Application of a Methanotrophic Immobilized Soil Bioreactor to Trichloroethylene Degradation
Trichloroethylene , Fermentation , Biodegradation , Methanotroph
Trichloroethylene (TCE) is a major groundwater contaminant and is a cause of serious health concern. Methanotrophic TCE degradation is very promising compared with other treatments. Methanotrophs produce methane monooxygenases (MMOs) which catalyze methane oxidation and cometabolize chlorinated and aromatic compounds. High rate of TCE degradation is attributed to only soluble MMO (sMMO) expressed mainly by type II methanotrophs under copper-deficient conditions. To make methanotrophic TCE degradation practical, high density methanotrophic biomass with high sMMO activity is required. Methane is the primary substrate for methanotrophs and sufficient quantities must be supplied to support biomass growth. Because of the poor water solubility of methane, mass transfer limitation essentially restricts high biomass production. When methanol was used as the growth substrate, biomass concentration of 7.4 g l-1 Methylosinus trichosporium OB3b was achieved in a 160-h fermentation using an exponential feeding strategy based on pre-determined . Even higher biomass density of 19 and 29 g l-1 biomass were obtained by a modified feeding strategy based on carbon dioxide production. It is concluded that methanol is a promising substrate for the production of large amounts of M. trichosporium OB3bbiomass. In addition, allylthiourea was applied to methanotrophs growth medium to circumvent the inhibitory effect of copper, which inhibits sMMO activity but not particulate MMO (pMMO). We successfully retained sMMO activity by supplementing allylthiourea. Even when M. trichosporium OB3b was grown with 4.5 M copper, which would completely block sMMO expression, addition of 15 M allylthiourea preserved half of the sMMO activity. It was also observed that switching the growth substrate from methane to methanol did not significantly affect sMMO activity. An immobilized soil bioreactor was developed to examine the efficiency of methanotrophic TCE degradation by combining the knowledge obtained on high biomass production and applying allylthiourea for sMMO expression. In a batch TCE degradation experiment, about 63% of TCE was removed in 5.75 h. The maximal TCE degradation rate of 1.40 mg l-1 h-1 was obtained in a continuous TCE degradation at a dilution rate of 0.15 h-1. This study demonstrated the effectiveness of a novel bioreactor system for methanotrophic TCE degradation.