Azo-dye conversion process by means of Pseudomonas sp. OX1 biofilm

The presence of azo-dyes in industrial effluents reduces wastewater transparency and oxygen solubility. Studies available in the literature on bacterial degradation of azo-dyes highlight the success of decolorization processes by microbial consortiums. The biodegradation of the monoazo-sulphonic dye Acid Orange 7 by means of Pseudomonas sp. OX1 was investigated both in gas-sparged flasks and in internal loop airlift reactors under controlled operating conditions. Bioremediation was accomplished by a sequence of aerobic and anaerobic phases. Cell growth took place only during the aerobic phase, while dye conversion occurred only during the anaerobic phase, according to a Michaelis–Menten-type kinetics. The maximum extent of decolorization was closely correlated with the amount of substrate carbon converted during the preceding aerobic phase; a limiting conversion of 70 mgdye per gram of substrate carbon was previously measured whatever the carbon source (Lodato et al., 2007). Present contribution regards further results about the conversion of Acid Orange 7 by means of P. sp. OX1 in an airlift biofilm reactor. A model for the simulation of the conversion process under a sequence of aerobic/anaerobic conditions is also presented. Experimental investigation was carried out in a bench-scale airlift reactor. Macro-porous pumice stones were adopted as biofilm carrier. The reactor was operated under continuous conditions – with respect to the liquid phase - during the aerobic stage and batch-wise during the anaerobic stage. Phenol supplemented synthetic medium was fed during the aerobic stages. Reference conditions assumed in the model were: spherical impervious particles wrapped of uniform biofilm in a three-phase internal loop airlift; well mixed reactor operated under continuous conditions with respect to both the liquid and the gas phases. Phenol was fed only during the aerobic phase whereas Acid Orange 7 was fed both during the aerobic and the anaerobic phase. The reaction network embodies: phenol conversion under aerobic conditions, oxygen depletion as a consequence of cells uptake – due to both growth and maintenance contributes - and diffusion in the biofilm, dye conversion under anaerobic conditions. The conversion kinetics were assumed in agreement with previous investigation (Viggiani et al., 2006; Lodato et al., 2007). A double substrate kinetic model was adopted for growth with phenol inhibition and oxygen Monod type kinetic. Instead as regards dye conversion a Michaelis-Menten type kinetic with oxygen inhibition was adopted when anaerobic conditions were established. Conversion process carried out with the biofilm reactor has been characterized in terms of time resolved concentration of biofilm, free cells, phenol and dye. Simulation results regard steady states conditions, aerobic or anaerobic. Results are in terms of phenol, oxygen and dye concentration in the liquid phase and throughout the bioparticle and effectiveness factors of biocatalyst. The influence of operating conditions on the effectiveness factors were also investigated. Lodato et al., 2007, Enz. Microb. Technol. 41, 646-652 Viggiani et al., 2006, J. Biotechnol. 123, 464-477