Remediation of groundwater contaminated with MTBE and benzene: the potential of vertical-flow soil filter systems

Abstract

Field investigations on the treatment of MTBE and benzene from contaminated groundwater in pilot or full-scale constructed wetlands are lacking hugely. The aim of this study was to develop a biological treatment technology that can be operated in an economic, reliable and robust mode over a long period of time. Two pilot-scale vertical-flow soil filter eco-technologies, a roughing filter (RF) and a polishing filter (PF) with plants (willows), were operated independently in a single-stage configuration and coupled together in a multistage (RF þ PF) configuration to investigate the MTBE and benzene removal performances. Both filters were loaded with groundwater from a refinery site contaminated with MTBE and benzene as the main contaminants, with a mean concentration of 2970 816 and 13,966 1998 mg L 1, respectively. Four different hydraulic loading rates (HLRs) with a stepwise increment of 60, 120, 240 and 480 L m 2 d 1 were applied over a period of 388 days in the single-stage operation. At the highest HLR of 480 L m 2 d 1, the mean concentrations of MTBE and benzene were found to be 550 133 and 65 123 mg L 1 in the effluent of the RF. In the effluent of the PF system, respective mean MTBE and benzene concentrations of 49 77 and 0.5 0.2 mg L 1 were obtained, which were well below the relevant MTBE and benzene limit values of 200 and 1 mg L 1 for drinking water quality. But a dynamic fluctuation in the effluent MTBE concentration showed a lack of stability in regards to the increase in the measured values by nearly 10%, which were higher than the limit value. Therefore, both (RF þ PF) filters were combined in a multi-stage configuration and the combined system proved to be more stable and effective with a highly efficient reduction of the MTBE and benzene concentrations in the effluent. Nearly 70% of MTBE and 98% of benzene were eliminated from the influent groundwater by the first vertical filter (RF) and the remaining amount was almost completely diminished (w100% reduction) after passing through the second filter (PF), with a mean MTBE and benzene concentration of 5 10 and 0.6 0.2 mg L 1 in the final effluent. The emission rate of volatile organic compounds mass into the air fromthe systems was less than 1% of the inflow mass loading rate. The results obtained in this study not only demonstrate the feasibility of vertical-flow soil filter systems for treating groundwater contaminated with MTBE and benzene, but can also be considered a major step forward towards their application under full-scale conditions for commercial purposes in the oil and gas industries.