Development Of Bioreactors Used For Remediation Of Azo Dye Containing Wastewater

White rot fungi have been proposed for remediation of soils contaminated with toxic organic pollutants such as DDT, polychlorinated phenols, and polycyclic aromatic hydrocarbons (Bumpus 1993). However, decolorization and remediation of wastewater from a variety of industries that produce colored wastewater may be the most promising place for use of these fungi in bioremediation systems. Processes developed to remediate colored wastewaters have been the subject of considerable research that has been reviewed recently by Knapp et al. (2001). Although many investigations have focused on the ability of white rot fungi to decolorize/degrade azo dyes, only a few investigations have specifically focused on development of bioreactors that could be scaled up and used for this purpose. Zhang et al. (1998) showed that an unidentified basidiomycete designated F29 was able to extensively and rapidly decolorize high concentrations of the azo dye Orange II in three different bioreactor configurations. Continuous packed-bed, fedbatch fluidized-bed, and continuous fluidized-bed bioreactors were studied. All of the bioreactors studied proved suitable for bioremediation of water-containing Orange II. Concentrations of dye up to 1000 mg/L were removed relatively rapidly (1-3.5 days) and the mycelium could be used repeatedly. Particular success was achieved with the fedbatch fluidized bed-immobilized reactor. In this system average decolorization rates of 40-50 mgl21hr21 were achieved resulting in 97% color removal in 24 hr. This study is significant as it demonstrates that rapid degradation occurs and the fungal mycelium are robust and unaffected by exposure to high concentrations of Orange II during repeated exposure over 1 -2 months.

Swamy and Ramsay (1999a,b) studied conditions, which promote azo dye degradation by Trametes versicolor, Bjerkundera adusta and Phanerochaete chrysosporium sequentially in batch reactors. Only T. versicolor proved to be able to maintain its ability to cause decolorization of water following repeated addition of the several dyes and dye mixtures.

Wang and Yu (1998) studied the ability of T. versicolor to decolorize water containing an anthroquinone dye, an indigo dye, and the azo dye Acid Violet 7. All of the dyes were degraded. These investigators also showed that adsorption to mycelium followed by dye degradation occurred. Adsorption was rapid and occurred with living mycelium and with heat killed controls. With live fungi, mycelium was regenerated by physical desorption and enzymatic degradation. The authors suggest that a sequential treatment system could be developed in which dyes in wastewater could be first adsorbed on fungal mycelium followed by decolorization/degradation. It is reasonable to suggest that sequencing batch reactor technology might function well in this situation. Indeed, Borchert and Libra (2001) have used T. versicolor in 4-L sequencing batch stirred tank reactors to decolorize water containing the azo dyes Reactive Black 5 and Reactive Red 198 and the anthroquinone dye Brilliant Blue R. Reactors were cycled repeatedly over the course of the experiments. During long-term (200 days) experiments under sterile conditions, 18 dye additions occurred. In each case substantial color removal occurred. When the initial concentration of dye was 100 mg/L, 97-99.5% decolorization occurred. When the initial concentration was 500 mg/L, 91-99% decolorization occurred. When similar experiments were performed under nonsterile conditions, only 5 decolorization cycles (during a 55-day experiment) were possible due to bacterial contamination, which appeared to interfere with peroxidase activity, resulting in decreased decolorization. Biodegradation of Acid Violet 7 was also studied in liquid batch cultures and in a fluidized bed reactor (Zhang and Yu 2000). In both cases, the ability of mycelial pellets to degrade this dye was studied. An interesting aspect of this research was that added activated charcoal had a positive effect on biodegradation. It was shown that fungal mycelium surrounded a core of activated charcoal to form complex pellets. These complex pellets appeared to enhance biodegradation by adsorbing both the dye and fungal enzymes responsible for biodegradation. Furthermore, decolorization by the complex pellets was superior to that observed for fungal pellets without activated charcoal, for fungal pellets to which activated charcoal was added (after pellet formation) and to activated charcoal alone. It was also shown that reactors operated in repeated-batch-fed mode resulted in greater and more efficient decolorization than reactors operated in a continuous flow mode. In one experiment a repeated-batch-fed reactor was able to decolorize 9 additions of dye over a period of 130 h. Batch-fed additions of dye ranged from 100 to 500 mg/L.

Mielgo et al. (2001) studied the use of immobilized P. chrysosporium in continuous flow packed bed bioreactors. At dye loading rates greater than 95%

decolorization was achieved at hydraulic retention times of 24 h. In these experiments, the temperature was maintained at 37°C and oxygen was supplied in a pulsed flow.

Trametes hirsuta was shown to effectively decolorize the azo dyes Reactive Black 5 and Direct Blue 71 as well as representative triphenylmethane, indigoid, and anthraquinone dyes (Abadulla et al. 2000). All of the dyes were decolorized by the laccase isolated from T. hirsuta. This research is important as it addressed the issue of recycling dye-contaminated water. Water decolorized using the intact fungus or laccase in solution was not suitable for reuse in dyeing operations due, presumably, to interference by soluble protein. However, when a reactor containing immobilized laccase was used, the recycled decolorized wastewater was suitable for dyeing operations as the laccase was retained in the bioreactor.

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