| A fibrous-bed bioreactor using a coculture of Pseudomonas putida and P. fluorescens immobilized in a fibrous matrix has been developed to effectively degrade benzene, toluene, ethylbenzene, and xylenes (collectively known as BTEX) in contaminated streams. BTEX compounds, selected as model pollutants, are usually inhibitory to cell growth. Therefore, they represent the characteristics of biodegradation kinetics of a wide range of pollutants present in industrial waste streams and contaminated groundwater. Both transient and steady-state behaviors of BTEX biodegradation in the fibrous-bed bioreactor operated under liquid-continuous and gas-continuous conditions were studied. BTEX compounds were used as the sole carbon sources, and the cultures were prescreened and preacclimated using toluene. Hydrogen peroxide {dollar}rm(Hsb2sb2){dollar} as an additional oxygen source showed a positive effect on BTEX biodegradation.; In comparison to free (suspended) cell systems, the immobilized cells in the liquid-continuous bioreactor tolerated higher concentrations ({dollar}>{dollar}1,000 mg/L) of benzene and toluene, and gave at least 16-fold higher degradation rates for benzene, ethylbenzene, and ortho-xylene, and a 9-fold higher degradation rate for toluene. Individual BTEX compound was efficiently and concurrently degraded in the liquid-continuous bioreactor at a retention time (RT) of less than 15 hours from a synthetic waste stream containing high BTEX concentrations. They were also efficiently degraded at an RT of less than 1 hour for the groundwater samples contaminated with low BTEX concentrations. Immobilized cells adapted in the bioreactor showed no preferential degradation of BTEX compounds in mixture, whereas the original cells grown as free-cell batch cultures showed degradation preference in the order of benzene, toluene, o-xylene, and ethylbenzene, when they were present in mixture. The bioreactor also maintained its ability for efficient BTEX degradation for more than one year. The superior performance of the fibrous-bed bioreactor can be attributed to the high cell density (4.433 g cell dry weight/L reactor liquid working volume) and its ability to adapt the cultures to the bioreactor environment and to select and enrich better BTEX degraders. The carbon mass balance study confirmed that benzene and toluene were completely mineralized without producing any identifiable intermediate metabolites. In the gas-continuous bioreactor operated as a trickling biofilter, benzene and toluene were efficiently and concurrently degraded at an empty bed retention time of less than 10 minutes from a synthetic waste gas stream containing a mixture of benzene and toluene. The biofilter also had a relatively low pressure drop at a high superficial gas velocity. This fibrous-bed bioreactor showed a long-term stability of more than one year by maintaining the efficient BTEX degradation ability, and the ability to adapt other microorganisms from the BTEX-contaminated groundwater to the reactor environment and subsequently enable them to efficiently degrade BTEX. |
