Abstract
Azo dyes are the largest class of dyes with a world market share of 60-70%. At least 3,000 different varieties of azo dyes are extensively used in industries for various purposes. Although the azo dyes do not generally display acute toxicity, they are regarded as water pollutants because of their high color intensity in the environment. Approximately 10-15% of the amount of synthetic dyes produced annually is discharged with factory aqueous effluents, and most dyes are resistant to degradation by traditional biological processes. In the past two decades, several physical or chemical decolorization techniques have been developed, but few of them are accepted by the industries. Their lack of implementation is due to a high cost. As a viable alternative, a novel biological process has received an increasing interest owing to their cost effectiveness and environmental benignity. The main aim of the research was to determine the feasibility of the combined methods using fluidized bed reactor with immobilized-cell beads in decolorization of azo dye. The description of this study is divided into five sections. The first section is focused on the evaluation of modified polyvinyl alcohol (PVA) immobilized technique in decolorization of textile dyes (Chapter 2). The results prove that the modified PVA immobilization technique has been successfully applied to the decolorization process in a continuous stirred tank reactor. The elasticity and high mechanical strength of PVA beads was also proven to be adequate for the high shear stresses encountered in the reactor. Microscopic observation revealed that the microbial consortium contained in the gel beads was at least made up of three kinds of bacterial species, i.e., circle-shaped bacteria, rod-shaped bacteria, and filamentous bacteria. The second section of this research, covered in two chapters (Chapter 3 and Chapter 4), deals with the isolation of the azo-degrading bacteria from the activated sludge of petrochemical plants and the mud of University Lake. The mechanism of dye degradation using the isolated strains and the factors affecting biodegradation of azo dyes were also investigated. Several bacterial strains with the capability of degrading textile dyes were isolated. Aeromonas hydrophila was selected and identified because it exhibited the greatest decolorization ability both in terms of extent and rapidity of decolorization for 24 kinds of azo, anthraquinone, and indigo dyes. Although A. hydrophila displayed good growth in aerobic or agitation culture, color removal was the best in anoxic or anaerobic culture. Dissolved oxygen (DO) concentration exceeding 0.45 mg l-1 inhibited significantly bacterial decolorization. Glucose inhibits bacterial decolorization activity because the consumed glucose was converted to organic acids that might decrease the pH of the culture medium, thus inhibiting the cell growth and decolorization activity. In addition, preliminary results indicated that decolorization proceeded primarily by enzymatic reduction associated with a minor portion of biosorption onto the inactivated microbial cells. The third section deals with the mass transfer phenomena and the degraded dynamics of azo dyes in the PVA immobilized-isolate beads (Chapter 5). The concentration of polymer and the density of the cell in the gel beads were the most significant influencing factors on dye diffusion at a negligibly external mass-transfer resistance condition. In addition, we have developed a procedure, which permit extraction of the intrinsic kinetic parameters from observed reaction rates. This method and concept may be used to analyze the performance of other immobilized-cell systems. On the other hand, the effectiveness factors of immobilized-cell beads have been calculated for a range of microenvironmental conditions and macroenvironmental conditions. The results showed that intraparticle diffusional resistance has a significant effect on the azo dye biodegradation rate. The calculated effectiveness factor (ηcal) approached unity at Thiele modulus (Φ) < 0.3 (dp < 0.475 mm). The only limitation was believed to be the enzymatic reaction. The internal diffusional limitations were neglected. The fourth section deals with the hydrodynamic behaviors of PVA immobilized-cell beads in the liquid-solid fluidized bed bioreactor (Chapter 6). Our new experimental results were presented and compared with published results for the drag coefficient-Reynolds number, velocity-voidage, and expansion index-Reynolds number relationships observed during fluidization of rigid particles in a fluidized bed. Predictions made from previous correlations were examined with our new experimental findings, revealing the inadequacy of most of these correlations. Thus, new correlations describing the above-mentioned relationships are suggested. For multiparticle systems, the correction factor, f(ε), was a function of the falling gel bead properties (Reynolds number) as well as the fluidized gel bead properties (Archimedes number), and depended strongly on the bed voidage (ε). A new simple relation was developed to predict easily theεvalue from 0.5 to 0.9 at 4986< Ar < 40745. The experimental results may also be used to analyze the hydrodynamic performance of other various entrapped materials in liquid-solid fluidized bed bioreactor (FBR). The final section deals with the operational performances and modeling study of the liquid-solid FBR using immobilized-cell beads on decolorization of azo dye. The hydrodynamic characteristics of particles (Chapter 6) and the internal diffusional limitations (Chapter 5) were taken into account (Chapter 7). It was found that azo dye degradation time initially reaching a steady state decreased with an increase in bed expansion, cell bead number density and hydraulic retention time (HRT). The mean cell residence time (θC) on liquid-solid FBR using PVA immobilized-cell beads increased insignificantly from 1014.1 to 1014.9 days as the HRT increased from 3 to 24 h, and thus the impact of conventional reducedθC can be minimized by using the same polymer as support carries. Additionally, the internal mass-transfer resistance rather than the film diffusion resistance played an important role in azo dye utilization in liquid-solid FBR when film modulus (mf) was smaller than 1. On the other hand, the model was validated through the comparisons with the experimental data that the azo dye biodegradation was at a steady state under well-mixed operating conditions. The results also showed that the model was close enough to be used in the design and simulation of liquid-solid FBR operated with immobilized-cell bead system.