(580d) Two Phase Biodegradation of Phenol in a Hollow Fiber Supported Liquid Membrane Bioreactor

During biodegradation of toxic
aromatic compounds, microorganisms often experience substrate inhibition
wherein the cell growth and metabolism are inhibited at higher concentration of
the toxic substrate. Several bioreactors have been proposed for the alleviation
of substrate inhibition. Most of these are based on the use of immobilized
cells but with the advent of two phase partitioning bioreactor (TPPB), this is
set to change. TPPBs are based on the equilibrium distribution of pollutants
between cell culture medium and an immiscible, biocompatible and
non-biodegradable organic solvent with high affinity for the substrate.
Although TBBPs attain very high growth and biodegradation rates and would have
made an ideal system; but various problems such as foaming and emulsification,
arising from phase dispersion limit their applicability.

The operational challenges
encountered in conventional TPPBs can be obviated by non-dispersive contacting
between the two phases using semi-permeable membranes. In this research, a
hollow fiber supported liquid membrane bioreactor (HFSLMB) was developed for
simultaneous extraction and biodegradation of phenol from wastewater using Pseudomonas putida. In a semi-dispersive
approach, extraction was carried out by dispersing the organic solvent into
feed solution whereas the transport of substrate from solvent to the cells
through the liquid membrane was non-dispersive. The dispersed solvent droplets
facilitated liquid membrane renewal and resulted in a stable mass transfer flux
while in the absence of any solvent, cell growth environment resembled that in
a monophasic system. P. putida in
suspension was able to biodegrade inhibitory phenol concentrations at 1000-4000
mg/L without experiencing severe substrate inhibition. For example,
biodegradation of 4000 mg/L was achieved within 76 hours while the specific
growth rate and biomass yield were 0.31 hr^-1 and 0.26 g/g,
respectively. Phenol biodegradation in the HFSLMB occurred in two stages: an
exponential removal rate was observed in the beginning which subsequently
assumed a linear profile under diffusion limitation. The specific growth rates
during the exponential growth phase could be controlled by increasing the
volume of the solvent while the removal rate under mass transfer limitation
improved by changing the operating conditions and the interfacial area. In
evaluating the long-term stability of the HFSLMB, the bioreactor was repeatedly
operated in batch mode with 1000 mg/L phenol. The biodegradation performance
improved during the first few runs due to the presence of attached cells on the
membrane. But the performance deteriorated subsequently at higher biofilm
thickness. The biofilms could be removed by alkaline washing lasting less than
5% of the operation time and resulting in the restoration of the biodegradation
performance. With two washing cycles, HFSLMB operation could be sustained for
416 hours in which 20 batch runs were conducted.

The HFSLMB offered many
advantages that included a better growth environment for the microorganisms,
lower energy demand, high separation factor, high removal rates and elimination
of secondary waste. The modular design is easier to scale-up and can also be
used for continuous operation. Modeling the HFSLMB will provide further
insights into substrate mass transfer and metabolism.