Two-Phase Biodegradation of Phenol in a Hollow Fiber Membrane Bioreactor

Research in the field of biodegradation of toxic industrial organics has made significant advances in the past two decades. These developments have been propelled mainly by the discovery of microbial strains capable of metabolizing, hitherto, non-biodegradable xenobiotics, genetic and metabolic engineering and proteomics analysis of cellular responses to biodegradation. On the engineering front, several innovative bioreactor designs have been proposed with potential to metabolize high inhibitory concentrations of the pollutants. Most of these bioreactors, however, are based on cell immobilization and therefore, exhibit very low biodegradation rates.

Recently, aqueous-organic two-phase biodegradation systems have been investigated as an alternative to cell immobilization to avert substrate inhibition. Biphasic bioreactors are also capable of attaining high rates of biodegradation and would have made an ideal system; but the plethora of problems resulting from the dispersion of the two phases render the application relatively impractical.

The objective of this research was to mitigate the operating challenges encountered in a biphasic biodegradation system by designing a dispersion-free two phase membrane bioreactor for the biodegradation of high strength phenol using Pseudomonas putida. The strategy was to use hydrophobic hollow fiber membranes to physically separate the aqueous and organic phases. The high surface area and porosity of the fibers facilitated the mass transfer of the substrate between the phases while the low pore size prevented any cell immobilization and helped in retaining the bacteria in suspension. The results from the bioreactor in which up to 2000 mgl^-1 phenol was biodegraded indicated that there was little effect of substrate inhibition on cell growth. Microorganisms did not exhibit any lag phase and the cell growth was independent of the total phenol concentration. The specific growth rates varied with aqueous phase phenol concentrations as in a monophasic aqueous culture. Biodegradation rate was controlled by the exponential cell growth in the beginning and by phenol mass transfer towards the end. It was also found that a high mass transfer rate of substrate was critical to achieve high biodegradation rates by delaying the onset of diffusion limitation. By doubling the number of fibers, it was possible to achieve biodegradation of 2000 mgl^-1 phenol in a mere 43 hours with an improved growth and biodegradation rates.

The biodegradation kinetics of the membrane bioreactor was comparable to that of dispersion-based two phase biodegradation systems. However, during the operation of the membrane bioreactor, no foaming or emulsification of the liquid phases was observed. The organic solvent - segregated from the aqueous phase, could be easily recycled and reused; making the process environment friendlier. Another advantage of the membrane bioreactor was the low-energy demand of the setup as the phenol mass transfer was independent of the speed of agitation. Further studies on the hollow fiber membrane bioreactor will be aimed at carrying out simultaneous extraction and biodegradation of phenol from waste water.