL OF AZO DYE
FROM MUNICIPAL WASTEWATERS
Christopher Harmer, Project Manager
CH2M-Hill, Milwaukee, Wisconsin 53201
Paul Bishop, Professor
Yongjie He, Graduate Student
Edith Holder, Research Associate
P.V. Scarpino, Professor
University of Cincinnati
Cincinnati, Ohio 45221-0071
INTRODUCTION
Dyes with the azo linkage (-N = N-)represent a significant portion of dyes in use today. Commercial
dyes are selected in part for their ability to resist deterioration due to sunlight, washing, and microbial
action. As a class, azos are difficult molecules for municipal biological treatment systems to degrade.
Shaul et al.1 found that of 18 azo dyes tested in activated sludge systems, only three were degraded to
a significant extent. Acid Orange 7 (AO-7,C.I. 15510, the dye used in this study) was one of these
degradable dyes, showing removals as high as 96%.
Degradation of azo dyes consists of two steps, the first being the cleavage of the azo bond. The
second step is the mineralization of the intermediates. The intermediates of many azo dyes are
aromatic amines, which are potential carcinogens.2 Thus, while the dyes themselves are not particularly toxic, their incomplete metabolism — particularly under anaerobic conditions— can lead to
increased levels of mutagens and carcinogens in the environment.3
Aerobic biofilms have a number of potential advantages for the treatment of xenobiotics in general,
and azo dyes in particular. They provide an environment which allows the development of slow-
growing organisms which may be responsible for degradation of some chemicals. Biofilms also
provide a diffusion barrier against the effects of degradable inhibitory compounds.4 They can also
provide both anaerobic and aerobic zones, which may facilitate complete mineralization in one
process operation.
RESEARCH OBJECTIVES
Work to date on azo biodegradation has focused on suspended phase removal in conventional
systems' or enzyme studies of axenic cultures.5,6 This study examined whether initial degradation of
AO-7 (azo bond cleavage) can be controlled using bulk-phase parameters. The objectives of this study
were to:
1. Develop an empirical model for the rate of the azo cleavage reaction in biofilms as a
function of bulk-phase parameters;
2. Evaluate the production and degradation of intermediate metabolites; and
3. Evaluate the impact and interaction of the azo dye on the normal biofilm removal
processes.
MATERIALS AND METHODS
Statistical Design and Evaluation of Experiments
The study involved running statistically designed kinetics experiments on three biofilm reactors. In
each run, the reactors were seeded from nonimpacted treatment plant mixed liquor and fed a synthetic
wastewater. In two of these reactors, the feed was spiked with Acid Orange 7 (AO-7). Flow, feed
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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