Two phase biodegradation of phenol in a hollow fiber supported liquid membrane bioreactor

Abstract

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.