(426e) Two-Phase Biodegradation of Phenol in a Hollow Fiber Membrane Bioreactor

Authors: 
Praveen, P., National University of Singapore
Loh, K. C., National University of Singapore


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.