Biodegradation of trichloroethylene by Methylocystis sp. strain M immobilized in gel beads in a fluidized-bed bioreactor

A fluidized-bed bioreactor (FBB) containing Methylocystis sp. strain M (strain M) isolated from Japanese soil was developed for treating synthetic groundwater containing trichloroethylene (TCE). Strain M bacteria were immobilized in 2% calcium alginate gel beads and introduced into the reactor, which was supplied with a methane/air gas mixture. TCE concentrations in the reactor were reduced from 0.9–1.6 to 0.1–0.2 mg/litre after a residence time of 2.56 h. Thus, 80–90% of the influent TCE was degraded in the reactor, and the resulting effluent gas contained only 0.02–0.04 mg/litre TCE. The ability of the gel-immobilized cells to degrade TCE declined rapidly during the TCE degradation process. To compensate for this decrease in activity, the reactor was operated for TCE degradation and reactivation of the immobilized strain M cells on alternate days. TCE degradation activity was completely recovered within 24 h after supplying a methane/air mixture (2:8 v/v) and a mineral salt solution to the bacteria. The degradation capacity of the reactor was kept in a steady state for 10 days in this way, but the rate of recovery of TCE degradation activity gradually decreased. TCE degradation activity fell from 6.0 litres/g dry weight (gdw) per hour on the sixth day to 1.4 litres/gdw per hour on the 24th day. Interestingly, there was no corresponding decrease in methane consumption by the bacteria