Aerobic Cometabolism of Chlorinated Solvents and 1,4-Dioxane in Continuous-Flow Columns Packed with Gellan-Gum Hydrogels Coencapsulated with ATCC Strain 21198 and TBOS or T2BOS as Slow-Release Compounds

The continuous aerobic cometabolic treatment of a mixture of chlorinated aliphatic hydrocarbons and 1,4-dioxane (1,4-D) was studied in laboratory columns packed with hydrogel beads containing bacteria and slow-release substrates. Three columns were packed with gellan-gum hydrogel beads that coencapsulate the bacterium Rhodococcus rhodochrous 21198 (ATCC strain 21198) and 8% (w/w) tetrabutyl-orthosilicate (TBOS) in columns 1 and 2 and tetrabutyl-s-orthosilicate (T2BOS) in column 3. TBOS and T2BOS slowly hydrolyze as slow-release compounds to produce 1-butanol and 2-butanol, respectively, as growth-supporting substrates for ATCC strain 21198. A mixture of 1,1,1-trichloroethane (1,1,1-TCA), cis-dichloroethene (cis-DCE), and 1,4-D was continuously fed into the columns. The addition of hydrogen peroxide (H2O2) at 50-100 ppm as an additional source of dissolved oxygen (DO) was required for the effective utilization of 1-butanol and biostimulation of ATCC strain 21198 within the beads throughout columns 1 and 2. H2O2 addition was never required in column 3, which was packed with beads containing T2BOS. Over 99% removal of all three contaminants was achieved with a hydraulic residence time of 12 h. The cometabolic transformation was confirmed by stopping H2O2 and DO addition, which resulted in an increase in the effluent contaminant concentrations to the influent levels. Transformation resumed when DO addition was restarted. Replacing cis-DCE addition with 1,1-DCE addition (100-250 mu g/L), while continuing to add 1,1,1-TCA and 1,4-D, resulted in the cessation in the cometabolic activity in the columns. At the end of the column studies, the beads were sampled and assayed for the amount of TBOS and T2BOS remaining. Approximately 56% of TBOS and 97.5% of T2BOS originally encapsulated in the beads were still present. Therefore, TBOS and T2BOS limitations were not responsible for the cessation in transformation activity in the columns. The estimated rate of hydrolysis of T2BOS was a factor of 15 lower than that of TBOS, which was consistent with the batch incubations of the hydrogel beads. The results from the column tests indicate that a passive cometabolic permeable treatment barrier might be created using the coencapsulated technology that was developed.