Benzene/toluene/p-xylene degradation. Part II. Effect of substrate interactions and feeding strategies in toluene/benzene and toluene/p-xylene fermentations in a partitioning bioreactor

A two-phase aqueous;organic partitioning bioreactor scheme was used to degr ade mixtures of toluene and benzene, and toluene and p-xylene, using simult aneous and sequential feeding strategies. The aqueous phase of the partitio ning bioreactor contained Pseudomonas sp. ATCC 55595, an organism able to d egrade benzene, toluene and p-xylene simultaneously. An industrial grade of oleyl alcohol served as the organic phase. In each experiment, the organic phase of the bioreactor was loaded with 10.15 g toluene, and either, 2.0 g benzene or 2.1 g p-xylene. The resulting aqueous phase concentrations were 50 mg/l, 25 mg/l and 8 mg/l toluene, benzene and p-xylene respectively. Th e simultaneous fermentation of benzene and toluene consumed these compounds at volumetric rates of 0.024 g l(-1) h-(1) and 0.067 g l(-1) h(-1), respec tively. The simultaneous fermentation of toluene and p-xylene consumed thes e xenobiotics at volumetric rates of 0.066 g l(-1) h(-1) and 0.018 g l(-1) h(-1), respectively. A sequential feeding strategy was employed in which to luene was added initially, but the benzene or p-xylene aliquot was added on ly after the cells had consumed half of the initial toluene concentration. This strategy was shown to improve overall degradation rates, and to reduce the stress on the microorganisms. In the sequential fermentation of benzen e and toluene, the volumetric degradation rates were 0.056 g l(-1) h(-1) an d 0.079 g l(-1) h(-1), respectively. In the toluene/p-xylene sequential fer mentation, the initial toluene load was consumed before the p-xylene aliquo t was consumed. After 12 h in which no p-xylene degradation was observed, a 4.0-g toluene aliquot was added, and p-xylene degradation resumed. Excludi ng that 12-h period, the microbes consumed toluene and p-xylene at volumetr ic rates of 0.079 g l(-1) h(-1) and 0.025 g l(-1) h(-1), respectively. Oxyg en limitation occurred in all fermentations during the rapid growth phase.